Quenching Rate Constants of the Xe(5p56p and 6p') States and the

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J. Phys. Chem. 1995,99, 7482-7494

Quenching Rate Constants of the Xe(5p56p and 6p’) States and the Energy-Pooling Ionization Reaction of Xe(5p56s)Atoms T. 0. Nelson,? D. W. Setser,* and M. K. Richmand Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received: June 24, 1994; In Final Form: December 23, 1994@

Two-photon excitation by a pulsed dye laser was used to prepare the Xe(5p56p[1/2]o, [3/2]2, [5/212) and Xe(5p56p’[1/2]o, [3/2]2) states in the presence of several reagent gases to measure total quenching rate constants at 300 K. The new measurements together with rate constants from the literature provide a general overview of Xe(6p) atom reactivity, especially with halogen- and oxygen-containing molecules. Reactive quenching and/or excitation transfer to the reagent molecule are the dominant quenching mechanisms, rather than intramultiplet transfer to other states of the Xe(6p) manifold. The XeCl(B and D) and XeF(B and D) product state distributions from reactions of the Xe(5~~(~P1,2)6p’) states with HC1, Ch, F2, N F 3 , CCL and CClzFz were observed in order to record the degree of conservation of the x e + ( * P ~core ~ ) as indicated by XeCl(D) or XeF(D) formation. Except for the HCl reaction, the Xe+(*Pm) core does not have a high degree of conservation in reactions of Xe(6p’) atoms. For conditions of low reagent pressure, radiative decay of the Xe(6p) states can produce a high local concentration of metastable Xe(6s) atoms, which subsequently undergo bimolecular, energy-pooling, associative-ionization reactions. The Xe2+ ions subsequently recombine with electrons to generate a variety of excited Xe* states.

Introduction The reactions of Xe(5p5(*P3/2)6p) atoms, which lie in the energy range from 9.58 to 9.93 eV, can be important in plasmas containing Xe because these states are generated from the Xe(6s,8.3 eV) states by collisions with electrons and by electron recombination with Xe*+. Modeling of the kinetic processes in the atomic Xe laser’ and Xe excimer lasers2 requires knowledge about these states. Collisional processes involving the Xe(6p) atoms also may affect the generation of shortwavelength light using various nonlinear optical properties of Xe and Xe-rare gas mixture^.^ The relaxation kinetics of Xe(6p) states in rare gases have been ~haracterized;~ however, much less is known about the reactions of Xe(6p) atoms with molecular reagents. In the present work, we report thermal quenching rate constants and XeX (X = F, C1) product distributions for reactions of Xe(5p5eP3n)6p) and Xe(5p5(2P~n)6p’) atoms, with several halogen containing molecules (RX). This work also is related to our effort to understand the two-photon, laser-assisted reaction between Xe and RX pairs? because the results from the full collision between RX and Xe(6p or 6p’) atoms often are needed for comparison. Excitation of the Xe(6p[5/2]2,9.68 eV; [3/2]2,9.82 eV; [1/210, 9.93 eV; and 6p’[3/2]2, 11.05 eV; [1/2]0, 11.14 eV) states is readily a c c ~ m p l i s h e dby ~ , ~two-photon excitation of Xe atoms in a gas mixture using a pulsed dye laser, and reactions with F2, NF3, Cl2, HCl, CF2Cl2, and CC14 were investigated in the present work. Combining these Xe(6p) and Xe(6p’) results with data in the literature6s7and with some unpublished quenching measurements8 for N2, H2, NO, and CF4 provides an overview of the Xe(6p and 6p’) atom reactivity. The degree of conservation of the Xe+(*PI/*)or Xe+(2P3/2)core state in reactions of Xe(6p) and Xe(6p’) states with RC1 and RF molecules was investigated by observing the XeCl(B,C,D) and XeF(B,C,D) emission spectra; the B and C states have the Xef(*P3/2) core Present address: Los Alamos National Lab, Los Alamos, NM 87545. Present address: Argonne National Lab, Argonne, IL 60439. @Abstractpublished in Advance ACS Absrructs, May 1, 1995. +

and the D state has the Xe+(2Pl/*) core. Previous work established that the total quenching rate constants, ko, and the branching fractions for XeX* or XeO* formation are larger for Xe(6p) reactions6 than for the Xe(6s[3/2]2 or [3/2]1) states interacting with the same RX or RO reagent.6 The 6p[5/2]2 to 6p’[1/2]0 states range from 1.46 to 2.83 eV above the first excited Xe(6s[3/2]2,8.3 eV) state; the higher energy of the entrance channel allows access to the (Xe+;RX-) ion-pair potential at larger internuclear distance, R(Xe*-RX), which enhances the reactive quenching channel giving XeX* in competition with excitation-transfer channels giving RX*. This point is illustrated in Figure 1. The principal question to be addressed for the Xe(6p’) reactions is whether the trajectories couple to the V(Xe+(*P3,2);RX-) potential or do they pass through the outer crossing and continue to the inner V(Xe+(2P1/2);FS-) potential, Le., is the Xe+(2P1/2) core conserved in reactive quenching? Based upon analogy to Ar(4s) vs Ar(4s’) and Kr(5s) vs Kr(5s’) reactions9-” and state-resolved studies of ion-recombination reactions,’* Xe* states with the Xe+(*P1/2) core could have a propensity to give XeF(D) and XeCl(D), rather than only XeF(B,C) and XeCl(B,C). The states of Xe are identified in Figure 8 for the aid of the reader; the JJ coupling or Racah notation is used to label the states. The two-photon excitation technique can generate significant concentrations of Xe(6p) atoms, which decay radiatively (t = 35-40 ns)43637at low reagent pressure to the metastable Xe(6s[3/2]2 or 3P2) and the resonance Xe(6s[3/2]1 or 3Pl) states. According to our interpretations, these metastable Xe(6s[3/2]2) and radiatively trapped Xe(6s[3/2] 1) atoms undergo energypooling, associative-ionization, or Penning ionization reactions: Xe(6s)

+ Xe(6s) - Xe2+ -te-Xe’ + Xe + e-

(14 (lb)

The three-photon (2+1) ionization of Xe atoms also can produce Xe*+ via three-body recombination of Xe+. These two pathways for Xe*+ formation can be distinguished by the

0022-3654/95/2099-7482$09.00/0 0 1995 American Chemical Society

Quenching Rate Constants of the Xe(5p56p and 6p’) States Energy ( e V ) pe+(*P,,2) I 1343 eV

li 11.13 eV ,=‘[3/21,)

Xd6P[’/2l0) Xd6P[5/2I2) Xe(6.’[1/2:0)

.......................................................

Xe(6s[3/2I2)

9

/

Figure 1. Schematic representation of several covalent entrance channel potentials and the two ion-pair potentials for Xe with a reagent,

Rx. The ion-pair potentials are drawn for an EAv(RX) = 1.0 eV, which closely resembles EAV(C1z). For smaller EAv(RX), the ion-pair potentials are corresponding higher in energy. The Xe(7p and 6d) states closest in energy to Xe(6p’[3/2]2) - 89 163 cm-I are the 7p[1/2]0 88 843 cm-I and 6d[5/2I2 - 89 244 cm-’ states. The zero point vibrational motion of Rx(X,v=O) actually leads to a crossing seam; see ref 7 for a discussion of the coupling between entrance and exit channel potentials for Cl2.

dependence of their formation rates on time and pressure; the three-body recombination rate is slow for our low pressure experiments. Electron recombination with Xe2+ generates a variety of Xe** states that subsequently can react with RX:

For experiments designed to isolate the Xe(6p) atom reactions, the pulse energy was constrained to be below the ionization regime. Two-photon excitation of Xe(6p) atoms can generate amplified spontaneous emission (ASE) to the Xe(6s) levels. Direct tests for ASE were not made, but no evidence for such processes was detected from the sideband emission for the low laser pulse energies used in these and p r e v i ~ u s ~experiments ~-~ in our laboratory with the Xe(6p) levels. The ASE problem is much more difficult to avoid when exciting Xe(6p’) state^.'^^,^ Experiments with direct monitoring of the ASE to be published in the near future’3c have shown that, even if weak ASE was present, it did not affect the quenching rate constant data or the XeX(B,C,D) product distributions from the Xe(6p’) experiments to be reported here. The experimental measurements of this work consist of the following: (i) the first-order decay constants of the Xe(6p) and Xe(6p’) atoms observed by monitoring the Xe(6p-6s) or Xe(6p’-6s’) emission intensity vs time, (ii) the XeC1* or XeF* product spectra from the Xe(6p and 6p’) atom reactions, (iii) the fluorescence of the Xe** states following electron recombination with Xe2+, and (iv) the ionization current collected from electrodes installed in the laser photolysis cell. A model calculation also was done for the ionization and recombination processes in order to confirm that (la) was the major source of the Xe2+ for our experiments.

Experimental Methods These experiments were performed by passing the doubled output from a pulsed dye laser through a mixture of Xe and a reagent gas in a static cell. The experiments with F2 were done

J. Phys. Chent., Vol. 99, No. 19, 1995 7483

in a stainless steel cell, which had 30 cm long baffle arms to reduce the scattered laser light. The stainless steel cell was thoroughly passivated with F2 before it was used to collect data. Experiments with all other reagents were done in a similar cell made from Pyrex glass. Pressures were measured by MKS Baratron transducers. A XeCl excimer laser pumped the dye laser, which employed an Inrad Autotracker with a BBO crystal to obtain the second harmonic frequency. The doubled laser pulse had a full width at half-maximum of -15 ns with a 22 GHz bandwidth. A Laser Precision (Model FUP-35) energy meter was used to measure the pulse energy at the window of the exit baffle arm of the cell. For some experiments, the laser beam was focused with a 0.5 or 1.0 m focal length quartz lens. Coumarin 500 and 440 dyes produced the fundamental frequencies that were doubled to the 256.0, 252.5, 249.6, 224.3, and 222.6 nm wavelengths required for excitation of the Xe(6p and 6p’) states. The Xe*, XeCl*, and XeF* emission spectra were observed with a 0.5 m monochromator which viewed the emissions perpendicular to the direction of propagation of the laser beam. The emission was collected with a short focal length quartz lens. Time resolved emission signals were recorded with a Hamamatsu 955 photomultiplier tube. The decay curves were recorded using the monochromator and photomultiplier tube with a Biomation digitizer (Model 6500 with a minimum sampling interval of 2 ns); the waveforms were accumulated and analyzed on a personal computer. The decay rates of the Xe(6p) and Xe(6p’) atoms were monitored by observing Xe(6p-6s) and Xe(6p’-6sf) atomic lines. The majority of the spectra were recorded with a Princeton Instruments optical spectrum multichannel analyzer (OSMA) enhanced for UV response. A 300 grooves/mm grating, giving a spectral window of 167 nm, replaced the higher resolution grating, 1200 grooved mm, for observations with the OSMA. The response of the OSMA was calibrated with a standard deuterium lamp; the spectra shown in Figuores 3-7 have been corrected for variation in spectral response. The experiments to measure the current associated with photoionization were done in a specially constructed Pyrex glass cell that contained electrodes placed 1 cm above and below the laser beam. The electrodes, which were made from rolled Ta foil, were soldered to tungsten rods that extended through the glass cell via standard tungsten-glass feed-throughs. The ioncollection circuit was a balanced-bias differential am~1ifier.I~

Experimental Results

A. Xe(6p and 6p’) Quenching Rate Constants. These experiments were done with 0.2 Torr of Xe to avoid the mixing of Xe(6p’ and 6p) states by collisions with Xe. Low laser pulse energies (= C I R Xexp[-C2[[(2IP)Ii2 (2EA)1i2]/2]R~] to fit the Xe* Cl2 data. The EA for HCl is 5 0 and Rx is smaller than for C12; thus H12 is larger for HCl than for Cl2 unless the C2 coefficient is adjusted. (8) Xu, J. Unpublished measurements acquired at the same time as the work in ref. 6c. (9) (a) Kolts, J. H.; Velazco, J. E.; Setser, D. W. 1.Chem. Phys. 1978, 69, 4357. (b) Chen, X.; Setser, D. W. J. Phys. Chem. 1991, 95, 8473. (c) Lin, D.; Yu, Y . C.; Setser, D. W. J. Chem. Phys. 1984, 81, 5830. (d) Sobczynski, R.; Slagle, A. R.; Setser, D. W. J. Chem. Phys. 1990, 92, 1132. (e) Yu. Y . C. J. Photochem. Photobiol. A 1989, 47, 259. (10) Sadeghi, N.; Cheaib, M.; Setser, D. W. J. Chem. Phys. 1989, 90, 219. (11) Sobczynski, R.; Beaman, R.; Setser, D. W.; Sadeghi, N. Chem. Phys. Lett. 1989, 154, 349. (12) (a) Tsuji, M.; Muraoka, T.; Kouno, H.; Nishimura, Y . J. Chem. Phys. 1992, 97, 1079. (b) Tsuji, M.; Furusawa, M.; Kouno, H.; Nishimura, Y . J. Chem. Phys. 1991, 94,4291. (c) Tsuji, M.; Furusawa, M.; Nishimura, V. J. Chem. Phys. 1990, 92, 6502. (d) Tsuji, M.; Ide, M.; Muraoka, T.; Nishimura, Y . J. Chem. Phys. 1993, 99, 1710. (13) (a) Rankin, M. B.; Davis, J. P.; Giranda, C.; Bobb, L. G. Opt. Commun. 1989, 70, 345. (b) Miller, J. C. Phys. Rev. A 1989, 40, 6969. (c) Alekseev, V.; Setser, D. W. J. Chem. Phys. to be published. (14) Adams, T. E.; Morrison, R. J. S.; Grant, E. R. Rev. Sci. Instrum. 1980, 51, 141.

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