An Efficient Intracavity Laser Raman Spectrometer

An Efficient Intracavity Laser Raman Spectrometer

MICHAEL HERCHER, WILLIAM MUELLER, STANLEY KLAINER, ROBERT F. ADAMOWICZ, RONALD E. MEYERS, and STEPHEN E. SCHWARTZ Block Engineering, Inc., 19 Blackstone Street, Cambridge, Massachusetts 02139 and Atmospheric Sciences Division, Department of Energy and Environment, Brookhaven National Laboratory, Upton, New York 11973

The design of an optimized intracavity laser Raman spectrometer is described, and representative Raman scattering data are presented. An intracavity laser power of 160 W was attained with an argon ion laser whose normal output power was 1 W.

Index Headings: Laser Raman; Aerosol analysis; Gas analysis; Pol- lutant monitoring; lntracavity lasers.

INTRODUCTION

Laser Raman spectrometry has appeared to be an attractive technique for the determination of trace con- stituents in the air (i.e., gases and aerosol species such as sulfates) which do not have a spectroscopic signature in the visible other than by Raman scattering. How- ever, the low signal intensities attainable with conven- tional laser Raman spectrometers have limited this application of Raman technique. To overcome the prob- lem of low sensitivity, the technique of placing the Raman scattering sample within the laser cavity (to take advantage of the increased power), has been suc- cessfully exploited by a number of spectroscopists. ~'= This technique has been refined to maximize the intracavity laser power, minimize the amount of laser plasma radiation reaching the sample volume, and optimize the optical coupling between the sample volume and the spectrometer. As an initial result, an intracavity power of 160 W at 488 nm has been obtained from an argon ion laser whose normal output at this wavelength was 1 W. At this power level Raman scattering from atmospheric nitrogen or oxygen at the intracavity beam waist was visible by looking through a yellow glass filter. The corresponding Raman line for the ~4N~SN Q-branch could be readily detected at 2293 cm -~ using a double monochromator and cooled photomultiplier. The most important considerations in achieving this performance were the minimization of losses within the laser cavity and the proper coupling of the intracavity beam waist to the spectrometer slit.

I. COLLECTION OPTICS

Fig. 1 is a schematic drawing showing the collection of 90 ° Raman-scattered light from a sample located in the vicinity of a focused laser beam. A short length of the laser beam waist is imaged onto the entrance slit of a spectrometer. The relevant parameters of the optical system are given in Table I.

To match the acceptance angle of the spectrometer to that of the collection optics properly, the magnification should be:

M = ~/SSo~ (1)

Received 18 November 1977; revision received 28 December 1977.

298 Volume 32, Number 3, 1978

This magnification defines the length L of the laser beam waist which is imaged onto the entrance slit of the spectrometer:

L = h / M = h ~ (2)

This value of L can now be used to determine the optimum diameter of the laser beam waist. Assuming that the laser is operating in the TEMoo mode, it is straightforward to show from laser resonator theory 3 that the diameter D of the laser beam at a distance ± L / 2 from the beam waist is given by:

D = 2Wo [1 + (LX/.,rrwo2)2]½ (3)

We next wish to define a value for the laser waist diameter, 2wo, which minimizes D so that all of the Raman-scattered light can be imaged onto a spectrometer slit of minimum width. By differentiating Eq. (3), the beam waist diameter which minimizes D is given by:

2wo = 2[LX/2~-]½ (4)

LASER I BEAM WAIST ~,-']

i I

[MAGE IN lii . ~ , ~ \ . . . . ~ \ .__ / ' ~ ~ f/5.6 SINGLET PLANE OF

I/~Z~ ___ II , ~ r ~ ) - j 1 ~ f= 20cm ENTRANCE SLII

\ ~( I f/O.9 PROJECTION \ I~\ j,'~ LENS SPHERICAL Ill f = 2.5cm MIRROR I I

",F Fro. 1. Collection optics.

TABLE I. Symbol definitions.

Symbol Definition

h d & & M P E

L

Wo

D p (T

T To Q

Height of spectrometer slit Width of spectrometer slit Solid angle accepted by spectrometer a Solid angle accepted by collection optics Magnification of collection optics Lase~ power Laser photon energy Length of laser beam waist imaged at spectrometer slit

(L = h/M) Laser beam waist radius (TEM~,~ mode) Laser beam diameter at beam waist ±L/2 Density of Raman-scattering molecules (molecules/cm 3) Raman scattering cross-section (cm ~ sr -1 molecule -1) Wavelength of laser radiation Optical transmission of spectrometer Optical transmission of collection optics Quantum efficiency of photomultiplier tube photocathode

a S s ~ ~/T{1 -- c o s [ t a n -~ d/2f/no.)]}.

APPLIED SPECTROSCOPY

The resulting value of D is given by:

Dmin -- 2½(2wo) (5)

and the expression for the corresponding spectrometer slit width, which defines the spectral resolution, is:

d = Dmi,M = [4hk(So/Ss)'~/~']~ (6) tz

The Raman scatter photoelectron count rate for this optical system is given by:

m (photoelectrons/s)= 2[P/E]LSopo'QTTo, or (7)

2[P/E] (SsSo)½hpo-QTTo

where the parameters are those defined in Table I and where the factor of 2 comes from the use of an auxiliary spherical mirror which doubles the solid angle of scattered light collected (Fig. 1). From this equation it is fairly clear that there are only a limited number of steps which can be taken to increase the Raman signal from a given gas sample: (1) select a spectrometer which maximizes S~, T, and h; (2) use as high a laser power as possible; (3) use a large solid angle of collection, So, and; (4) use a detector with a high quantum efficiency, Q.

In addition to these steps, one can also consider increasing the sample density and using a wavelength which maximizes the Raman cross-section. The major emphasis in designing the laser Raman system dis- cussed herein has been on the maximization of the laser power at the sample and of the collection solid angle, and the optimization of the coupling between the illu- minated sample region and the spectrometer.

The system described here had a collection solid angle of approximately 0.79 sr obtained by using an f/0.9 projection lens with a 2.54 cm focal length. This lens approximately collimated the light from each point in the sample volume. The collected light was focused onto the entrance slit of the spectrometer by a 20 cm focal length f/5.6 singlet (Fig. 1). By using two lenses in this way (rather than a single lens) either lens could be individually focused while a constant separation between the laser beam waist and the spectrometer slit was maintained.

II. THE LASER

There are two aspects to the advantage to be gained by placing the Raman scattering sample within the laser resonator: first, the laser power is immediately higher by a factor of [1 + R]/T, where R and T are the reflectivity and transmission of the laser output mirror. Second, the intracavity power can be further increased by a significant factor if the output mirror is replaced by a mirror having a higher reflectivity.

Consider a laser which has a round trip optical gain of~7. nepers, and a round trip loss of (~/, + ~2) nepers, where ~1 and ~/2 are the components of the loss due to (a) transmission through the output mirror and (b) all other losses-such as scattering, absorption, diffraction, etc. The intracavity laser power can be writ ten 4

P~ = A[(,y1 + T2) -~ - ('yT) -1] (8)

If we now consider two different values for ~/~, say ~, and .y,', we can write the ratio of the corresponding intracavity powers as:

p,/p = [ ' y i ' + "Y2] -1 - [')/7'] --1 + c (9)

For the usual 1 W ion laser at 488 nm, the transmission of the output mirrors is close to 5% (~, ~0.05) and the round trip gain, ~T, is on the order of 0.5. Fig. 2 shows the intracavity power enhancement factor (P'/P) as a function of ~/2 for three different values of ~1'. This factor can be greater than 10 and depends critically on the value of the residual loss parameter ~2. If~/2 has the reasonable value of 0.005 and if the output mirror is replaced by a mirror whose transmission is between 0.1 and 1%, then, from Fig. 2, the intracavity power will be enhanced by a factor of between 4 and 10. Using an argon laser at 488 nm which had a 1 W output when used with the original mirror, we were able to realize a value of 3.5 for this factor when the laser was reconfig- ured as described in the following paragraphs. This somewhat low value for the improvement factor is attributed to a high value of the residual loss parameter, ~2, due to deposits on the inside of the Brewster windows of the laser tube, which had seen many hours of prior use.

Fig. 3 shows the layout of the argon ion laser designed to excite intracavity Raman scattering. The two Brew- ster angle prisms disperse plasma radiation from the laser tube so that only the wavelength of interest enters the sample region. In the more usual laser configura- tion, in which the dispersing prism is located at the other end of the laser tube adjacent to the plane mirror, plasma radiation at the Raman-scattered wavelength undergoes Rayleigh and Mie scattering in the sample volume and thereby produces an undesirable background level. The two fused silica Brewster angle prisms provide an angular dispersion of 5.9 mrad between radiation at 488 and 514 nm. This dispersion

103

500 I

200

I00

z I{ = 0.0001 / ~, = 0.5 2° ½ J

({ = 0.01 - .

0 0,01 0,02

Y5

Fro. 2. Intracavity power enhancement as a function of the residual intracavity loss parameter, ~/~, for different values of the mirror transmission loss, 3' / . The transmission loss for the original output mirror is taken to be 0.05 Np and the round-trip laser gain, 5,,,. is taken to be 0.5 Np.

APPLIED SPECTROSCOPY 2gg

Ml R,,

I rA I USED S,L CA

. . . . . . . . . . . . . . . . . . . . . . . . PHISMS :OR.S° L SPECTRA-PHYSICS MODEL 165 LASER ._I \ ~

DETECTOR Ill' TO MONITOR I J

I NTRA-CAVI TY Iq POWER I!j

R:io~m ~ ', i ! I!I \ \'~I Irl

~IOcm \~I li,

,NTRA-CAV, TY __..~.----f"""~"~.! 11 LASER f ~ ~il

BEAM WAIST ', 5cm I ~

MR R=lOcm

FIG. 3. Optical layout of the laser used for intracavity Raman scattering.

separated the beam waists at these two wavelengths by about 300 t~ at the observation region. Fused silica was used to minimize absorption and scattering losses within the prisms and at their surfaces. The laser was turned on its side so that the polarization vector would lie in the horizontal plane of incidence shown in Fig. 3.

The use of two spherical mirrors, as shown in Fig. 3, provided a convenient means to form an intracavity beam waist while maintaining the optimum transverse mode diameter within the laser tube. The optimum beam waist diameter is given by Eq. (4), with the value of M being given by Eq. (1). For a 0.79 sr collection solid angle and an f/8 spectrometer (Ss = 0.012) with a 2 cm high entrance slit, Eq. (1) gives a value of 8 fo rM and a corresponding value of 0.25 cm for L. Eq. (4) thus defines the optimum beam waist diameter to be 28 t~. A pair of 10 cm radius, high reflectance (R > 99%), spherical Fabry-Perot interferometer mirrors were on hand and were used to form the beam waist and complete the laser resonator. With these mirrors, and maintaining the TEMoo mode diameter ( -1.4 mm) within the laser tube, an intracavity beam waist diameter of approximately 19 ~ was obtained as expected on the basis of resonator theory2 (The beam diameter was measured by viewing Rayleigh scattering at the beam waist through a microscope equipped with a measuring reticle.) This particular optical arrangement was very convenient in that mirror M8 (Fig. 3) could be fixed, and x, y, and z adjustments to the position of mirror M~ were sufficient both to align the laser resonator and to adjust the TEMoo mode size at the laser tube (i.e., to maximize the laser power in the TEMpo mode).

In practice, as predicted by Eq. (9), the intracavity laser power depends critically on the cleanliness of the optical surfaces within the resonator. It was desirable to keep all airborne dust and aerosols out of the path of the intracavity laser beam; otherwise, these particles would not only scatter light, but would also be trans- ported along the beam by photophoresis until they encountered the surface of an optical component, where they would be deposited. The dominant residual intracavity loss in the laser appeared to be scattering by inaccessible deposits on the inner surfaces of the laser tube Brewster angle windows. As shown in Fig. 3, the

300 Volume 32, Number 3, 1978

intracavity laser power was monitored by detecting the small amount of light transmitted by mirror M,. The detector was calibrated for intracavity power by measuring the output power of the laser when it was equipped with the standard output mirror whose transmission was accurately known, calculating the corresponding intracavity power, and relating that number to the output of the detector behind M,. Using a Spectra- Physics model 165 argon ion laser with an output of 1 W at 488 nm, we were able to attain an intracavity power of 160 W. Since the normal output mirror had a transmission of 4.3%, there was a factor of [1 + R]/T, or 46, increase in the laser power simply by working inside the laser resonator rather than outside. The remaining factor of [160/46] = 3.5 was due to the replacement of the output mirror by the pair of high, but not optimum, reflectors as described above.

IIl. SPE C T R OME T E R AND DETECTOR

The spectrometer was a double 1-m Jarrell-Ash f/8 Czerny-Turner monochromator with 2 cm high slits. The dispersion in each monochromator was 8 A/mm (or about 12 cm-1/mm at 580 nm). The entrance and exit slits were kept equal in width and the middle slit was 50% wider. The transmission of the spectrometer for horizontally polarized light at 488 nm was approximately 1.8%. The exit slit of the double monochromator was imaged onto the photocathode of an ITT type FW130 photomultiplier which was cooled to reduce the dark current to less than I cps.

IV. MEASUREMENTS WITH THE INTRACAVITY LASER RAMAN SYSTEM

A drawing of the entire system is shown in Fig. 4. All of the components were rigidly mounted on a vibration- isolated optical table and contained within an enclosure which could be purged with filtered air. In spite of the precautions taken to keep dust and aerosols from the vicinity of the instrument, it was necessary to clean the optical components of the laser resonator periodically in order to maintain a high laser power. The laser power was peaked by making fine adjustments to the position

Fro. 4. Drawing of the intracavity laser Raman scattering spectrometer.

of mirror M2 (Fig. 3). The collection optics were aligned by adjusting the positions of the two lenses to form a sharp image of the laser beam waist on the entrance slit of the monochromator, and by adjusting the position of the auxiliary collection mirror to image the beam waist accurately back on itself. Final adjustments to all of these alignments were made while the Raman-scattered signal from atmospheric nitrogen or oxygen was monitored. This signal level was 770 000 cps for N2 in air with fully open slits. As shown in Table II, this is in reasonable agreement with the expected value given by Eq. (7).

Fig. 5 shows the Raman-scattered 488 nm line of the argon ion laser for the 0-1 transition in atmospheric

TABLE II. Observation of Raman scattering by atmospheric nitrogen at 2330 cm L

Intracavi ty laser power Excitation photon energy Spectrometer slit he ight Collection solid angle Spectrometer solid angle Spectrometer slit width

(full open) Concentrat ion of ni trogen

molecules in air corrected for 20°C laboratory temperature)

Raman scat ter ing cross-section a Transmission of collection optics Transmission of spectrometer Quantum efficiency of photo-

mult ipl ier

Count ra te predicted by Eq. (7) Observed count rate

105 W (P) 4.1 x 10-t~ (e) 2.0 cm (h) 0.79 sr (So) 0.012 sr (Ss) 0.30 mm

2.0 X 10'" cm -a (p)

5.4 x 10 -31 cm 2 sr -1 (~) 0.65 ( T O ) 0.018 (T,) 0.10 (Q)

1.26 x 106 cps 7.7 x 10 ~ cps

a Based on data published by Fouche and Chang '~ and corrected for wavelengths (488 vs 514.5 nm).

NI4N 15 PEAK

2293

NI4N 14 PEAK

42 cm "1

23'31 RAMAN SIIIFT, cm - I

FIo. 5. Raman spectrum for the 0-1 t ransi t ion in atmospheric nitrogen showing the 14Nl'~N peak at 2293 cm -1.

20-

C <

L~ 10"

5

o

550

H 2 LINE AT 5gOcm - l

(APPROX. 30 ppm)

, ~ - 5,8cm "I

BACKGROUND: 0,2cps

, .

6~5 RAMAN SHIFT, cm "I

Fro. 6. Raman spectrum for molecular hydrogen.

nitrogen in air at room temperature. The resolution which may be achieved by the system is illustrated by the detection of the Raman-scattered line from 14NL~N, the nominal concentration of which is approximately 3 x 103 ppm. The spectral resolution was approximately 1.2 cm -1, corresponding to entrance and exit slit widths of 100/x.

To demonstrate the sensitivity of this instrument further, a Raman spectrum for molecular hydrogen in the vicinity of 590 cm -1 was run. A small amount of hydrogen (-100 ml at STP) was introduced into the laser resonator enclosure ( -5 × 105 ml) and allowed to diffuse for about 5 min. A spectral scan for the Raman- scattered light (in the vicinity of 590 cm -1) produced the data shown in Fig. 6. By comparing these data to those obtained from the Raman-scattered light from nitrogen in air under identical conditions, it was determined that the concentration of molecular hydrogen was approximately 30 ppm.* The background count during these runs was only 0.2 cps for an intracavity laser power of 140 W. This low background level would not have been possible without the dispersion of intracavity plasma radiation provided by the two Brewster angle prisms.

V. SUMMARY

We have described the design of an efficient intracavity laser Raman spectrometer and have documented its performance with Raman spectra for atmospheric nitrogen and for a low concentration of molecular hydrogen.

ACKNOWLEDGMENT

This work was performed under the auspices of the U.S. Department of Energy under Contract EY-76-C-02-0016.

* In making this calculation, the observed photoelectron count ra te for hydrogen was normalized to tha t of nitrogen. The concentration of hydrogen was then inferred from the relative Raman scat ter ing cross-sections.:',"

APPLIED SPECTROSCOPY 301

1. R. S. Hickman and L. Liang,.Appl. Spectrosc. 27, 425 (1973). 2. J. J. Barrett and N. I. Adams III, J. Opt. Soc. Am. 58, 311 (1968). 3. H. Kogelnik and T. Li, Proc. IEEE 54, 1312 (1966).

4. A. Yariv, Quantum Electronics, 2nd ed (Wiley, New York, 1975), p. 178. 5. D. G. Fouche add R. K. Chang, Appl. Phys. Letters 18, 579 (1971). 6. R. C. Harney, J. E. Randolph, and F. P. Milanovich, Lawrence Livermore

Laboratory Preprint No. UCRL-76681 (1975).

Measurement of Electronic and Resonance Raman Spectra from the Same Gas Matrix or Thin Film Sample

W. SCHEUERMANN* and K. NAKAMOTO Todd Wehr Chemistry Building, Marquette University, Milwaukee, Wisconsin 53233

A new method is described to measure effective electronic absorption spectra by means of the Raman spectrometer. The spectra originate from the same sample spot of a gas matrix or thin film that was used for the Raman measurement. The influence of the substrate material and the angle of incidence was investigated. Octaethylporphinatocobalt(lI) was used to demonstrate the method. It was observed that the a and fl absorption bands shift as much as 700 cm -1 depending on sample preparation, thermal treat- ment, and the resulting physical state. A variation of the angle of incidence from a = 45 ° to e~ = 200 had no influence on the position of the a and fl bands. Five significantly different resonance Raman spectra were recorded with 514.5 nm excitation. It was found that the relative intensities of the Raman bands were dependent on the position of the fl band with respect to the frequency of the exciting radiation. This result led to the conclusion that Raman band intensities should only be correlated with an effective electronic absorption spectrum obtained from the same sample spot.

Index Headings: Electronic absorption; Resonance Raman; Matrix; Thin film; Octaethylporphinatocobalt(II).

INTRODUCTION

In resonance Raman spectroscopy, knowledge of electronic spectra is indispensable since resonance enhancement is governed by the nature of the electronic transi- tions involved. In order to discuss the excitation profiles of resonance-enhanced bands, both Raman and electronic spectra should be obtained from the same sample under the same experimental conditions. In the case of solid samples (thin films and inert gas matrices) this condition is rather difficult to achieve unless both spectra are measured by using the same sample with the same experimental set-up. We have developed a new method to measure both spectra of the same solid sample using our Raman spectrometer with slight mod- ifications. This paper reports the experimental procedure and the results obtained for octaethylporphinatocobalt(II) (abbreviated as Co(OEP)).

I. EXPERIMENTAL

Fig. 1 shows the optical arrangement used for the

Received 5 December 1977. * On sabbatical leave from the Department of Chemistry, University

of South Africa, P. O. Box 392, Pretoria, 0001, South Africa.

measurement of electronic spectra. It is essentially the same as that used for Raman measurements of matrix- isolated compounds ~ except for the insertion of a small incandescent lamp (LA), powered by a regulated power supply, into the path of the laser beam.

The procedure to obtain electronic spectra is as fol- lows. First a matrix or a thin film is deposited onto the inclined front surface of the cold-tip (CT) as already described, ~ and the resonance Raman spectrum is recorded in the normal manner. Then the polarization analyzer (AN) is inserted so that the electric vector of the transmitted radiation is oriented parallel to the slit (SL) and the scattering plane. The scattering plane is defined by the optical axis passing through the slit and the imaging lens (IL), and the direction of the exciting laser beam. The function of the polarization analyzer is to allow only radiation which is polarized parallel to the slit and the grating grooves to reach the double monochromator. The sharp dip in the response of the double monochromator at 15 750 cm -~ for light polarized normal

~ . - - - - - - ~ FL, f = 8 0 mm

LA 0 mm 5 0

FIO. 1. Experimental set-up for the recording of electronic absorption spectra.

302 Volume 32, Number 3, 1978 APPLIED SPECTROSCOPY

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An Efficient Intracavity Laser Raman Spectrometer