Formation and decay kinetics of the 2p levels of neon, argon, krypton

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J O U R N A L

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PHYSICAL CHEMISTRY Registered rn U S P a t e n t Offrce 0 C o p y r i g h t , 1977, b y t h e American Chemical Society

VOLUME 81, NUMBER 24

DECEMBER 1,1977

Formation and Decay Kinetics of the 2p Levels of Neon, Argon, Krypton, and Xenon Produced by Electron-Beam Pulses’ R. Cooper, F. Grleser, Deparfment of fhyslcal Cbemistry, University of Melbourne, Parkville, Victorla, Australla 3052

Myran C. Sauer, Jr.,” Chemistry Division, Argonne National Laboratory, Argonne, Illlnois 60439

and David F. Sangster Australian Atomic Energy Commission, Lucas Heights, New South Wales 2232, Australia (Recelved April 1, 1977) Publication costs assisted by Argonne National Laboratory

Observations of the kinetics of light emission show that the 2p levels of the rare gases Ne, Ar, Kr, and Xe are produced by three different processes: (1)direct excitation by the electron beam; (2) degradation of higher energy levels; (3) ion recombination. The time dependences of the visible and near-IR emission spectra observed from these gases reveal that process 3 produces a significantlydifferent distribution of 2p levels than does the combination of processes 1 and 2.

plates. The light emitted was focussed onto a 0.75-m Jarrell-Ash Model 75-000 spectrograph. The bandpass was -2 nm. To avoid possible interference from second- or third-order transmission, an appropriate filter was placed in front of the entrance slits. The emission signals were monitored with an RCA 4832 photomultiplier tube, the output of which was amplified 100 times with a HP-462A amplifier and displayed on a Tektronix 7904 oscilloscope. This display was photographed using a Polaroid camera with 10000 ASA film. The photomultiplier was shielded from x rays produced by the pulse, and the cell windows were out of the electron beam path; thus, no Cerenkov light was detected. The overall rise time of the equipment was 6-8 ns, which was due mainly to the photomultiplier. The spectrograph and all of the electronics were enclosed in a double-screened room8 so that electromagnetic interference from the Febetron was essentially eliminated. Computer simulation studies were performed with a Data General “Nova” Computer.

Introduction Pulse radiolysis has proven to be a useful technique for studying the production and decay of excited states and intermediate species in the gas p h a ~ e . ~ - ~ This paper presents a study of the kinetics of rare gas atoms in their 2p electronic states which resolves apparent incongruities in the results of previous studies5t6of the production and decay of these states. Experimental Section The optical system used in this gas phase pulse radiolysis study is similar to one described in detail elsewhere.’ The perturbation source was a Field Emission Corp. 706 Febetron system, which emits an intense pulse of electrons (7000 A) having a maximum energy of 0.5 MeV. The pulse shape is approximately triangular with a half-width of -3 ns. The electron pulse entered a l-L stainless steel cell containing the rare gas. The dose was sometimes reduced by passing the electron beam through perforated steel 2215

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Cooper et al. I

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Flgure 1. Emission vs. time at 725 nm (2p,, Is,) in 840 Torr of Ne. The time scale is 100 ns/division. The arrow indicates the end of the approximately 5 4 s wide electron pulse.

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WAVELENGTH (nrn)

Figure 4. Emission spectrum of 420 Torr of Kr. The spectrum was measured using a lower dose per pulse than in the case of neon or argon (see text). The full curve represents the spectrum of the "fast" emission and dashed curve the delayed emission. The insert is an expansion of delayed emission spectrum. I

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Flgure 2. Emission spectrum of 840 Torr of Ne. The full curve represents the spectrum of the "fast" emission and dashed curve the delayed emission. The insert is an expansion of the delayed emission spectrum.

WAVELENGTH (nm)

Figure 5. Emission spectrum of 560 Torr of Xe. The spectrum was measured using a lower dose per pulse than in the case of neon or argon (see text). The full curve represents the spectrum of the "fast" emission and the dashed curve the delayed emission.

WAVELENGTH (nm)

Figure 3. Emission spectrum of 600 Torr of Ar. The full curve represents the spectrum of the "fast" emission and dashed curve the delayed emission.

Materials. The rare gases used were Matheson Research Grade, except for argon, which was Airco Ultrapure (99.999%). All these gases were used as supplied. SF6 (Matheson Research Grade) was purified by a few freeze-pump-thaw cycles. Triply distilled water was degassed in a similar manner. Results and Discussion The emission signal depicted in Figure 1was obtained at 725 nm from a sample of 840 Torr of neon. The trace distinctly shows that the excited rare gas species is produced by two reaction sequences which are partially resolved in time. This behavior was common to all observed 2p emissions. Emission spectra for the four rare gases are shown in Figures 2-5, and were plotted using the intensity maxima of the primary and secondary peaks from the emission vs. time traces. These figures are presented mainly to indicate the nature of the variation of primary to secondary The Journal of Physical Chemistty, Vol. 81, No. 24, 1977

emission with wavelength, and no correction has been made for the variation in photomultiplier sensitivity with wavelength. For krypton and xenon, the two intensity maxima were not very well resolved at the pressures used; however, by reducing the pulse intensity, good resolution was achieved (this will be described in greater detail in the following section). The spectra reported for Kr and Xe were therefore obtained at lower dose per pulse. All the emission lines have been previously observed in a pulse radiolysis study by Arai and F i r e ~ t o n e ,and ~ correspond with tabulated 2p-1s atomic transition^.^ The time dependence of the two types of processes exemplified in Figure 1was studied as a function of rare gas pressure, dose per pulse, and additives for all of the 2p 1s transitions which were of great enough intensity. I. "Delayed" Excited State Formation. The qualitative effects of changes in the aforementioned experimental parameters on the delayed emission (e.g., the second peak in Figure 1) were the same for all four gases and independent of the emission band monitored. The decay rate (e.g., 7 N 140 ns for 840 Torr of Ne) decreased linearly with decreasing pressure and the emission maxima shifted to longer times. Figures 6a and 6b show that decreasing the dose causes the emission maximum to move to longer times, but causes no appreciable change in decay rate. Figures 7a and 7b show that 1 Torr of SF6, a good electron scavenger, completely eliminates the delayed emission. The effects of dose and SF6strongly suggest that ion recombination is responsible for the production of the excited rare gas

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Formation and Decay Kinetics of the 2p Level of Rare Gases

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Figure 8. Emission vs. time at 830 nm (2p5 ls.,)in Xe at various pressures. The time scale is 10 nsldivision (a) and 20 ns/division (b). -+

The x-ray signals (dashed curves) are amplified two times.

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Flgure 6. Emission vs. time at 585 nm (2p, Is2)in 840 Torr of neon using the maximum dose per pulse (a), and using 114 of the maximum dose per pulse (b). The time scale is 100 nddivision, and the signal has been amplified by a factor of 7 in (b). In both cases the “fast” emission is off-scale and is not shown.

We have not been able to devise a mechanism which quantitatively explains the delayed emission. The kinetics may be complicated by the fact that electron thermalization in argon is ~ l o w , which ~ ~ J ~would delay the recombination, and by the likelihood that impurities are present which could react with either the positive ion or the electron. 11. “Fast” Excited State Formation. By using low pressures (5100 Torr) or lowering the radiation dose to the rare gas samples, the delayed excited state production could be sufficiently reduced or separated from the prompt formation to allow the latter to be studied without significant interference from the ionic processes. The following observations were common to the four rare gases studied (1)The formation and decay kinetics of the fast emission are independent of dose. (2) Addition of small amounts of SF6 ( 1Torr) or HzO (-0.2 Torr) had essentially no effect on either the kinetics or the timeintegrated intensity of the fast emission. Certain other characteristics of the light emission depended on the particular rare gas; hence each gas will be discussed separately. 1. Xenon. The two transitions, 2p5 ls4 at 830 nm, and 2p8 ls5at 884 nm, behave similarly with change in experimental conditions. In both cases the maximum intensity is reached near the end of the pulse and the decay rate decreases with decreasing pressure. Figure 8 shows the time dependence of the 830-nm emission as a function of xenon pressure. The x-ray signal (which is due to x rays produced by the electron beam striking the photomultiplier) has been shown to compare the pulse shape, modified by the rise time, with the growth of the light emission. It can be seen that the maximum emission intensity is reached about 7 ns after the maximum in the x-ray pulse. Part of this delay can be ascribed to the rise time of the system, so it is reasonable to conclude that the half-time for the formation of the emission is 5 5 ns. At pressures below -8 Torr, the decay of both the 830- and 884-nm emissions becomes complex, showing neither first- nor second-order behavior. The first-order decay rate of the 830-nm emission, at a xenon pressure of 8 Torr, was about 1.5 X lo7s - ~ ( T 67 ne). The latter is much greater than the reported radiative lifetime (30-40 ns) of the 2p5 l e ~ e l . ~Similarly, ~ J ~ the decay rate of the 884-nm emission, at low pressures, is slower than predicted from the radiative lifetime of the 2p81e~el.l~ At a xenon pressure of 1 Torr, where the decay is no longer first order, the initial T (measured beginning at the

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Flgure 7. Emission vs. time at 826 nm (2p2 1s2)in Ar: (a) 600 Torr of Ar, (b) 600 Torr of Ar i- 1 Torr of SFe. The time scale is 100 nddivision, and the gain is the same in both cases. -+

atoms giving the delayed emission. The temporal characteristics of the delayed emission are in qualitative accord with the time scale expected for ion recombination on the basis of measured recombination coefficients for electrons with rare gas dimer ionslOJ1and our estimated ion concentrations (e.g., =2 X 1013ions/cm3 for 600 Torr of Ar). Delayed production of excited argon has been observed in Ar/N2 mixtures by LeCalv6 and Bour6ne,12 which they conclude is due to dissociative recombination of electrons with Arz+.

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maximum emission intensity) is -110 ns. These long lifetimes suggest that the 2p levels are being repopulated from higher precursor levels with lifetimes longer than those of the 2p levels. A further point to note is that even at the lower pressure limit of this study, the emission maxima (for the two transitions being discussed) are still observed to occur near the end of the electron pulse. If the 2p levels were exclusively populated by cascade from higher levels, this would not be the case. This can be shown by considering the following simplified mechanism: k

k

A f 2p level -.% lower electronic state

where A represents the precursor levels formed by the electron pulse. The expression for the time of maximum intensity of the 2p state is given by tm,,

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- In [h1'h21 - k1- k2

for h l > k2 or kl < k2 0.03

From the results of the 1 Torr of xenon sample, using kl 1 X lo7 s-l, and taking the radiative lifetime of the 830-nm emission to be 30 ns (collisionalremoval of the 2p6 level at this pressure is slow compared to the radiative lifetime, and the correction to kz can therefore be ignored), one obtains t,,, N 52 ns. Therefore, if the emission maximum occurs within 5-10 ns of the pulse, a major fraction of the 2p level must be formed by direct excitation during the electron pulse. The nonlinearity in the decay a t lower pressures can be ascribed to the combination of two processes forming the 2p level, the first being the prompt formation during the electron pulse and the other being a cascade from either one or more upper levels which have appreciably longer lifetimes than the 2p level. Computer simulation results (section 111) also support this interpretation. 2. Neon. In the case of neon, all of the 2p 1s emission lines show variation in decay rate with pressure, although the 2p1 Isz emission is only slightly affected (see Figure 9). The 2p10 1s6and the 2p10 1s4emissions show the same kinetics, as expected, and the time of the maximum in the emission signal was observed to increase with decreasing neon pressure, reaching 70 ns at 26 Torr (compare with Figure 1, 840 Torr of Ne, t,,, = 30 ns). The large value of t,,, indicates that the 2p10 level is produced predominantly from higher energy levels. The variation oft, with pressure indicates that the decay of the higher levels is accelerated by increasing the pressure. At low neon pressures (