Bremsstrahlung and photoconduction in dense gases

BREMSSTRAHLUNG

15 September 1982

OPTICS COMMUNICATIONS

Volume 43, number 2

AND PHOTOCONDUCTION

GCrard HAUCHECORNE,

IN DENSE GASES

Franqois KERHERVE! and Guy MAYER

Laboratoire d’optique Quantique, Groupe de Physique des Solides de I’E.N.S., Universitd Paris VI, 75230 Paris Cedex 05, France Received 6 July 1982

We report bremsstrahlung experiments in dense gases. The first conduction electrons required for these observations are obtained by a two-steps ionization process acting on dopant molecules.

1. Introduction

877 vi uf2

U(f, w) = 3

The experiments here described involve two light beams of different frequencies acting simultaneously or successively in a high pressure cell fitted with MgF2 windows and with three electrodes. The low-power (“1 kW) short-wave light (X = 0.354 pm or h = 0.266 pm) gives to a few electrons (- 106) enough energy to divorce from their parent molecule and to get some freedom of motion. Subsequently a more powerful (- 100 kW) longwave light (h = 1.06pm or h = 0.53 pm) gives or takes energy by quantas ?iw to these electrons through “Bremsstrahlung” effects: when “free” electrons interact with surrounding atoms they are able to absorb or emit light [l-3]. Along the energy scale the electron “motion” can be described [4] as a random walk: steps +??o by absorption or steps -fro by stimulated emission with unequal probabilities both dependent on energy E. Upon this discrete kind of motion is superimposed a quasi-continous downward flow induced by kinetic energy transfer to the cold surrounding atoms. The spontaneous bremsstrahlung emission which we measure to get informations about the electron energy distribution function is rather weak and does not affect significantly the energy balance. At low densities, when electron interaction with an atom is a rare event well defined in time the calculation of the absorption cross-section U(E, w) is not too difficult and many authors [2,3,5] agree about such an expression:

114

,3

e2

NQWnc

(1)

Viand vf are the initial and final electron velocities,fV the density of surrounding atoms. The functions Q(E) momentum transfer cross-sections can be obtained from electrical measurements [6]. For an electron of energy e, the probability to gain ho (absorption) is proportional to Q(E) and the probability to loose fiiw (emission) is proportional to Q(E - Aw). For N- 10z2 cm-3 in argon, u is equal to a few lo-l7 cm2 when e is of the eV order. We are mainly interested in the high density domain up to liquid state density. When the electron is always in interaction with close surrounding atoms, the model from which eq. (1) is derived becomes questionnable. May be we shall discuss this point in a future theoretical work; for the present time we report here experimental observations.

2. Electron multiplication If the irradiation of the “free” electrons inbedded in the dense gas is intense and long enough, they will eventually get from their random walk enough energy to excite or ionize other atoms. These successive ionizations leads to multiplication of their number. Fig. 1 shows the multiplication factor G induced by pulses of different intensities (A = 1.06 pm; duration t = 10P8 s) in argon. Similar plots have been obtained

0 030-4018/82/0000-0000/$02.75

0 1982 North-Holland

Log2G

L-10 G ,,.-%

15 September 1982

OPTICS COMMUNICATIONS

Volume 43, number 2

Log

G

_.._...c. . ..-.

I-

I 1

2

3

I"

1

Fig. 2. Effect of various amount of CO on electron multiplication. Abscissas: irradiation intensity; one unit = 1 X 10’ W cm-? ; A = 1.06 pm; t = 1 X loss s; concentration lens f = 35 cm. Ordinates: logte G. Fig. 1. Electron multiplication induced by bremsstrahlung in argon. Abscissas: irradiation intensity; one unit = 1.6 X lo8 W crne2., A = 1.06 pm; t = 3 X 1Oa s; concentration lensf= 15 cm. Ordinates: decimal logarithm of multiplication factor. -o- density 275 Am, +: density 515 Am.

in krypton; for instance in Kr at density 140 amagats, one gets a log G versus I, curve similar to the 275 Am argon curve of fig. 1 with abscissas reduced by a factor 2, -Intensities near 1O8 W cmm2 about which large G are observed are more than an order of magnitude [5] lower than the intensities required to get the same effect near atmospheric pressure. In all experiments, we check that the pulse of wave length 1.06 pm acting alone (without the ionizing uv pulse) does not produce any measurable electronic signal. At high electronic densities, space charges and recombination effects bring some experimental troubles. Fig. 2 shows the quenching effects of small CO amounts on multiplication in argon at 300 bars. Let us consider the conditions in which G = 103 (pure argon). Since IO3 - 21°, it does mean that in these conditions, one electron creates another free electron in an average time of 10eg s during which it ex-

periences

about

lo6 collisions

before

it gets enough

energy to ionize an argon atom. Some of these collisions will occur against CO mole-

cules if the relative density of CO is larger than 10m6. For E - 9.7 eV, the reaction e- + CO + C t O- is known [7] from low density experiments to have a large cross-section (z - 2 X lo-l9 cm2), The curves of fig. 2 suggest that this reaction which takes the electron energy to break the molecule is efficient to quench the ionization process. We observed electron multiplication in all the monoatomic gases investigated: He, A, Kr. We did not observe multiplication in the polyatomic gases we tried: CO, CH4, CO2, N2. We think it could be a rather general rule for polyatomic molecules that the channels leading to dissociation under electron impact are more efficient that the channels leading to ionization.

3. Emission spectras In the spectral range 1300-3000

A we observe the 115

Volume

43, number

light emitted by the threadbare volume in which electrons have gained energy by bremsstrahlung processes. There is a rather flat component of spontaneous bremsstrahlung induced by hot electrons and fluorescence lines or bands from excited atoms or molecules. Fig. 3 shows the most intense component from krypton at 130 Am. The number of electrons heated by bremsstrahlung in the 10h4 cm3 hot space was near lo9 at the end of the 1.06 pm pulse which induced a multiplication of a few 103. The observed band is the fluorescence of Kr2 molecules [8]. It happens that our commercial krypton contains about 2 ppm of Xe whose resonance line falls inside the fluorescence band of Kr2. At the density of 130 Am of Kr the absorption line of atomic Xe has its peak at 1460 A (attenuation /3 is 0.3 cm-l) and a line width of 45 A. On fig. 3 is also plotted the emission spectrum of Kr2 corrected from Xe absorption. Our detection system is calibrated in number of photons per unit solid angle. From our measurements in this spectral range the total amount of fluorescence photons emitted in all

1400

1500

1600

116

4. Photoionization

in argon, krypton

and methane

In a former article [9] we described photoconductive effects induced in high density argon by light of wavelength 3540 A (AU = 3.5 eV): the number of free electrons produced is proportional to the square of light intensity; the efficiency of the process is a fast rising function of density. Above one hundred amagats, the effect is easily measured with intensities of the order of 1 kW cm-2. We repeated these experiments with light of wavelength 2660 A (fiw = 4.7 eV) and investigated other gases. In A, Kr, CH, the number of conduction electrons produced is again proportional to the square of light intensity, but it is too large to be accounted for by direct two-photon ionization of any conceivable amount of impurities. We come to the conclusion that in A, Kr, CH4 we observe two-steps ionization of impurities according to the fig. 4 scheme. Calling n the density of ionizable molecules, oA the cross-section of the transition connecting the fundamen. tal to the real states A, and oB the ionization cross section of the excited state B, the number ?Z of free elec-

A

Fig. 3. Ultraviolet fluorescence of krypton at 130 amagats volume observed 1 O4 cm3 ; number of electrons after multiplication 109; number of photons integrated over frequency and all space lOlo. .: Direct observations; o: spectrum corrected for Xe atomic absorption.

1982

space is calculated to be lOlo while the number of electrons recorded is 109. When the irradiation intensity is varied, the fluorescence intensity stays proportional to the electrons number. In these experiments the density of Kr2 molecules is about 1014 cm -3. If one assumes a stimulated crosssection of lo-l7 cm2, a gain index of 10e3 cm-l can be expected. The rather flat spectrum due to spontaneous bremsstrahlung is two orders of magnitude less intense than this fluorescence; it can be observed around 2000 A.

.--a...._ I -.+/

.___...’

1300

15 September

OPTICS COMMUNICATIONS

2

Fig. 4. Two steps ionization

in A, Kr and CH4.

OPTICS COMMUNCATIONS

Volume 43, number 2

For our bremsstrahlung studies, the impurities are useful to provide the first free electron but also source of trouble for the optical observations in the far uv where they are strongly absorbing. A relative content of 1O-7 seems a good compromise.

Table 1 Two-steps ionization process fiiw (eV)

Gas

Density (amagats)

noA @m-l)

3.5 4.1 4.1 4.1

Kr Kr Kr Kr

350 350 150 120

1x 5x 5x 2.5 X

4.1 4.7

A A

335 260

2.5x 1O-2 1 x 10-2

OB (cm-?) 10-Z 10-l 10-Z 1O-2

7 ; ;“,::: 2.7 2.7 ;;

X X

15 September 1982

4.2. Effect of gas density on

10-l’ lo-18 ii:::

trons produced in unit volume by a pulse of duration I and intensity I is

%! a n UA~B I2 t2/2

(2)

ifog If < 1. To avoid space charges problems in the measurement of %! we had to work in conditions satisfying the above inequality. The quantity n oA (w) is measured as an optical attenuation index; on (w) is then obtained from electronic photoconduction experiments provided the electron mobility is known, or from ionic currents measurements [9]. Table 1 gives some figures relative to argon and krypton.

CrOSS-SeCtiOnS

oA

and (Tg

In our experiments the steep increase of photoconduction with density N is only partly due to the increase of impurity density n. N has also an effect on oA and oB . We measure the product noA (w) as an optical attenuation without knowing yet separately n and DA(w) but we get good values of oB (w). Table 1 shows the fast increase of oB with density on Kr and A. In CH, noA and oB are of the same order of magnitude and photoconduction is very efficient. In N2 at 800 bars, for hw = 4.7 ev, ncA is close to 10-l cm-l but no photoconduction is observed; so og < 1O-23 cm2. In He up to 950 bars we observe neither attenuation nor ionization. In A, Kr and CH4 the high values of ‘Jg (w) for rather small fro iS an interesting fact. We shall investigate more thoroughly these two-steps ionization processes with an improved control on the number and the nature of the molecular species involved.

4. I. Effect of gas density N on impurity density n 5. Conclusion It should be well known [lo], yet it is sometimes forgotten that high pressure gases are good solvents for organic molecules. The saturation gaseous density above a substance at temperature T which would be no in an empty cell becomes in presence of an other gas of density N:

n = no exp

4nN .!m r2 [exp(L 0

V(r)/kT) - 11dr , (3) I

V(r) describes the potential energy between the two species. The volume 4n Jrr2 [exp(- V(r)/kT) - l] dr is of the order of a few 1O-22 cm3 for a couple organic molecule-krypton. So, as far as our experimental cell was not perfectly clean from minute grease spots or fingerprints, the steep dependence of n versus N predicted by eq. (3) does account for our observations.

It is possible to obtain in dense gases controlled electron multiplication and fluorescence excitation by bremsstrahlung effects. One can expect a better efficiency with shorter light pulses since less time will be left for kinetic energy transfer losses. A possible application is the detection of charged particles. The first electrons of the ionized track can be multiplied and fluorescence can be induced in the surrounding atoms by intense short light pulses. The optical field will have the same effect as the static field of conventional “streamers chambers” [l l] Two kinds of improvements can be expected from the optical process: a better time resolution and a lower spatial drift. It is a duty and a pleasure to thank the staff of our Direction des Recherches, Etudes et Techniques for its interest in this research and for its financial support. 117

.

Volume 43, number 2

OPTICS COMMUNICATIONS

References ill H.A. Bethe and E.E. Salpeter, Handbuch der Physik, Vol. 35, ed. S. Fltlgge (1957) p. 88. 121 M. Ashkin, Phys. Rev. 141 (1966) 41. [31 R.R. Johnston, J. Quant. Spectr. Radiat. Transfer 7 (1967) 815. I41 Ya.B. Zel’dovich and Yu.P. Raiser, Sov. Phys. JETP 20 (1965) 772. [51 M. Louis-Jacquet, Avalanche Electronique induite par laser dans un gaz, These Paris 1977.

118

15 September 1982

[61 L.S. Frost and A.V. Phelps, Phys. Rev. 136A (1964) 1538 [71 H. Massey, Atomic and molecular collisions (Taylor and Francis, London 1979) p. 131. 181 Y. Tanaka, J. Opt. Sot. Am. 45 (1955) 710. 191 G. Hauchecorne and G. Mayer, Optics Comm. 38 (1981) 185. [lOI S. Robin, These Paris 1951; J. de Chimie Physique 48 (1951)415,501;49 (1952) 1. (111 K. Kleinknecht, Physics Reports 84, nr. 2 (1982).