Spectroscopic measurements of vapor clouds due to oblique … · trograph, ICCD camera unit, and a data acquisition unit. The ICCD cameras (Oriel Corporation, InstaSpec V)have 256

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. E8, PAGES 19,427-19,441, AUGUST 25, 1998

Spectroscopic measurements of vapor clouds due to oblique impacts

Seiji Sugita and Peter H. Schultz Department of Geological Sciences, Brown University, Providence, Rhode Island

Marc A. Adams

Jet Propulsion Laboratory, Pasadena, California

Abstract. Vapor clouds generated by oblique impacts were observed spectroscopically. The observations here concentrate on the earliest downrange-moving component, among multiple components of vapor clouds generated by hypervelocity impacts of quartz projectiles into dolomite targets. The spectrum of impact vapor simultaneously exhibits blackbody radiation, molecular band emission, and atomic line emission, but their relative ratios change with time and space. The spectrum of the earliest component is dominated by the line/band emissions. The strong band/line emissions demonstrate the presence of a gas phase. The ratio of line/band emission to blackbody radiation is higher in near-vertical impacts than shallower angle impacts. Ratios of normalized intensities of emission lines indicate that a Boltzmann distribution with an equilibrium temperature ranging from 4000 K to 6000 K approximates well the distribution of calcium atoms in energy levels in the impact vapor. Furthermore, the temperature of impact vapor appears to be controlled by the vertical component of the impact velocity. The degree of self-absorption of a calcium emission line, which comes from vaporized dolomite target, indicates that only a very small mass of impact vapor is involved in the observed atomic radiation process. This observation, as well as the short lifetime and extremely high temperature, suggests that jetting may be the dominant source of the line emissions in the earliest stage vapor component at impact velocities less than 6 km/s. The experimental techniques and analysis methods presented in this study are applicable to other components of impact-generated vapor and will provide new information on vaporization mechanisms in hypervelocity impacts.

1. Introduction

Intense flashes of light from high-speed impact events have been observed for decades [e.g., Atkins, 1954; Charters, 1960], but the physics of the phenomena has not been studied or understood well yet, perhaps due to the complexity and variety. For example, there are at least three different kinds of radiation sources in hypervelocity impact events: solid/melt under intense shock, typically observed in flyer plate experiments [e.g., Kondo and Ahrens, 1983]; high speed jetting material [Yang et al., 1992]; and impact-induced vapor clouds [Schultz, 1996; Kadono and Fujiwara, 1996; Schultz et al., 1996; Adams et al., 1997; Sugita et al., 1997].

There are also a variety of experimental techniques used to measure the impact flash. Each method has its own advantages and disadvantages. One of the most widely used detectors is a photomultiplier tube (PMT). PMTs are often combined with ei- ther interference filters or monochromators to measure a spe- cific wavelength (or a narrow band of wavelength) of light [Gehring and Warnica, 1963; Jean and Rollins, 1970; Eich- horn, 1975, 1976, 1978ab; Kadono and Fujiwara, 1996]. An advantage of PMTs is their capability for monitoring the time history of an impact flash. But they do not record many differ-

Copyfight 1998 by American Geophysical Union.

Paper Number 98JE02026. 0148-0227/98/98JE-02026509.00

ent wavelengths simultaneously. A spectrograph using photographic film is much more effective for capturing a wide range of spectral information. Although this approach allows observing the elements and molecules in the flash [Gehring and Warnica, 1963; MacCormack, 1963; Jean and Rollins, 1970; Schultz, 1996], the time evolution cannot be monitored. The minimum exposure time allowed by such measurement systems is much longer than the timescale of impact flash phenomena, i.e., only time-integrated signals can be obtained. A strategy to limit the effective exposure time to a microsecond level is to place the slit of a spectrograph perpendicular to the trajectory of a rapidly moving vapor cloud [Schultz, 1996]. The film is then exposed to the vapor radiation only when the cloud is passing in front of the slit. Although this approach captures short-duration spectra, quantitative analysis of spectral data, particularly line/band emission, is difficult or ambiguous. Thus high resolution in both time and spectral dimensions are necessary to interpret spectra created by impact vapor (impact flash spectra). Such a requirement is fulfilled with more ad- vanced electronic devices such as a multichannel analyzer [e.g., Kondo and Ahrens, 1983] and an intensified charge cou- pled device (ICCD) detectors [e.g., Schultz et al., 1996; Adams et al., 1997; Sugita et al., 1997]. The high sensitivity and rapid electric shutter system of such detectors allow obtaining "snap shots" of impact flash spectra.

The purpose of the present study is to analyze quantitatively spectra of the impact vapor (the impact flash spectra) and to determine the temperature of the vapor using high-speed spec-

19,427

19,428 SUGITA ET AL.: SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS

trometers. After establishing the method of temperature measurement, we investigate the effects of impact angle on flash temperature and assess the mass of the source of flash light. Photometric and photographic measurements of vapor cloud expansion indicate that the impact vapor production process is strongly controlled by impact angle [Schultz, 1996], but little quantitative spectral analysis has been possible without ad- vanced high-speed spectrometers.

Temperature measurements of the impact flash are important for two reasons. They provide fundamental data for understanding the physics of impact vaporization. When the temperature of such impact-induced vapor is known as a function of impact conditions, the energy partitioning mechanism into vapor phase during hypervelocity impact events can be better understood. Although impact vaporization phenomena are often assumed to be controlled only by local peak shock pressure, they may be controlled by other mechanisms as well, particularly in oblique impacts [Schultz and Gault, 1990; Schultz, 1996]. More accurate knowledge of the energy partitioning mechanism also will benefit understanding the atmospheric evolution of terrestrial planets in their early history because the evolution is most likely controlled by impact degassing and vaporization [e.g., Matsui and Abe, 1986; Ahrens et al., 1989; Melosh atwl Vickery, 1989; Vickery atwl Melosh, 1990; Abe, 1993].

A second reason for understanding the impact flash is its application to future planetary explorations (e.g., characteriz- ing asteroid surface compositions using impact flash spectroscopy) [Schultz et al., 1996; Adams et al., 1997]. One of the most important goals of such applications is to know the chemical composition of a target surface from intensities of line emissions of different elements in the flash lights created by artificial impacts. But the relative intensities of emission lines of different elements and different energy levels change with temperature, even if the elemental abundance in the flash- source vapor cloud is constant [Sugita et al., 1997]. The temperature of impact-induced vapor may be influenced by many factors such as impact velocity, angle, surface roughness, and porosity. Thus temperature measurements of impact-induced vapor under known conditions are critical for such applications.

Because one of the primary goals of this study is to estab- lish a basic technique to measure the temperature of impact vapor, we concentrate on observing emission spectra with strong line emissions relative to blackbody continuum. Such "clean" flash spectra do not occur in all impacts nor through the entire time history of the process; rather, they occur only in early stages of impact events with relatively high impact velocities. Thus only the earliest component of impact vapor clouds is analyzed in this study. The technique developed in this study, however, ' is not restricted to such clean spectra but can be used for any spectra with significant line emissions.

2. Experimental Conditions

The impact experiments were conducted with the two-stage light gas gun at the NASA Ames Vertical Gun Range (AVGR). Impact velocity ranged from 4.7 to 5.6 km/s. Impact angle was from 15 ø to 90 ø , measured from the horizontal. Spherical quartz projectiles were used because previous studies had shown that other projectiles such as aluminum generate strong line/band emissions, which would interfere with target signals [Schultz et al., 1996; Adams et al., 1997]. The diameter and the mass of

the projectiles are 6.35 mm and 0.288 g, respectively, with dolomite used as the target material. Advantages of dolomite are its high volatility, high concentration of both calcium and magnesium, and knowledge on its impact vaporization and impact flash generation obtained in previous investigation [Schultz, 1996].

Two spectrometers are used simultaneously in order to cover a wide range of spectrum. Each spectrometer consists of a spectrograph, ICCD camera unit, and a data acquisition unit. The ICCD cameras (Oriel Corporation, InstaSpec V)have 256 X 1024 pixels of CCD array and 16 bit read-out capability. The vertical column of 256 pixels is binned to maximize the signal-to-noise (S/N) ratio of spectrograms. The detector is sensitive to light with wavelength from 180 to 800 nm and has a minimum exposure time of 5 ns. Exposure is triggered by a phototransistor placed near the point of impact that captures the flash at the first contact between an impactor and the target. The resulting electric signal is sent to a digital delay/pulse generator (Stanford Research Systems, Model DG535), which converts the analogue electric signal to a square TIL signal. The TTL signal, which has a prescribed delay and a duration, i s sent to both the spectrometers and an oscilloscope. Compari- son between times of flight calculated from the velocity meters of AVGR and times estimated from a phototransistor signal indicates that the response of the triggering system is well within a microsecond, which is comparable to the velocity measurement error (-- 10 m/s).

The spectral resolution is determined by the spectrographs (Oriel Corporation, Imaging Spectrograph, MS127i) attached in front of the ICCD cameras. The spectrographs are Czerny- Turner type and have a focal length of 12.5 cm. Gratings with ruling density of 1200 1/mm are typically used in this study. The full width at half maximum (FWHM) of the spectrometers with this configuration is about 0.9 nm. Emission lines from calcium atoms occur over a broad spectral range. Conse- quently, one spectrometer is designated to cover shorter wavelengths (425 - 530 nm); the other, longer wavelengths (525 - 650 nm). The two spectral ranges overlap a strong calcium emission line at 526.5 nm, thereby allowing relative intensity calibration for the two spectrometers. When a wider range of wavelength needed to be covered, gratings with a lower ruling density (600 l/mm) are used. Wavelength calibration of the spectrometers was done manually with a mercury-argon lamp and a neon lamp each time the wavelength range was changed. Fidelity tests of wavelength precision before and after a series of impact experiments indicate that the error and temporal drift of the wavelength calibration is about 0.1 nm, which corresponds to a pixel or less of the CCD arrays.

The spectral sensitivity of the spectrometers was calibrated with a 200 W quartz-tungsten-halogen lamp (Oriel Corpora- tion, Model 63355). The intensity calibration lamp is Na- tional Institute of Standards and Technology (NIST) traceable over the spectral range of 250 - 25,000 nm. Because the size of the filament of the lamp is much smaller than that of the view spot of spectrometers, the light source can be approximated as a point source. The calibration lamp was placed inside the impact vacuum chamber in order to account for the entire optical path from the impact site to the detectors. This gives intensity calibration for the spectral measurement system. However, cross examination of the calibration results using an extended uniform light source with calibrated intensity of radiation (Labsphere, Model USS-600) indicates that the spectrometers have significant nonuniform spatial sensitivity over the view

SUGITA ET AL.: SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS 19,429

spot. The nonuniform spatial sensitivity results from the structure of the bundled optical fibers in the image slicer units in the spectrometers, which reshape the circular light beam projected from foreoptics into a narrow slit shape. Since the image slicer greatly enhances optical throughput of the spectrometer system, it is an indispensable part of the measurement system. But the entrance of bundled optical fibers has both "active" core areas of optical fibers and "dead" peripheral areas around the fiber cores. This causes an undesired spatial nonuniformity. Thus the absolute scale of radiance has significant uncertainty. Nevertheless, cross examination also indicates that there is no significant variation in spectral sensitivity throughout the view spot of the spectrometers. In other words, the spectrometers give the same spectral pattern when the same light source is placed at any point in the view spot. But the magnitude of the signal may significantly dependent on the location of the light source within the view spot. Con- sequently, only relative intensity of emission lines are used in the analyses of this study.

The foreoptics collect light with a view spot size of ap- proximately 4 cm in diameter on the target plane. The view spot is located downrange from the point of impact in order to isolate the downrange-moving self-luminous component of an impact vapor cloud and avoid the point of impact itself, ex- cepting the 90 ø impact cases.

The spectrometers are set on the top of the impact vacuum chamber and view the target plane through a window. The distance between the impact site and the spectrometers is about 2 m. The latitudinal angle of the viewing vector of the spectrometer is about 75 ø measured from the horizontal. The az-

imuthal angle of the vector is about 45 ø measured from the azimuth of the impact trajectory.

3. Principles of Measurements

In this section, we describe the theoretical basis of our

quantitative analyses of impact flash spectra. First, a method is derived to determine a temperature from relative intensities of emission lines. Then a method is developed to estimate the mass of a self-luminous gas cloud from the degree of self- absorption of an emission line.

3.1. Temperature

The temperature measured with the method used here is called excitation temperature, and has been widely used in emission spectroscopy [e.g., Griem, 1964; Gaydon and Wolf- hard, 1970]. The radiant intensity In, (W/sr) of a line emission due to an electron transition of neutral atoms in an optically thin gas body is given by

h v t,, I,,, = -•-• A,,,N u (1)

where I and u are the indices of the lower and upper energy levels of the electron transition responsible for the line emission, respectively [e.g., Griem, 1964; Cannon, 1985]. The other symbols, h, vn,, Atu, and N,,, indicate the Planck constant, the frequency of photons in the line emission, the Einstein A coefficient of the transition, and the number of atoms in the

upper energy. Energy levels of neutral calcium atoms are shown in Figure 1 as well as the transitions for the observed emission lines in our impact experiments. The number of at-

oms in an energy state is given by a Boltzmann distribution if thermodynamical equilibrium is achieved:

N,, = g'-2-' exp - (2) go kr) o

where g,,, E,,, k, and T are the statistical weight and the energy level of the energy state u, Boltzmann constant, and temperature, respectively. The subscript "o" denotes the ground state of atoms. The assumption of thermal equilibrium in the vapor cloud may break down [e.g., Gaydon and Wolfhard, 1970], e.g., when an exothermic chemical reaction takes place or if the density is too small (i.e., a collisionally thin cloud). Even under disequilibrium conditions, however, the electron energy distribution is often approximated by Boltzmann function [e.g., Gaydon and Wolfhard, 1970]. The temperature defined by the atomic energy distribution (i.e., the electron energy distribution) is different from the temperature defined by the random translational motion of atoms. The former, often called excitation temperature, is what is measured in this study. As long as an apparent Boltzmann distribution is observed in the atomic energy distribution (i.e., equation (2)), the excitation temperature can be determined. Such a temperature always can be used to estimate the number N o of atoms in the ground state in a vapor cloud from line emission intensities with equation (1). The relation between the number of ground-state atoms and the total number of atoms is discussed in detail below. The ratio of

the two numbers is usually very close to unity at moderate temperatures.

Although equations (1) and (2) contain many parameters, most of them are constants independent of the physical conditions of the vapor and depend only on the type of atoms emit- ting light. Those constants can be obtained in National Bureau of Standard (NBS) tables and other references. Adopted values and their sources are listed in Table 1. By introducing dimen- sionless normalized radiant intensity It, , equations (1) and (2) can be rewritten into a much simpler form:

In •n, = In N O - E,._._•, (3) kT

where

I lu

]t,,-- hV,u gu At,, 4n: go

(4)

Equation (3) indicates that the intensity of line emissions de- pends on both the number of atoms and temperature. The temperature dependence of intensity is different for line emissions with different upper energy level E,. But if the natural loga- rithm ln]n, of normalized intensities is plotted against their upper energy levels Ee/k, then the data points should line up along a straight line for a single excitation temperature. The extrapolated value of In ]•u at the energy level of zero gives the number No of atoms in the ground state. The slope of the straight line gives the inverse of temperature.

In reality, the temperature of impact vapor captured within the view spot of a spectrometer may not be uniform. The temperature variation within the view spot, however, cannot be measured with the current configuration of the spectrometer because all the light within the view spot is summed together to attain a high S/N ratio. Thus a result of the above spectral analysis method gives an average temperature of a vapor cloud.

19,430 SUGITA ET AL.' SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS

70000

60000-

50000-

40000-

30000-

20000-

10000-

.

452.69

.i•• 5 •__s ._, 5 __/-\s2•6 504 X /_•_• 560

6116 --

610 - 616 42• 61

6

422, •/ / 4142 / •446

Ground State

487.81 458 - 459

Neutral Calcium

-6

-5

-4

-3

-2

-1

-0

Figure 1. Energy diagram of neutral calcium atoms. The horizontal bars indicate energy levels of electrons. The arrows indicate transitions whose emission light is observed in this study. The numbers associated to the arrows are the wavelengths of emission lines in nanometer. The vertical axes are energy level in Kelvin (left) and electron volt (right).

It should be noted that an average temperature tends to be shifted toward the highest temperature within a vapor cloud. This shift results from the nature of gas radiation that gas at higher temperature emits more radiation and tends to dominate the emission spectrum of an inhomogeneous gas body when the body is optically thin.

Figure 2 shows synthetic diagrams of line emissions from neutral calcium atoms with spectral width comparable to the instrumental resolution. The intensities of emission lines are

calculated with equations (1) and (2); i.e., emission-source gas is assumed to be optically thin and to be in thermodynamical equilibrium. The relative line intensities change with temperature. The intensity of emission lines around 610-620 nm and 640-650 nm, whose upper energy states are relatively low, is higher at a lower temperature (Figure 2a). But they becomes lower at a higher temperature (Figure 2b). The intensity of the strongest line at 422.7 nm is greater than the other lines by a factor of more than ten in the synthetic diagrams. The anoma- lous strength comes from both its large Einstein A coefficient and low excitation energy (see Table 1 and Figure 1). But the intensity of the 422.7 nm line actually observed in laboratory experiments rarely exceeds several times the intensities of other strong lines because of self-absorption. Such discrepancy between theoretically predicted intensities and actual observations requires particular caution in spectral analysis of impact vapor clouds. As shown below, however, the observed intensities of the other emission lines of calcium is predicted well by a low-opacity model (i.e., equations (3) and (4)). Thus the effect of self-absorption is inferred to be insignificant for

the observed emission lines of calcium other than the 422.7

nm line.

3.2. Mass Estimate

Self-absorption becomes significant when the optical thickness of the radiating gas body becomes large. Equation (1) assumes no absorption and holds only when the optical thickness of radiating source is small. Thus the semilogarith- mic relation (equation (3)) is not applicable for an optically thick gas body. When the degree of self-absorption is quanti- fied, however, it constrains the column density of radiating atoms in a vapor cloud. This procedure involves some detailed discussions about the radiative transfer equation of line emission, spectral line broadening, the partition functions of atoms, and ionization.

When light scattering is negligible, the radiative transfer equation in a given direction is written as

8x

where Ldx) is spectral radiance (W/m2/sr/Hz) with frequency v at position x [e.g., Griem, 1964; Cannon, 1985]. The absorption coefficient kv of the gas, the source function By, and the number density no of atoms in the ground state are assumed to be homogeneous within a gas body. Thermodynamical equilibrium is also assumed here. The absorption coefficient k• and source function (W/m2/sr/Hz) B• of gas in thermal equilibrium are given by

SUGITA ET AL.: SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS 19,431

Table 1. Einstein A Coefficients of Neutral Calcium Atoms

(nm) g,, Ee/k At, , Erl 27 guAtuVtu Er2 )•

(K) (108s '•) (%) (1022 s '•) (%) (nm) gu Egtk Atu Erl 27 guAtuYtu Er2

(K) (108s '•) (%) (1022 s 4) (%)

422.67 3 34032 2.18 a 10 46.3 10 534.94 7 58329 0.163 c 5.0 6.40 5

428.30 5 55471 0.47 b 10 119 428.93 3 55346 0.64 b 10 429.89 3 55346 0.51 • 10 430.25 5 55471 1.45 • 10 430.77 1 55279 1.87 • 10 431.86 3 55346 0.76 • 10

442.54 3 54314 0.50 a 13 443.49 5 54320 0.65 a 7 443.56 3 54314 0.36 a 13

445.47 7 54327 0.87 a 7 445.58 5 54320 0.30 a 13 445.66 3 54314 0.025 a 13

452.69 3 63214 0.307 c 2

457.85 5 60677 0.128 c 2.6 458.14 7 60679 0.176 a 15 458.14 5 60677 0.025 a 50 458.58 9 60679 0.188 a 15 458.59 7 60679 0.018 a 50 458.61 5 60677 0.0007 a >50

468.52 5 64735 0.0802 c 3.5

487.81 7 60928 0.274 c 1.9

504.16 3 59971 0.296 ½ 3

518.88 5 61755 0.4620 5.0

91

6.10

25

10 551.29 1 60125 0.784 c 2.1 4.26 2

558.19 7 55050 0.0852 c 2.6 558.87 7 55050 0.696 c 2.6 559.01 5 54992 0.115 ½ 2.6

559.44 5 54992 0.533 c 2.6 559.84 3 54953 0.581 c 2.6 560.12 5 54992 0.127 c 2.6 560.28 3 55817 0.193 c 2.6

62.5 3

585.74 5 58591 0.6840 5.0 17.5 5

610.27 3 45381 0.097 b 12 612.22 3 45381 0.28 • 11 616.21 3 45381 0.47 • 10

1.3 10

15 615.60 5 52627 0.0022 a >50 5.80 3.6 616.12 5 52627 0.0190 c 3.0 616.37 3 52598 0.0304 c 2.6

616.64 1 52588 0.126 c 2.8 616.90 3 52598 0.0955 c 2.8

616.95 5 52627 0.117 c 2.6

2.57 4 643.90 9 51651 0.439 c 2.5 646.25 7 51539 0.418 c 2.6

11.8 2 647.16 7 51539 0.0468 c 1.9

526.03 5 56605 0.0093 c 2.3

526.17 3 56598 0.212 c 2.1 526.22 1 56595 0.817 c 2.6 526.42 5 56605 0.128 c 2.8 526.55 3 56598 0.618 c 2.3 527.02 5 56605 0.696 c 2.6

5.27 3 649.37 5 51411 0.246 c 2.1 649.96 5 51411 0.0480 c 2.1

13.4 5 650.88 5 51411 0.0024 a >50

33.5 3

6.83 3

42.5 3 644.98 5 51562 0.101 c 2.0 2.69 3 645.56 5 51562 0.015 ½ 7.0

Here •., gu, Eu/k, Alu, Erl, 27 guAtuVtu, Er2, and vtu are wavelength in nanometer, the statistical weight of the upper energy state of electron transition, the energy of the upper state divided by Boltzmann constant in Kelvin, Einstein A coefficient in 108 s '•, estimated errors of Einstein A coefficient of each line in percent, the sum of gAv in a cluster of emission lines. in 10 22 s '2, estimated errors of the sum in percent, and frequency in s 4, respectively. Clusters of emission lines are set off with line spaces. The sources of the data are indicated by the symbols in the column of Einstein A coefficients:

aWiese et al. [ 1969]. •Ueda et al. [1982]. CSrnith and Raggett [1981]. øHunter et al. [ 1985]. ½Srnith [ 1988].

k v = Alu exp - 1 exp - gt(v- v, ) Too ' (6)

2hv 3 2

By = c (7) exp -1

where c, h, gt, and Vl,, are the speed of light, Planck constant, the spectral profile of an emission line, and the central frequency of an emission line, respectively [e.g., Griem, 1964; Cannon, 1985]. Note that the absorption coefficients of different emission lines are different because they are controlled by both the Einstein A coefficients and the upper energy levels of the electron transitions. Thus the optical thickness of a

given gas body is different for different emission lines. This is the reason why only the 422.7 nm line, which has the largest Einstein A coefficient and the lowest energy level among the atomic lines of neutral calcium, is subject to very strong self- absorption and others are not.

When the radiating gas body is assumed to be homogeneous and to have no incident light from outside, equation (5) is integrated easily and gives the spectral radiance of emerging radiation from the gas body:

L•(xo)=B•(1-e -•v(xø)) (8) where ,• and Xo are the optical thickness and the linear thickness of a gas body along the line of sight, respectively. Note that the optical thickness is measured along the line of sight. Optical thickness ,• is defined as

ß v(x)=k•noX (9)


0.8

0.6

0.4

0.2

' ' ' '

a 3000

ß

ß

ß

ß

ß

ß

ß

ß

ß ß ß ,

400 450 500 550 600 650

Wavelength (nm)

0.8

0.6

0.4

0.2

' ' ' '

ß b 6000 ß

ß

ß

ß

ß

ß ß ß ß

400 450 500 550 600 650

Wavelength (nm)

Figure 2. Synthetic spectrograms of emission due to neutral calcium atoms. The temperature of the source calcium vapor is assumed to be 3000 K in Figure 2a and 6000 K in Figure 2b. No self-absorption is considered. The FWHM of each emission line profile due to instrumental resolution is assumed to be 0.8 nm.

The spectral irradiance received by a light detector is the integral of spectral radiance over the solid angle which the radiating gas body occupies with respect to the detector. But when a detector is very far from the radiation source, virtually only one direction of light reaches the detector. The distance between the spectrometers and the target position in our experimental conditions is about 2 m, which is much larger than the size (<0. t m)of expanding vapor clouds in the first 2-20 gs. Thus the integral over the solid angle can be replaced with an integral over the cross-sectional surface area of the gas body. The radiation emerging from each portion of the gas body is described by equation (8). When the shape of the gas body is assumed to be spherical, the spectral irradiance FdR) (W/m2/Hz) which a detector at the distance of R receives from the body is given by

and

SB•I l_ (t + •.•,ma,) e ....... ) =R-- T- t- 2 •'vm•x / 2 (lO)

S=m 2 (lt)

•'vmax ---- 2k•nor (12)

where r, •, S, and •%max are the radius of a sphere, distance from the center of the sphere along the line of sight, the cross- sectional surface area of a sphere, and the maximum optical thickness of the gas sphere, respectively. Although equation

SUGITA ET AL.' SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS 19,433

(10) is derived for a spherical gas body, it can be used for any ellipsoidal homogeneous gas body. Note that both the projectional surface area S and the maximum optical thickness are measured with respect to the direction of the line of sight. The quantity 2nor is the column density of a vapor cloud at its maximum thickness along this line of sight. When the optical thickness •'•ax of a gas body is small, equation (10) asymp- totically approaches

F•(R)=k• vN/R 2 (13)

where No is the total number of ground-state atoms in the entire volume of a gas body. When equations (6) and (8) are substi- tuted, equation (13) becomes identical to equation (1) except for the line profile factor grand geometric attenuation factor R 2. Radiant intensity It, in equations (1) and spectral irradiance F•,(R) in equation (13) are connected through the following relation:

fVl. +AV . . _

It. = R 2 Jv,,_,vF•(R)dv (14) where integral width Av needs to be large enough to cover the spectral broadening of an emission line.

A spectral line profile gr(v-vt,,)in equation (6) is controlled by both temperature and pressure of radiation source gas. When only the temperature effect is significant, an emission line profile is described by a Gaussian function (Doppler broadening), and when only the pressure effect is significant, the profile usually can be approximated by Lorentz broadening [e.g.,

Mitchell and Zemansky, 1961]. When both effects are significant, the spectral profile becomes a convolution of the two profiles, i.e., a Voigt function. The Voigt function is of a form of an integral and is expensive to compute. Consequently, an analytical approximation by Whiting [1968] is used here. Al- though the Doppler broadening is easily estimated from a temperature measurement described above, Lorentz broadening is very difficult to assess because it is unknown how much pressure each gas species in impact vapor has. But because the width of emission lines in spectrograms obtained in impact experiments is indistinguishable from the instrumental line width (-0.9 nm), the real line widths are significantly smaller than 0.9 nm. This gives an upper limit of the Lorentz broadening.

Figure 3 shows the degree of self-absorption of the emission line at 422.7 nm as a function of column density of ground-state neutral atoms and Lorentz line width. The shape of a vapor cloud is assumed to be ellipsoidal, though the result is not very different from a slab symmetric case. The average column density of the vapor cloud, which is 2/3 of its maximum column density, is used in F. igure 3. The degree of self- absorption is expressed as apparent opacity •'a• defined as

*avv =-In(I/1,,,) (15)

where I and l, hi, are the total radiant intensities of a line emission when the effect of self-absorption is taken into account (i.e., equation (10)) and when the effect is not considered (i.e., equation (13)), respectively. Radiant intensity I is obtained as

10

0.1

• / Avt./Av o !

, / • •0.01 • / ......... 0.1

' / ..... 1 / - - -10

0.01 , ,, Z' .... • ........ • ........ ' ........ ' ........ ' ....... 0.001 0.01 O. 1 1 1 0 1 O0 1000

Column Density (ng/cm 2)

Figure 3. Apparent optical thickness of a calcium vapor cloud with respect to the Ca emission line at 422.7 nm. The temperature of an impact vapor cloud is assumed to be 6000 K. The emission line profile is assumed t o be a Voigt function due to both Doppler and Lorentz broadening. The FWHM Av,• of Lorentz broadening is given as multiples of that of Doppler broadening Avt•. An ellipsoidal shape of an impact vapor cloud is assumed; i.e., equation (10) is used. The horizontal thin line indicates the apparent opacity of an observed vapor cloud, with the upper and lower limits of the estimate indicated by dashed lines. The spectrum of the observed vapor cloud is shown in Figure 5f.


an actually measured radiant intensity in an impact experiment. The other radiant intensity I,h,, is obtained as an extrapolated value from equation (3) with other weaker emission lines which are free from significant self-absorption. Note that apparent opacity •app in equation (15) is different from optical thickness •v defined by equation (12). The former •app is a quantity defined for an entire emission line and is not necessarily proportional to the number of atoms in a vapor cloud, whereas the latter •v is defined for a wavelength within an emission line and always proportional to the number of atoms by its defini- tion.

When apparent opacity, temperature, and the degree of line broadening are given, the column density of a vapor cloud can be determined from Figure 3. The column density is 2no r in equation (12). But this density includes only neutral atoms i n the ground state. Estimation of the total number of calcium atoms requires the atomic population in the other energy states, including ionization states. The ratio •gr of the number of neutral atoms in the ground state to the that of the total atoms is given by

1

•g, = Ni (16) Z+ "Z •

N,,

where Z, Z •, N o, and N o' are the partition functions of neutral atoms and singly ionized atoms and the numbers of neutral and singly ionized atoms in the ground states, respectively. The number of atoms in higher stages of ionization is assumed to be negligible here, because it requires exceedingly high temperature. The ratio No'/N o can be estimated by the intensity of emission lines of ions

• = -A-exp 'J (17) N o lb, kT

where the superscript I indicates the first stage of ionization and the subscript i and j are energy levels of lower state and upper state of the transition of the singly ionized atoms, respectively. The partition function Z of neutral atoms in equation (16) is given by

L', < E,,,,,

The effective ionization energy of an atom E,o . is given by

E ..... = Eoo -AEoo (19)

where E• and AE•. are the ionization limit and reduction in ionization energy of neutral atoms, respectively. The ionization limit of neutral calcium atoms is 70,941 K or 6.1132 eV

[Sugar and Corliss, 1985]. The reduction in ionization energy AE• is derived from a plasma theory [Griem, 1964] and given by

2 e

AE• = • (20) 41rSoP D

J eøkT (21) fid = '2e2n-•. where Po, e, eo, and r/e are Debye radius, electron charge, dielec- tric constant of vacuum, and electron density, respectively.

Here it is assumed that macroscopic electric neutrality holds and that the charge of all the ions is +1. The partition function Z • of singly ionized atoms is given by a similar summation as equation (18), though the reduction •Eoo in ionization energy of singly ionized atoms is twice of that given by equation (20) [Griem, 1964]. Comprehensive tables of energy levels and statistical weights of both neutral calcium atoms and ions can be found in the work by Sugar and Corliss [1985]. Such tables, however, do not exhaust all the energy states required by equation (21). Then an approximate expression by Griem [1964] based on hydrogenic structure of outer electron orbits is used for higher energy states. The approximation is

where

Z = g• exp --• ,=1

3

+--(2S + 1)(2L + 1) exp - kT J 3

(22)

m e e 4 E• = atto2---- • (23)

for E•, S, L, and m• are the ionization limit of a hydrogen atom, spin and orbital angular momentum of the inner electron shell of atoms, and electron mass, respectively. The partition function of singly ionized atoms is given similarly. The cutoff quantum number i .... of individual counting of energy levels in equation (22) is 8 for neutral atoms and 10 for singly ionized atoms in this study. This choice of i ..... does not affect the result significantly [Griem, 1964]. Figure 4 shows the partition functions of both neutral and singly ionized calcium atoms. The partition functions of atoms are generally functions of both temperature and electron density. When temperature is lower than 7000 K, the partition functions of calcium atoms are almost independent of electron density over a wide range of its values.

4. Results and Discussion

This section first describes the spectral content of the impact flash obtained in the experiments. Quantitative temperature measurements and mass estimates for the impact vapor clouds are then presented. Finally, the source of the atomic line emission observed in the experiments is discussed based on these results.

Figure 5 illustrates some of the impact flash spectra. Most impact flash spectra show strong continuum background radiation. The ratio of the intensity of line emissions to that of the continuum is the highest immediately after impact (Figure 5a) and decreases with time (Figures 5b and 5c). For a given exposure time, the line emission intensities relative to the contin-

uum background increase with impact angle. Figures 5d and 5e show that shallower impacts (30 ø and 15 ø ) produce much lower line emission intensity relative to the continuum background than in 60 ø cases (Figures 5a, 5b, and 5f). Most of the continua can be fitted well by single-temperature blackbody curves (3000 - 4000 K), though some of continua with very low intensity are difficult to fit with a blackbody curve because of strong interference with molecular band emission. Thus the continuum background is not an instrumental effect but real blackbody radiation. The source of the blackbody radiation is

SUGITA ET AL.' SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS 19,435

•. Electron Density

c• 8 •- (X1023m '3) / 0 !- x 0.01 / • L .... x-...O.03 ,.. / --x--0.1

,- ::--:• lO

= 4

•E• ." x" .';"5" X

2

0 t , I , , , I , , , I , , , 2000 4000 6000 8000

Temperature(K)

10000

10

Electron Density (X 1023m '3)

x 0.01 .... x .... 0.03 - -x- .0.1 •0.3

..... 3 - - -10

, I , • , I , , • I , , , I , , • 2000 4000 6000 8000 0000

Temperature (K)

Figure 4. Partition functions of (a) neutral calcium atoms and (b) singly ionized calcium atoms as functions of temperature and electron density. Note that all the curves in Figure 4b coincide with each other because the electron density has little influence over the calculated range of temperature.

most likely incandescent melt or solid fragments, rather than an optically thick hot vapor. It is possible that there is a hot vapor cloud with large amount of molecular gas such as CO2 and CO. Such gases, however, should show molecular bands Oust as MgO and CaO do) in the observed spectrograms (Figure 5). In addition, strong emission bands of CO2 and CO are re- ported at much shorter wavelengths than the present experimental coverage [e.g., Pearse and Gaydon, 1976].

Except for the 15 ø impact cases with an extremely high thermal background level, all impact flash spectra exhibit

measurable atomic emission lines from calcium, magnesium, and sodium. Molecular band emissions from calcium oxide and

magnesium oxide are also observed. In some spectrograms (such as Figure 5b), individual band heads are observed in the MgO band emission. These atomic/molecular emissions clearly indicate the presence of a gas phase as previously observed in time-integrated spectra [Schultz, 1996]. The very weak atomic/molecular emission in the 15 ø impacts is not co- incidental but reproducible, because a couple of 15 ø impacts show very similar emission spectra with predominant blackbody background for different exposure times. The vapor may be mixed with (or masked by) considerable fragmental impactor debris directed downrange as documented in other studies of impactor survival during oblique impacts [Schultz and Gault, 1990; Schultz and Sugita, 1997]. Nevertheless, the reduced intensity of emission lines in lower angle impacts may indicate a fundamental change in the vapor production process. Both possibilities will be explored in separate contributions.

4.1. Temperature Measurements

In order to assess the temperature of the gas phase, the emission intensities of calcium lines were measured and

Boltzmann plots were made based on equations (3) and (4). The sum of the photons captured by the CCD pixels within the spectral width of a line emission determined by instrumental resolution is taken as an intensity measurement of the line emission after the contributions of blackbody continuum and molecular band emission have been subtracted. This integral intensity is more accurate than a peak intensity because it is not as sensitive to instrumental spectral resolution. A peak intensity, which is much easier to measure, is roughly inversely proportional to the instrumental resolution.

Because the intensity calibration of the spectrometers is accurate only in a relative scale, the measured intensities of two spectrometers differ by a certain factor. Differences also come from the slight mismatches in positioning of the spectrometer view spots. These effects were assessed by comparing measurements of 526.5 nm calcium line, which is covered by both of the spectrometers.

Due to the limitation of the spectrographs, not all the emission lines are spectrally resolved. The intensities of those lines are measured together if they are from the same or very close upper energy states. In such cases, the summed intensities are normalized with summed normalization factors as

listed in Table 1 accordingly. Two examples of the intensity measurements are shown in

Figure 6. Although some scatter remains, the normalized intensities are closely approximated by straight lines determined by least squares fits. This strongly suggests that the flash- source vapor is in thermal equilibrium, or at least the electrons in a cloud are in an equilibrium state over the exposure time. The linear distribution of normalized emission intensities also

supports the validity of equation (1), i.e., the approximation of optical thinness of the vapor clouds. The inverse of the slope of the straight line in Figure 6 represents the temperature of flash-source vapor as equation (3) shows. The shallower slope of the straight line for the vertical impact (slightly higher velocity) indicates that this impact produced higher temperature vapor than the 30 ø impact.

The errors in temperature are estimated by two methods. The first approach is a standard error analysis method [Press et al., 1992]. Because the measurement error for the intensity of each emission line is difficult to assess, it was estimated from the


2000

1500

1000

r• 500

0

a 60 ø, 5.27km/s, 0 - 2 !•s

54.3 63.2 60.7 60.9 60.9

Ca Ca Ca Ca Ca

ner y level (1000K) 45.4 59.3 56.6 58.3 60.1 55.6 ,• Mg Ca Ca Ca Ca

,, 1 ,,

I

450 500 550

58.6 24.4 45.4 52+.6 51.6 Ca Na Ca Ca Ca

, 600 650

Wavelength (nm)

4OOO

35OO

= 3000 ß

o 2500

• 2000

1500

1000

500

I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

b 60ø, 5.25 km/s, 0-20 gs

450 500 550 600 650

Wavelength (nm)

Figure 5. Impact flash spectra produced by impacts with quartz projectiles and dolomite block targetsß The impact velocity, impact angle measured from the horizontal, and exposure time measured from the first contact of impact are noted in the figures. Indicated in Figure 5a and 5b are source atoms/molecules of emission lines/bands and the energy levels of the upper states of the electron transitions in 1000 K. The spectral irradiance is in an arbitrary unit, which is calibrated for relative intensity within a spectrogram but has uncertainty in absolute scale across different spectrograms. Magnesium emission lines at 517 and 518 nm in Figures 5b and 5c are truncated because of saturation of CCD elements. Calcium lines around 610-612 nm are not truncated

despite of their higher intensities because the ICCD system is more sensitive to shorter wavelength light and reaches the saturation count for smaller intensity of light in a shorter wavelength range. Note that the spectral resolution (FWHM) in Figure 5f is about 1.5 nm, whereas the resolution for Figure 5a through 5e is about 0.9 nm.

scatter of the data points in the Boltzmann plots. Overall error (lo') in each data point is roughly 25%, including the uncer- tainties in Einstein A coefficients, effects of self-absorption, and intensity calibration.

The second method is a "removal" method. Although this is not a standard method for an error analysis, it provides an ob- jective measure for the robustness of a linear regression analy-

sis in this study. Although we use the intensity of many emission lines of calcium atoms for the analysis, there are many other emission lines out of our spectral range and detection limit. Weaker emission lines also sometimes may not be measured with significant accuracy and have to be discarded from an analysis. Thus the choice of emission lines is not unique or constant but is rather arbitrary. The removal method

SUGITA ET AL.: SPECTROSCOPIC MEASUREME•S OF VAPOR CLOUDS 19,437

35OO

3OOO

25OO

2000

1500

1000

5OO

' I ' ' ' ' I ' '

C 60 ø, 5.22krn/s, 1.4 gs - 25 ms

450 500 550 600 650

Wavelength (nm)

o

o

2OOO

1500

IOO0

5OO

d 30 ø, 5.32 km/s, 0 - 20 gs

, I , i , t I , , , • I , t , , I , , , ,

450 500 550 600

Wavelength (nrn)

650

Figure 5. (Continued)

can evaluate how such an arbitrary choice affects the temperature estimate. First, two data points are removed from an ob- servational data set. This calculation is repeated for all the possible combinations of two data points to remove. Then the mean value and standard deviation of the estimate temperature are calculated. Consequently, the approach resembles simu- lated experiments. This error analysis gives a temperature estimate for a fictitious case as if we had spectrometers with ei- ther a narrower spectral coverage or lower sensitivity so that fewer emission lines can be measured. If the temperature estimate is insensitive to the choice of the removed emission

lines, then we should expect that the results will not be altered when additional emission lines are measured with a better spectrometer. The mean temperature obtained by the removal method is the mean value of estimated temperatures when the intensities of two emission lines less than a real experiment

are used for an analysis. The standard deviation for the removal method is the breadth of the distribution of the temperatures estimated from data sets with two points less data. Calculation results indicate that this distribution has a similar form to a

Gaussian distribution in most cases. From this distribution, we can assess how the best fit temperature may change when more data points become available.

Since the choice for the number of removed data points is rather arbitrary, we tried similar analyses with one and three data points removed. For our particular data sets, however, the standard deviation is generally largest when two data points are removed and this is presented in Figure 7, along with results of the standard error analysis. Comparison of the errors by the two methods indicates that the mean temperatures by removal method and the best fit temperatures by the standard method correlate well and that the standard deviations of the

19,438 SUGITA ET AL.' SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS

c-

.--

2OOO

1500

IOO0

5OO

4.71 km/s, 0 - 20 !xs •

450 500 550 600

Wavelength (nm)

65o

c-

8000 '

6000

4OOO

2000

[ , i i I I

f 60 ø, 5.19km/s, O-21xs

4OO

i i i

450 500 550 600 65O

Wavelength(nm)

Figure 5. (continued)

temperature estimates by the removal method are generally smaller than those by the standard method. This means that the arbitrary selection of emission lines does not significantly affect the results of the temperature estimates.

Figure 8 compares the flash temperatures for different impact conditions, such as impact angle 0, velocity V, and the vertical component Vsin0 of impact velocity. The error bars in Figure 8 represent lo of confidence level estimated by the standard method for error analysis. Although all three vari- ables (V, sin0, and Vsin0) show a correlation with vapor temperature, the vertical component of impact velocity appears to have the best correlation. When this trend is approximated by a power law, the power-law index/• is about 0.3:

T,x:V•_ t• , /• = 0.3 (24) where

V•_ = V sin 0 (25)

and V• is the vertical component of impact velocity. Although the vapor temperature clearly increases with the vertical component of impact velocity, the precise value of the power-law index may be subject to future revision when more data points are available. It is noteworthy, however, that vapor temperature is a rather weak function of the vertical component V•_ of impact velocity, particularly compared with the radiation intensities of emission lines. Experiments by Gehring and Warnica [1963] and Jean and Rollins [1970] indicate that the intensities of radiation of emission lines in impact flashes are proportional to 4th to 8th power of impact velocity for vertical impacts.

SUG1TA ET AL.: SPECTROSCOPIC MEASUREMENTS OF VAPOR CLOUDS 19,439

6 -

5 -

4 -

3 -

2 i ,

16' [ [ [ I ] ' [ [ I [ [ ' ' I [ ] ]

45000 50000 55000 60000 65000

E•/k: Energy Level (K)

Figure 6. Boltzmann plot of impact flash emission. The normalized intensity is defined by equation (4). The energy level, E,,/k, normalized by Boltzmann constant is given in Ta- ble 1. The solid circles• indicate emission intensity in an impact flash due to a vertical quartz impact into a dolomite block with 5.54 km/s of velocity. The slope of the regression line corresponds to 5600+_500K. The open circles are due to a quartz impact into a dolomite block with 30 ø of impact angle measured from the horizontal and 5.32 km/s of impact velocity. The estimated temperature indicated by the dashed line i s 4200_+300 K.

4.2. Mass Assessment

The mass of the earliest component of a vapor cloud can be assessed by measuring the degree of self-absorption of the line at 422.7 nm and the intensity of the ion lines at 393.4 nm and 396.9 nm. But because such measurements require a wider spectral range, a grating with a lower ruling density was used in the shorter wavelength range at the expense of spectral resolution. One of the spectrograms from such experiments is shown is Figure 5f. The apparent opacity •' of 422.7 nm of the impact flash is 3.0 +_ 0.3. The estimated temperature is 5600 +_ 500 K.

The uncertainty in the estimated column density comes largely from that created by pressure broadening (Figure 3). But the FWHM of pressure broadening must be less than 10 times of that of Doppler broadening in this particular impact vapor; otherwise, the effective line width of the 422.7 nm line would exceed the instrumental line width. With the uncertain-

ties in apparent opacity and pressure broadening, the average column density is estimated to be 0.3 - 100 ng/cm 2 (Figure 3).

Intensity measurements of calcium ion lines at 393.4 nm and 396.9 nm indicate that the number ratio Not/No of ionized to neutral atoms are 6 +_ 3 % (cf. equation (17)). Substituting this ratio and partition functions obtained from Figure 4 into equation (16), the column density of all calcium atoms is estimated to be about 1.5 times that of neutral calcium atoms in

the ground state. High-speed photographs show that the radius of the down-

range-moving self-luminous component of an impact vapor cloud 2 •ts after the impact in a vacuum condition has grown to 1 - 3 cm [Schultz, 1996]. If the radiation from the component of a vapor cloud is assumed to be somewhat constant throughout the 2 •ts of exposure time of the spectrometers and an mean projectional surface area of 10 cm 2 is adopted, the mass of calcium is estimated to be 5 ng to 2 •tg. Thus the mass of atomic calcium, including ionized atoms, is 10 -8 to 10 -5 of a projec-

tile mass (0.288g). Because this does not include other gas species in the impact vapor cloud such as Mg, CaO, MgO, and CO, the total mass of this early-time vapor component may be much more than 10 -'• projectile mass. However, it is very un- likely that the total vapor mass is comparable to a projectile mass, in contrast to vapor released later in the cratering process [e.g., Schultz, 1996].

4.3. Source of Atomic Line Emission

Experiments by Gehring and Warnica [1963] show that flash radiation is independent of the target thickness for impacts into thin sheets. This also suggests that the material involved in the early-time impact flash processes is very small amount and that it is from only near the surface of contact. Temporal variation of intensity of emission lines obtained by PMTs attached to monochromaters indicates that the timescale

of impact flash is comparable to the timescale of projectile penetration [Jean and Rollins, 1970]. Other much broader band observations with PMTs obtained similar timescales for im-

pact flash phenomena with aluminum and steel projectiles [Gehring and Warnica, 1963], although similar measurements by Kadono and Fujiwara [ 1996] using nylon projectile yielded much longer duration of light. The large discrepancy in the lifetime of the impact flash between metal and nylon projectiles may be caused by the large difference in heat of vaporization. The heat of vaporization of nylon is roughly an order of magnitude less than that of metals and silicates. The vaporization phenomena by nylon impacts may be controlled by processes that metals and silicates experience only at much higher impact velocity.

The short-lived, small-mass, high-temperature gas discussed here is most likely produced by the jetting process [e.g., Kieffer, 1977]. Theoretical calculations of jetting by Ang [1993] show that certain combinations of projectile and

7000

6000

5000

4000

ß _

ß

, -

: Standard Error

"I•-- "Removal" Error [', , i , I, I, ,, ,I I ,I I I , I ..... I .........

4000 5000 6000 7000

Best-Fit Temperature

Figure 7. Comparison between best fit temperatures by a standard linear regression method and mean temperatures estimated by a "removal" method. See the text for details of the two methods. The horizontal and vertical error bars indicate 1 o'

confidence levels estimated from the standard linear regression

metl•od and the removal method, respectively.


7000

6OOO

5000

4OOO

0.4

30 ø 45 ø 60 ø 90 ø 8000 !

• Qz, O-20us FR Qz, 1.4us-25ms /• Qz, O-2us

0.5 0.6

7000

6000

5000

4000

3000 0.8 0.9 1 4

E• Qz, O-20us Qz, 1.4 us - 25 ms /• Qz, O-2us

5

b

sin0

8OOO

7000

6OOO

5OOO

4OOO

I Qz, O- 20 us FR Qz, 1.4us-25ms /• Qz, O-2us

3000 .......

2 3 4 5 6 7 8 9

V sin0

Velocity (km/s)

Figure 8. Impact flash temperature as a function of (a) impact angle, (b) impact velocity, and (c) vertical component of impact velocity. The impactors and targets are quartz spheres and dolomite blocks, respectively. The exposure times are indicated in the diagrams. Note that both vertical and horizontal axes are in logarithmic scales.

target materials may have an impact velocity range where flash intensity decreases with impact velocity, which Eichhorn [1976] had found experimentally. Although Yang et al. [1992] observed only blackbody radiation in the jetting material, the estimated temperature of a jet is very high (>3000 K) even for relatively low impact velocity (<2 km/s). It is rather surpris- ing that extremely high-velocity impacts (up to 35 km/s) by a Van De Graaf accelerator for nanograms to micrograms masses yielded only blackbody radiation [Eichhorn, 1975, 1976, 1978ab]. But it is very likely that any early time emission containing line/bands is obliterated by later blackbody radiation because the lifetime of the early time emission is expected to be nanoseconds for such small particulates. It is also possi-

ble, however, that the process producing the impact flash by such high-velocity submicron impacts is different.

5. Conclusions

Through the use of high-speed spectrometers, we have been able to probe the compositional and temperature characteris- tics of early-stage impact vapor. Several conclusions can be drawn.

1. Measurements of line emission intensities indicate that

the population of calcium atoms in different energy levels is approximated well by a Boltzmann distribution.

2. Temperatures inferred from Boltzmann distributions range from 4000 K to 6000 K.

SUGITA ET AL.: SPECTROSCOPIC MEASUREME•S OF VAPOR CLOUDS 19,441

3. Vapor temperature shows the best correlation with the vertical component of impact velocity among the examined parameters over the experimented velocity range.

4. Both large self-absorption of the line at 422.7 nm and insignificant self-absorption of other lines are observed simultaneously.

5. The degree of self-absorption of the emission line at 422.7 nm suggests that the mass of the earliest component of an impact vapor cloud is much less than a projectile mass.

6. These observations, as well as previous literature, sug- gest that jetting may be the source of the early-stage impact "flash."

On the basis of these experiments and analyses, we will be able to probe the rapidly evolving thermodynamic conditions and chemical reactions within other components of an impact- generated vapor cloud, which have been observed in high- speed photography by Schultz [1996]. Such experimental data could be used as calibration references for theoretical models of

impact cratering phenomena.

Acknowledgments. The authors would like to thank Wayne Logsdon and John Vongrey at the NASA Ames Vertical Gun Range for their indispensable and excellent technical support. J. T. Heineck provided critical and creative help. This study has benefited greatly from discussions with T. Hiroi, O. S. Bamouin-Jha, A. Sugita and D. Fletcher and constructive reviews by J. Moses and G. Eichhom. This research was supported by NASA grants NAGW-705, NAGS-3877, and Jet Propul- sion Laboratory Director's Discretionary Fund (JPL-960879).

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(Received November 21, 1997; revised June 8, 1998; accepted June 12, 1998.)

Documents

Spectroscopic measurements of vapor clouds due to oblique … · trograph, ICCD camera unit, and a data acquisition unit. The ICCD cameras (Oriel Corporation, InstaSpec V)have 256