Physica 1OlC (1980) 139-144 0 North-Holland Publishing Company

LUMINESCENCE OF HIGH PRESSURE ARGON EXCITED BY PULSED ELECTRIC DISCHARGES

G. KLEIN and M.J. CARVALHO Centre de Recherches Nuclcinires et Universite’ Louis Pasteur, Laboratoire de Physique des Rayonnements et d’Electronique Nuckfaire, 23, rue du Loess 67037 Strasbourg Cedex, France

Received 22 October 1979

The hnninescence produced by a pulsed electric d&charge in hi pressure argon has been analyzed and compared to the luminescence obtained with Q and p particles exitation. Qualitative differences have been observed and discussed. The time resolved emission spectra of the lowest singlet IX: and triplet 32i excited states (“second continuum”) have been measured and an energy gap of 0.04 eV between these levels has been deduced. A “second continuum” delayed emission with a decay time constant of 500-600 ns, which is not observed with ionizing particles excitation, was attributed to secondary emission from the lowest molecular state 1 I$. One assumes that this state results from the excitation of argon atoms by accelerated free electrons. A plasma radiation could also be observed in the spectral range from 1130 A to 5000 A, with a temporal decay which has a FWHM comprised between 1.5 -2 ns.

1. Introduction

The light emission of high pressure argon (above 1 atm) has several continua with various origins. The principal feature, the ‘“second continuum“, centred at 1270 A (9.76 ev), has about 90 A width at half maxi- mum, and results from the radiative transition from the two lowest (singlet “Et and triplet 3Z=) excited molecular states to the ground state ‘Es [ 1,2]. The corresponding lifetimes are respectively in the nano- second and the microsecond range f 31. Besides this continuum two other important continuous emissions exist: between 1600 A and 2900 A in argon excited’ by ~11 particles [4,5] or by protons [6] and between 1100 A and 5000 A in plasma produced by electric discharges in high pressure argon (l-30 atm) [7-8 1.

This paper deals mainly with the study of the excited states produced in high pressure argon (1 S- 15 atm) by a pulsed electric discharge. The experi- mental data are compared with the results obtained with other excitation conditions, as arises with CY and 0 particles, to get more information on the origin of the various emission bands and on the kinetics involved in the deexcitation processes. The experi- mental technique is based on the analysis of the light intensity laws. Time resolved spectra and time con- stants were measured.

2. Experimental

The experimental set-up is shown in fig. 1. The chamber cont~~g the argon is made of stainless steel, and before filling it was pumped and baked at about 120°C during 24 hour. The lithium fluoride window is sealed with indium. Argon N60 from “Air Liquide” was used without any further purifica- tion. According to the supplier’s speci~cations it con- tains less than 2 ppm impurities. The electric dis- charge occurs between two cylindrical (2 mm diam- eter) thoriated tungsten electrodes, generally 0.2 mm apart. The stray capacitor betwen the high voltage electrode and the grounded chamber is charged slowly through a 50 MS2 resistor from a dc power supply. When the disruptive voltage (between 1 and 2 kV) is obtained, a conducting plasma is created between the electrodes, and the capacitor is dis- charged very rapidly to the ground through a 20 St resistor. Thus the sparks are obtained in a free run- ning mode. The power supply high voltage was taken so that the spark repetition rate was around IO4 s-r, The spark was analyzed by a V.U.V. monochromator (“Hilger E 720”) and a photomultiplier (“413 F Intertechnique”) with a magnesium fluoride window. The delayed single photo-electron coincidence tech- nique has been used for the emission spectra and

139

140 G. Klein and M.J. Carvalho /Electric discharge luminescence of high pressure argon

Fig. 1. Experimental set-up. HV: high voltage power supply; C: argon chamber; M: vacuum monochromator; PM: 413 F photomultiplier; A: fast amplifier; AT: attenuator; D: fast discriminator; TAC: time to amplitude converter; MCA: mul- tichannel analyzer; SCA: single channel analyzer.

decay time measurements. The zero time pulse picked up at the 20 Q resistor corresponds to the quasi instantaneous excitation of the gas. The delayed timing signal is given by the pulse corresponding to a single photon impinging on the photomultiplier. The time intervals between these two pulses are measured and stored by a time to pulse amplitude converter fol- lowed by a multichannel analyzer. The light intensity decay of the spark was measured betwen 0 and 20 ~.ts at various wavelengths. The time resolved emission spectra were obtained by a single channel analyzer which selects a given time interval. The standard pulses were then counted and stored in a memory, each channel corresponding to a given wavelength. The wavelength resolution was 2 A.

The excitation time of the gas, which corresponds to the plasma lifetime, is given by the width of the electric pulse at the 20 !YJ resistor. The full width at half maximum (FWHM) measured with a sampling oscilloscope is comprised between 1.5-2 ns, and is nearly independent of the pressure (3-15 atm) and the spacing of the electrodes (0.05-0.5 mm). Other plasma characteristics, such as temperature and charge densities were evaluated only roughly. Their precise determination was not possible with our experimental conditions and, furthermore, is out of the scope of this paper.

All measurements of the spectra and of the inten- sity decay curves were reproducible. The aspect of

the electrodes remained unchanged throughout the whole experiment.

3. Results

In figs. 2 and 3 the luminescence decay curves are shown measured respectively at 1270 A (9.76 eV) and 1600 A (7.75 ev) in argon at 15 atm, 7 atm and 1.5 atm. At 1270 A four components can be ob- served:

Component Z is very rapid and has a decay time constant of about 1.3 ns.

Component ZZ is also in the nanosecond range but is slower; it has a decay time constant which decreases with increasing gas pressures. For our pres- sure range, typical values are comprised between 50 and 7 ns.

Component ZZZ has a time constant, around 500 ns, apparently independent of the pressure; it depends however on the spacing of the electrodes, shortening when that distance decreases. At lower pressures, this component has a bump at the begin- ning of its decay, as can be seen in fig. 2 at 1.5 atm.

ComponentZV is exponential after 5 PS. Its decay time constant is 3.4 PS, independent of the pressure.

At 1600 A a fast decay curve has been measured which is identical to component I at 1270 A. Figs. 4 and 5 show time resolved spectra corresponding to

0 50 loo 0 5 10 t (nr) t(rr)

Fig. 2. Light intensity decay versus time measured at 1270 A with 1.5 atm, 7 atm and 15 atm argon pressure.

G. Klein and M. J. Carvalho /Electric dischade luminescence of high pressure argon 141

7atm

\ .

i i i i i

\ i ? !15atm i

\

Fig. 3. Light intensity decay versus time measured at 1600 A, with 7 atm and 15 atm argon pressure.

the four components I, II, III and IV measured in the time intervals (O-l ns), (3-7 ns), (0.4-l j&h (4-1.5 q), respectively.

The shape of the spectrum of component I is nearly independent of the pressure; however its struc- ture is better visible at lower pressures (3 atm). In fig. 4, the little maximum observed around 1270 A in the spectra of component 1 is actually a part of compo- nent II, because these two components for a gas pres- sure higher than 7 atm cannot be completely separ-

Fig. 4. Emission spectra of component I, and of the total luminescence at 15 atm.

I # I 1 1100 1300 h(i) 1500

Fig. 5. Emission spectra of components II, III and IV at 15 atm.

ated. At lower pressures this separation becomes pos- sible .

Components II, III and IV have almost the same spectral distribution as the broad continuum whose maximum is at 1270 A in the spectra of the total luminescence. Their spectra contain only the well known “second continuum” characteristic of the emission spectra of argon. Nevertheless, some little differences can be observed between them. The FWHM increases with pressure in the spectra of com- ponent III, whereas in component IV it remains un- changed. For component II the situation is less clear: at low pressures (<7 atm) the spectra are broader than for the highest one (15 atm), but at this point it is not possible to ensure that the same component has been measured along the whole range of pressure. These results are plotted in fig. 6. The spectra are not corrected for the spectral response of the optical sys- tem, therefore the exact determination of the wave- length of the maxima is not possible, especially in the shortest wavelength region. The maxima in the spec- tra of components II, III and IV are probably red shifted, because in these spectra the blue wing almost coincides with the sharp absorption edge of the lithi- um fluoride windows. Nevertheless, it is possible to

142 G. Klein and M.J. Gwalho /Electric discharge luminescence of high pressure argon

100

t

L I I I

0 5 10 p(atm) 15

Fig. 6. Full widths at half maximum (FWHM) of the emission spectra of components II, III and IV, versus argon pressure (PI.

ensure that there is no noticeable difference between the positions of components II and III, to the limit of our spectral resolution (2 A). We measure that the maximum of component IV is located at 5 + 2 A from the one of component III towards longer wave- lengths, and we verified that this energy gap was independent of the pressure.

4. Discussion

Component I is characterized by a very wide emis- sion spectrum extending throughout all our measur- able wavelength region (1130 A-5000 A). This spec- trum is never observed with a or &particles excita- tion. The width of the luminescence decay curve measured at 1600 A (fig. 3) is about 3 ns, very close to the time resolution of the photomultiplier (413F). Measured at 4500 A with another photomultiplier (RCA 8850) which provides a temporal resolution at least as good as 1 ns, the luminescence decay curve has a FWHM nearly independent of the pressure and comprised between 1.5 and 2 ns. Such a width cannot be ascribed to the resolution of the apparatus. Since it is the same as that of the electric pulse, that corre- sponds to the excitation of the gas, we identify the continuous emission that decays like component I with the plasma radiation. The emission spectrum can be compared to that obtained from continuous dis-

charges in high pressure argon (l-30 atm) [7,8]. The observed structure may be due to the spectral response of the optical system.

The three other emission components, whose spec- tra only contain the second continuum, come from radiative transitions from the lowest singlet ‘X:(0=> and triplet 3Zz(1,) molecular excited states to the dissociative ‘X:(0;) ground state of argon. The emit- ting molecular states (‘Zz, 3ZD are formed via three body collisions involving two argon atoms in the ground state and an excited one [ 1,2], with a rate constant of about 1O-33 cm6 s-r for the singlet state [3]. The primary species (excited and ionized atoms) are created along the plasma and immediately after it had disappeared.

Because of their relative values, component II is difficult to separate from components I and III; it can be better isolated from component I at low pressures and from component III at high pressures, above 7 atm. The decay time constant of component II decreases from about 21 ns to about 7 ns when the gas pressure increases from 7 atm to 15 atm. These values and the global behaviour of this component under changing pressure are similar to the ones found for the fast component of the argon scintillation induced by electrons [3]. This emission component can thus be attributed to the radiative transition ‘Zi + ‘2:. The FWHM of its time resolved spec- trum, in these measurements, is always overestimated (106 A at 15 atm. for instance); we think that the true value is not far from the width of 92 A obtained for the same singlet emission induced by (Y particles [5]. This is a consequence of the difficulty, men- tioned above, to isolate component II from its two neighbours. At high pressures (15 atm) the back- ground of component I cannot be completely removed, as can be seen in fig. 5. For pressures lower than 15 atm the measured emission spectrum of com- ponent II is still broader (see fig. 6); its width has values (119 A, 122 A) similar to those of component III. We attribute this to the weak intensity of compo- nent II as compared with component III, even at the early decays. Therefore, below 7 atm the major con- tribution to the width of the spectra comes from component III and the measured values (119 A, 122 A) actually correspond to this component emission. Component II may be compared to the weak molecu- lar emission observed in short are discharges [8 1, in

G. Klein and M.J. Carvalho /Electric discharge luminescence of high pressure argon 143

the surrounding of the plasma, in those regions where already proposed, in cold regions-the boundaries of the temperature is lower than in the plasma. the plasma.

When the plasma disappears, the remaining free electrons are accelerated in the electric field between the electrodes, and loose their energy by collisions creating new argon excited states. The resulting mo- lecular excited states (lZ+ ’ X3 decay radiatively to the ground state, giving r&. , respectively, to compo- nent III and to one fraction of component IV. The decay time constant of component III (500-600 ns) corresponds to the time needed for collection of the electrons and its apparent independence of the pres- sure of the gas is a consequence of mutual compen- sation among the influence of the different parameters governing the discharge-breakdown voltage, free car- riers density, remaining electric field, etc. Component III emission can be compared to the secondary light observed in proportional scintillation counters [9,10]. The collection time scales involved are of the same order of magnitude [9]. Concerning the time resolved spectra, components II and III have almost the same spectra; within the experimental error (*2 A) their maxima coincide, although the width of the spectra of component III is larger. Besides this difference and bearing in mind the decay time results, we also attribute the third component emission to the radiative deexcitation of the lowest singlet state ‘Z~(O~): this one belonging to the secondary lumines- cence (delayed) and the component II to the primary luminescence [9]. The observed differences in the wavelength spreads of the emission spectra of compo- nents II and III can be explained in terms of geomet- rical effects: it is possible that the ‘Xi states giving rise to the component III are created in regions where the local temperature is enhanced and maintained by multiple collisions involving energetic electrons, which allows the formation of excited molecules with some amount of vibrational energy. This is coherent with the results of Cheshnovsky et al. [ 1 l] on the influence of the temperature on the second conti- nuum emission of argon. In the same way the broadening of the spectra of component III with the increasing pressure can also be explained by a local enhancement of the gas temperature, which is a con- sequence of the increase of the breakdown voltage and the free electron density. On the other hand, all of these statements do not apply to the singlet state source of component II if they are formed, as we

Component IV corresponds to the radiative decay of the lowest triplet excited states 3Bi which are produced either like the singlet states ‘LB: of compo- nent II in the surrounding of the plasma, or by colli- sions of accelerated free electrons like the ‘Ci states of component III. The 3.4 ~.ts decay constant of com- ponent IV is slightly larger than the 3 MS of the triplet state lifetime [3,5] . This difference may be due to a small delayed emission (not necessarily only triplet emission) produced by tertiary and higher order elec- trons. The FWHh4 of the time resolved emission spec- tra is 92 A, independent of the pressure, and was measured 4 PS after the excitation when most free electrons were collected, and the gas temperature sta- bilized to the room temperature. This value is very near to the 89 A measured with a-particle excitation [5] for the same slow component.

The 3Xt emission spectrum (component IV) is red shifted from the singlet ‘Xi emission spectra (components II and III). Their maxima are separated by about 5 A + 2 A (=0.04 ev), an energy gap that would correspond to the separation between the two lowest excited molecular states responsible for the main scintillation of gaseous argon. With an o-particle excitation source we found 6.8 A (0.052 eV) [S] . Both these values compare well with the correspond- ing energy gap (0.03 eV) measured in solid phase [ 121 and are of the same order as the one calculated (=0.09 eV) for gaseous argon [ 131.

5. Conclusion

Emission spectra as well as intensity decay curves of argon excited by an electric discharge present a greater complexity than those obtained under excita- tion by high energy ionizing particles. In the spark a new continuous emission appears (component I) and the second continuum (1270 A) has a spectral width dependent (component III) on the pressure, at con- stant gas temperature. Also the intensity decay curves at 1270 A are far from the sum of two exponential terms that describes the decay of the same scintllla- tion observed in argon excited by /I particles.

Furthermore we didn’t find here the “third con- tinuum” which extends from 1600 A (7.75 eV) to

144 G. Klein and M. J. Carvalho /Electric discharge luminescence of high pressure argon

I ,

0 100 P*(atm') 200

Fig. 7. Ratio R of component I intensity at 1700 A to the “second continuum” emission at the maximum (1270 A), versus the square of the pressure p2.

about 2900 A (4.35 eV) and can be seen under cr particles or protons excitation of gaseous argon [5,6]. The reason may be found in energetic considerations: in argon only a few of the electrons produced by the discharge and particularly the secondary and the higher orders electrons can attain kinetic energies higher than 11.5 eV [ 141 and those are the ones which can lead to the formation of the excited states responsible for the third continuum. These molecular excited states would lie above the ionization poten- tial.

From a practical point of view the plasma radia- tion (component I), produced in argon by an electric discharge as we report here, can be used as a short pulsed (FWHM = 1.5-2 ns) light source at wave- lengths above 1500 A (8.3 ev). Below 1500 A because of the second continuum emission, with

decay time constants up to 3.4 ~.ts, argon is no longer suitable. As can be seen from fig. 7 the intensity ratio between component I (measured at 1700 A) and the second continuum increases as the square of the pres- sure. Therefore, in argon, high pressures @lS atm) are recommended to get short light pulses, above 1500 A, with a high intensity.

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