Radiat. Phys. Chem. Vol. 24, No. 5/6, pp. 545-549, 1984 0145-5724/84 $3.00 + .00 Printed in the U.S.A. © Pergamon Press Ltd.

ION RECOMBINATION PROCESSES IN THE FORMATION OF KRYPTON FLUORIDE

RONALD COOPER, STEPHEN P. MEZYK and DAVID A. ARMSTRONGf Department of Physical Chemistry, University of Melbourne, Parkville, Victoria 3052,

Australia

(Received 9 July 1984)

Abstract--The formation of KrF* and XeF* has been studied in electron-beam irradiated Kr/ Xe/SF6 gas mixtures. An electron-beam-energy dependent process has been observed, and confirmed to be due to ion recombination processes between rare gas cations and halide anions. The anions are formed by thermal electron capture by SF6 and the cations either being directly formed in the electron pulse (Kr + ), or by subsequent charge transfer (Xe + ).

INTRODUCTION

ALTHOOGU evidence for rare gas-monohal ide ex- ciplex formation had been reported as early as 1964, "~ it was not until early 1974 t2-41 that further work in this field appeared in the literature. Since then, the rare gas-monohal ide systems have been studied extensively, both experimentally and the- oretically. However , most of the literature, to date, concerns the static condition effects on the effi- ciency and yields of exciplex lasers; t5-7~ very few reports are available on direct observation of time dependent exciplex formation and decay, t8-~2~ In these reports, there is clear evidence that two tem- poral processes are occurring, but until recently, no definite resolution, identification, or precise kinetic analysis of these processes had been achieved.

Maeda et al . , ~9) who studied exciplex formation using pulsed electron beam excitation in Ar/F2, Xe/ Br2, Xe/C12, and Xe/HCI gas mixtures, obtained data showing these two temporal processes. They were unable to do an independent study of either of these processes, but they attributed one process to ionic recombination, and the other to 'direct ' re- action.

The work of Cooper et al. " ~ on the KrF/KrCI/ KrBr exciplex systems showed that experimental conditions could be obtained so that the time-de- pendent processes forming these exciplexes could be resolved.

Of the two processes, the faster one was found to be independent of the total electron beam energy (dose per pulse) and they assigned this process to d i rec t reaction between high electronic excited

t Permanent address: Department of Chemistry, Uni- versity of Calgary, Alberta, Canada.

states of krypton and the halogen source. The other, slower, process was found to be dependent on the electron beam energy, and was attributed to ion recombination. The work also showed that the exciplex broad band emission wavelengths changed with the halogen source present, thus establishing a negative halide ion dependence in the formation process. They also presented evidence for the for- mation of these exciplexes in high vibrational lev- els.

This work seeks to further look at the slower for- mation processes proposed as ion recombination reactions.

E X PE RIM E N T A L

The excitation source used in this study was a Field Emission Corporation Febetron 706 electron pulser. This machine emits a 3 ns (FWHM) beam of 0.2-0.6 MeV electrons with a peak current of 7000 A.

The gas mixtures were prepared in a grease free, g lass-metal vacuum line, and irradiated in T- shaped brass cells; the electrons entered the vessel via a stainless steel foil window of thickness 2.5 × 10 -3 cm. The light emitted from the irradiated gas mixture passed through suprasil windows, and was observed perpendicular to the electron beam. The exciplex fluorescence was focused by a lithium fluoride lens onto the slits of a Spex-minimate grat- ing monochromator , whose band pass was either 2 nm (kinetic traces and integrated emission) or 1 nm (emission spectra).

The emission selected by the monochromator was monitored by a RCA IP28 photomultiplier tube, and the subsequent photo current produced was

545

546 R. COOPER et al.

displayed on a Tektronix 7633 storage oscilloscope. The oscilloscope trace was then digitised and stored via a T.V. camera interfaced with a LSI-II mini- computer. The resulting digitised data was later an- alysed using a nonlinear least squares fitting pro- gram. To shield against R.F. noise generated by the Febetron, the entire detection system was housed in a large copper mesh Faraday cage.

The intensity of the electron pulse was varied by placing perforated steel plates in an evacuated space between the Febetron and the gas cell.

The integrated yield data was obtained by re- placing the normal load resistor (50/93 1~) by a high resistance (106 gl) and a low capacitance (80 pF) which gave a step trace, whose height, when meas- ured at long times, is proportional to the total light emission.

They also showed that a trace amount (0.05 mmHg) of SF6 in a krypton/CF3C1 gas mixture (50.0/ 0.50 mmHg, respectively), upon irradiation at full electron beam intensity, resulted in a partial loss of emission at 222 nm (KrCI*) and the production of a new emission at 248 nm (KrF*). The remaining kinetics at 222 nm were identical to those obtained at low initial pulse intensities, and the new emission at 248 nm was shown to be kinetically identical to that lost from the slower process at 222 nm. These workers attributed this to the more efficient elec- tron capture by SF6 (k = 1.32 × 1014 M-~ s i) compared to CF3CI (k -- 3.1 × 107 M - j s - I ) . "3~

SF6 can clearly compete successfully against CFsCI for electrons, resulting in the formation of SF6 anions instead of chloride anions. Hence the ion recombination reaction will now be

MATERIALS

The gases used in this study, krypton and xenon (Matheson Research Grade, 99.999%) and SF6 (Matheson 'pure ' grade), were held in glass storage containers connected to a glass and metal vacuum line. The irradiation cells and gas cylinders were connected to the vacuum line by metal 'Cajon' con- nectors.

All the gases were purified by several successive freeze (77 K)-pump-thaw cycles prior to use.

RESULTS AND DISCUSSION

The earlier work done in this laboratory ~ showed that the kinetic traces of KrCI* at 222 nm comprised of an initial fast rise, not clearly resolved from a subsequent, slower growth and decay of emission. Reduction of the initial pulse intensity showed that the initial fast formation was relatively more pronounced, and decayed somewhat before the second formation process, which was much smaller than before, could be seen. At the lowest pulse intensity used, only the first process was ob- served, over the time region 0-300 ns.

The interpretation of these results was that the second, (intensity dependent), slower growth- decay curve was due to an ion recombination pro- cess of the form

Kr + + CI- ~ KrCI*

where the krypton ions are formed directly during the electron pulse, and the chloride ions formed by dissociative electron capture by CF3CI

e + CFsCI--~ CI + CF3.

Kr + + SF6----~ KrF* + SF5

resulting in a new emission at 248 nm. Halide anions are clearly precursors required for

the slower intensity dependent process to be ob- served.

If the second process is indeed ion recombina- tion, then krypton cations must also be essential. This can be checked by scavenging experiments with xenon added to an irradiated gas mixture of krypton and a halogen source, RX. If this is done, there is a possibility of a competitive charge trans- fer reaction occurring

Kr ÷ + Xe >Xe + + Kr

VS

Kr + + X - > KrX*

and this would result in a decrease in emission in- tensity from KrX*, and a corresponding production of an emission due to XeX* via the reaction

Xe ÷ + X - > XeX*.

The ionization potential for xenon is 12.130 V which is lower than that for krypton (13.996 V), t~4~ hence the charge transfer reaction is energetically feasible.

Figure l(a) shows the KrF* emission at 248 nm of an irradiated gas mixture containing 0.50 mmHg SF6 and 50.0 mmHg krypton, and the corresponding emission when 1.0 mmHg xenon was added to the gas mixture. Figure l(b) shows the corresponding emission at 351 nm due to XeF* under the same experimental conditions. As xenon was added to the gas mixture, a decrease in KrF* emission in-

600 F

500

~400

.15 300

z zoo

1oo.

NO Xe PRESENT 1.o

09 ,,"tl'm'Ir~,l,, &

," " I" , , o.a g "~- Xe PRESENT

m 0.6

t _-"-t :-. i - "- '- ~ oz m ~ % a 0./. ... - ---.._.

• ,r" 0.3 " t

.." 0.2

: / 0.1

, t ' : I " o . . "

25 SO 75 100 125 150 175 200 TIME (NSEC.)

Fro. l(a). KrF* emission at 248 nm of irradiated Kr/SF6- Kr/Xe/SF6 gas mixtures: abscissa, time (ns); ordinate, in-

tensity (arb. units).

tensity and concomitant increase in XeF* emission intensity was observed.

The pressure of xenon in the irradiated gas mix- ture initially was small, and hence direct excitation or ionization by the electron beam may be ignored. The emission of a gas mixture containing 1.0 mmHg xenon and 0.50 mmHg SF6 gave no detectable signal at 351 nm under the same experimental conditions.

The variation in KrF* and XeF* emission inten- sity (at constant absorbed electron beam energy), as a function of added xenon pressure is shown in Fig. 2. The values in this plot were obtained by measurement of the totally integrated yields of both KrF* and XeF* due only to the 'slow' process. At

Ion recombination processes in the formation of krypton fluoride 547

0~5 1;0 1 ;5 Z0 2~5 3;0 315 "i0 4.5 5.0 5;5 PRESSURE Xe (Torr)

FIG. 2. The xenon pressure dependence on the relative yields of KrF* and XeF*: abscissa, pressure xenon (Torr); ordinate, relative yields.

the higher xenon pressures, the energy deposited in the gas mixtures was kept constant by suitably varying the bulk krypton gas pressure according to relative stopping powers of the constituent gases, The data was normalised for KrF* emission inten- sity using data from 'blank' Kr/SF6 and Xe/SF6 gas mixtures under the same experimental conditions.

A plot of the decrease in KrF* intensity against the increase in XeF* intensity, as a function of in- creasing xenon pressure, is shown in Fig. 3. The gradient of this plot is unity, and hence directly as- sociates a process removing the KrF* emission in- tensity with one responsible for the production of the XeF*.

In Fig. (2), it can be seen that the emission in- tensities from KrF* and XeF* are equal at a xenon

200

160

.~120

Bo z

40,

o s6

Xe PRESENT

l

Z NO Xe PRESENT

100 150 200 250 300 TIME (NSEQ)

Fie. l(b). XeF* emission at 351 nm of irradiated Kr/SF6- Kr/Xe/SF6 gas mixtures: abscissa, time (ns); ordinate, in-

tensity (arb. units).

10

09

08

07

06

~o~

OL

0.3

(?2

0.1

0 01 02 0 3 0 L 0.5 06 07 018 019 ~u AXeF

Fie. 3. The relative changes in KrF* and XeF* yields as a function of xenon pressure: abscissa, relative change in XeF* (AXeF*); ordinate, relative change in KrF*

(A KrF*).

pressure of 1.61 mmHg. If a proposed competitive charge t ransfer - ion recombination mechanism is correct, then under these experimental conditions, the reactions

(1)

(2)

(4)

Kr + + X e - - ~ X e + + Kr

Kr + + SF6- - - -> KrF* + SF5

are proceeding at equal rates. Since reaction (1) is pseudo-first order ([Kr + ] ~ [Xe]), and reaction (2) is second order, there should be a dependence of the observed rate of these reactions on the initial pulse intensity. At high initial ion concentrations, reaction (2) should be dominant, resulting in a rel- atively high yield of KrF* emission compared to XeF*. Conversely, at low initial ion concentrations, reaction (1) should be dominant, resulting in a rel- atively high yield of XeF* emission.

The variation in KrF* emission intensity with in- itial pulse intensity is shown in Fig. 4. The predicted effect of a relatively high yield of KrF* at high pulse intensity, decreasing with decreasing pulse inten- sity, is observed, supporting the proposed mecha- nism of competit ive first and second order reac- tions.

From this, we conclude that recombination of ha- lide anions and rare gas cations is a mechanism for the slower process which is consistent with this data.

In this study, it was also noticed that, under fa- vourable experimental conditions (low initial pulse intensity and at times longer than 0.5 txs), the decay of both these exciplexes became second order.

At these long times, if we assume that the ion recombination process is rate controlling, then from

1000

800

--~ 600'

~,oo

..J

200

the proposed mechanism we have

Kr + + S F r - k,) KrF*

KrF* k2 Kr + F + h r .

Assuming KrF* is in a steady state, solving for [Kr + ] gives us

1 1 (3) - - - + k , t

[Kr + ] [Kr+ ]o

From the observed emission intensity, I = Kk2 [KrF*], where K is a proportionali ty constant for the light-detecting efficiency of the experimental set-up, and the steady state assumption, we have

5

E ~ 2

1.

I = K k l [Kr +][SF6 ].

At long times, these two concentrations are equal, hence

(5) I = K k l [Kr +]2

and using (3) and rearranging gives us

K k l [Kr + ]2 I =

(1 + kt t[Kr+]) g '

which transforms to

V7 X/K ~ [Kr + ]o + ~/ "

Thus a plot of 1/~/I " vs t should be a straight line with slope X/~/X/-K. Figure 5 shows such a plot for

548 R. COOPER et al.

200 /~0 600 800 1000 RELATIVE PULSE INTENSITY

FIG. 4. The initial pulse intensity dependence on the rel- ative KrF* yield: abscissa, relative pulse intensity; ordi- nate, relative KrF* intensity.

1000 1500 20bo 2:500 TIME (NSEE)

FIG. 5. Second-order plot of the decay of KrF* in a Kr (50.0 mm Hg), SF6 (0.50 mm Hg) gas mixture: abscissa,

time (ns); ordinate, 1/(intensity) ~/2 (arb. units).

Ion recombination processes in the formation of krypton fluoride 549

a gas mixture of 0.50 mmHg SF6 and 50.0 mmHg Kr. The straight line obtained is further evidence that the proposed mechanism is correct. If the in- itial concentrations of these ionic species can be obtained, we then have a method of determining absolutely the rate constants for exciplex forma- tion. Using a calibrated optical detection system, this situation is currently under study, and will be reported in a further communication.

C O N C L U S I O N

The electron pulse irradiation of Kr/Xe/SF6 gas mixtures has been shown to produce KrF* and XeF* by two distinct formation-decay processes.

The slower, electron-beam-energy dependent process, has been demonstrated to be due to ion recombination between rare gas positive ions and halide negative ions. The production of Xe + has been shown to be due to a competitive charge trans- fer reaction with Kr + . The overall mechanism, for this slower process, consistent with the obtained results is

Kr + + SF6- ~ KrF* + SF5

Kr + + Xe ~ Xe + + Kr

Xe + + SF6- ~ XeF* + SF5

KrF*---~ Kr + F + hv

XeF*--* Xe + F + hr.

Acknowledgements--R. C. and S. M. wish to acknowl- edge support from the Australian Research Grants

Scheme. One of us, D. A., wishes to acknowledge as- sistance from the National Research Council of Canada. S. M. also wishes to acknowledge assistance from a Mel- bourne University Postgraduate Scholarship.

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