J. Phys. Chem. 1992, 96, 7010-7013
7010
Reaction of Two-Photon-Excited Xenon and Krypton Atoms with Hydrogen Molecules M. Kawasaki, Y. Matsumi, Institute for Electronics Science, Hokkaido University, Sapporo 060, Japan
A. Chattopadhyay, N. Shafer, S. Satyapal, S. Tasaki, W. Yi, and R. Bersohn* Department of Chemistry, Columbia University, New York, New York 10027 (Received: December 13, 1991; In Final Form: May 11, 1992)
Xenon and krypton atoms were prepared in excited g states, 6p['/2]0 and 5 ~ [ ' / ~ ]respectively, 0, by two-photon absorption at the focus of a dye laser beam. In the presence of H2molecules, H atoms are formed with kinetic energy about that required for the reaction Xe* + H2 Xe + 2H. The rate constant for quenching of the Xe fluorescence by H2 is (1.0 f 0.1) X lo", cm3molecule-' s-I. The excited atoms react with HD to produce a large excess of H atoms Over D atoms. This is explained by assuming that the reaction of Xe* or Kr* with HD produces mainly a H atom and a rare gas monodeuteride in the A2ZIl2 state.
-
Xenon can be excited by one photon to a series of odd (u) 5pSm (n 1 6) states and by two photons to a series of even (g) 5pSnp' ( n 1 6) states. One of the lowest u states, 6 ~ [ ~ /is~excited ] ~ , by a xenon resonance lamp at 147.0 nm (68027 cm-' or 8.43 eV). The lowest excited g state is the 6 ~ [ ' / state ~ ] ~at 80 118 cm-' or 9.93 eV. Figure 1 illustrates these data. With this much internal energy, it is not surprising that when these excited atoms collide with molecules, interatomic bonds are broken. For example, it has been shown by chemical means'P2 that when H2 molecules collide, H atoms are formed; e.&, after mixtures of Xe, H2 and D2 are irradiated with a Xe resonance lamp, HD is found. Spectroscopic measurements such as electron-spin resonance of the H atoms3 and observation of emission from XeH and XeD molecules also prove that H atoms are This paper describes a study of the reaction of Xe*(6p[l/2]o) and K r * ( 5 ~ [ ' / ~ with ] ~ ) hydrogen molecules. Atoms in these states are generated from the ground-state atoms by a two-photon transition using a rather intense source at 249.63 nm (Xe) or 212.55 nm (Kr). Reactions of Xe atoms in this and other g states with C12have been extensively studied to try to understand the processes in the XeCl laser.8 Our aim was to study the dynamics of the Xe + H2 reaction using laser-induced fluorescence (LIF) to determine the kinetic energy of the H atoms.
Experimental Section This experiment was carried out in a conventional pumpprobe apparatus which is sketched in Figure 2. The 249.63- and 212.55-nm light needed to excite the Xe and Kr atoms, respectively, was obtained by frequency doubling of 499.26- and 425.1 l-nm light with two different BBO crystals. The fundamental wavelengths were generated by a Lambda Physik FL 2002E dye laser pumped by a Lumonics HYPEREX 400 XeCl excimer laser. The pulse energies of the coumarin and bis-MSB dye laser outputs at 500 and 425 nm, respectively, were typically 10-12 mJ. The doubled frequency light was focused in a cell containing a mixture of H2 and Xe or Kr. Typical pressures were rare gas, 900 mTorr, and H2, 100 mTorr. To probe the H(D) atoms, vacuum ultraviolet light was generated by frequency tripling of 364.7-nm (364.6-nm) light which was focused in a cell containing 60 Torr of krypton. A Lambda Physik MSC 201 XeCl excimer laser pumping DMQ in a Lambda Physik FL3002E dye laser produced a typical pulse energy at 10 Hz of 30 mJ. The LIF of hydrogen atoms was detected by a solar blind photomultiplier tube (PMT), EMR 542G-08-17-03900, whose output was preamplified, then averaged by a gated integrator and boxcar averager (Stanford Research System Model SR 250) and normalized by the output of another solar blind PMT (Thorn EM1 RFI/B215FV) monitoring the vacuum ultraviolet probe beam. When observing the fluorescenceof H and D atoms in Xe-containing systems, a tube with 20 Torr of Xe was placed 0022-3654/92/2096-7010$03.00/0
between the window of the reaction cell and the PMT to absorb the vacuum ultraviolet (147-nm) fluorescence of Xe (Figure 2). In Kr-containing systems, the tube was filled with about 125 Torr of 02,as Kr itself was found to be less effective in absorbing 123.6-nm light. All gases (except D2 and HD which were 98% pure) used had purities greater than 99%. Vacuum ultraviolet fluorescence of Xe and Kr was monitored by the EMR PMT, and the IR fluorescence was measured with a visiblenear-IR PMT.
Results and Discussion Xe*(6p['/&) + Hp When the dye laser at 249.63 nm was tuned to an exact resonance with the twephoton transition to the Xe*(6p['/2]o) state, Xe atom fluorescence both at 147 nm and at 828 nm could be seen. With the addition of Hz, a H-atom LIF signal could be seen as well but disappeared when the Xe atom was no longer in resonance. This is proof that an excited Xe atom is the reactant and not an excimer such as Xe2*. When the vacuum ultraviolet and the IR fluorescence were monitored in the presence of Hz, their fluorescence was quenched and by the same fraction at the same H2pressure. The implication is that most of the H atoms are formed by collision with the initially formed Xe*(6p['/2]o) rather than the Xe*5ps6s state to which it passes by fluorescence. A detailed proof of this statement is given in the Appendix. The lifetime of the Xe*6s state which can directly radiate to the ground state is lengthened to microseconds by resonance trapping, but nevertheless, relatively few of these atoms react in the time of the experiment. The SternVolmer plot of Figure 3 shows that the ratio of the fluorescent quenching rate constant to the rate constant for decay in the absence of quencher is 0.213 f 0.023 TOIT-'. The lifetime of the 6 ~ [ ' / state ~ ] ~is 28 ns when the Xe pressure is 800 mTorr? From these numbers, one calculates a quenching rate constant of (1.0 f 0.1) X 10-9 cm3molecule-' s-'. This corresponds to an average cross section of about 33 A2, which is reasonable for an atom in a Rydberg state. Figure 4 shows the LIF excitation curve of H atoms under typical conditions when the Xe pressure is 0.9 Torr, the H2pressure is 0.1 Torr, and the time delay between the firing of the pump and probe laser is 200 ns. The main reason for the poor signal/noise is the fact that the Xe* reactant which has to be formed by two-photon absorption is generated only in a very small focal volume. Given the poor signal/noise, the average kinetic energy was calculated in the following very rough way. The excitation curve was assumed to be Gaussian in shape, and from the full width at half-maximum (FWHM), one can extract the average square uncertainty, the second moment. The average kinetic energy is calculated from the equation ( 0 2 ) = 3c2((u - V O ) * ) / U 0 2 (1) where u is the absorption frequency of an atom with speed v, relative to the probe laser, yo is the absorption frequency of a H 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 7011
Reaction of Two-Photon-Excited Xe and Kr Atoms
1
1
0
W
"t 1
2 Delay / p s
Figure 5. Line width (FWHM) of excitation spectra of H (0)and D (0)
0
F i p e 1. Selected part of the Xe atom level scheme. Two photons of wavelength 249.63 nm were used to excite Xe(lSo) to Xe*(5p[l/2]o). a, twephoton excitation; b, fluorescence at 828 nm; c, fluorescence at 147 nm.
I'
1
atoms from HD as a function of the time between the pump and probe laser pulses. pxe = 0.9 Torr, PHD = 0.2 Torr; A, pxC = 0.45 Torr, PHD = 0.05 Torr.
/--A
Delay / rs
G
Figure 6. H/D ratio calculated from the ratio of the areas under the fluorescence excitation spectra of H and D atoms generated from Xe* HD as a function of time. pxc = 0.9 Torr, PHD= 0.1 Torr unless otherwise noted. For high Xe pressure, pxc = 10 Torr, PHD = 0.1 Torr. For high HD pressure, PHD = 0.8 Torr, pxc = 0.2 Torr. For high He pressure, pHc= 10 Torr, pxC = 0.9 Torr, p H D 0.1 Torr.
D
+
0 I '
V
2. Schematic diagram of part of the apparatus. A, PMT; B, tube, C,baffle; D, LiF window; E, reaction cell; G, connecting tube between the cell and the monochromator.
I c1n-l
Xe 09 Torr HO 01 Torr
i
dt
h
/
6
,p
\H
I
Figure 7. Doppler-broadened excitation spectrum of H as well as D atoms produced from Xe* + HD.
I
I 10
0
a0
Pressure of H2/ Torr
Figure 3. IR (open circles) and vacuum ultraviolet fluorescence (filled circles) of Xe* as a function of H2 pressure. Io is the intensity of vacuum ultraviolet or IR fluorescence without H2 in the cell, whereas I is the corresponding intensity at different H2 pressures. Xe
H,
--'
At
lcr 4
0 9 Torr 01 Torr 02 ps
H
d
"Y
Figure 4. Doppler-broadened fluorescence excitation spectrum of H atoms produced from Xe* + H1.
atom at rest, and c is the velocity of light. The factor of 3 takes into account the average kinetic energy in the two directions lateral to the probing beam whose direction is 2. The average kinetic energy calculated from a set of such curves is 79 f 19 kcal/mol. We can calculate what average kinetic energy must result from the reaction Xe*(6p[f/z]o)+ H2
-
Xe(lSo) + H
+H
(2)
The excitation energy of the Xe atom is 228.4 kcal/mol, and the bond energy of H2is 103.2 kcal/mol. Therefore, on average, each H atom must have 62.6 kcal of kinetic energy (neglecting the very small kinetic energy gained by the rare gas atom). This number is at the lower end of the experimental range. One of our initial reasons for undertaking the experiment was to verify if the two H atoms had about the same energy or if one was distinctly slower than the other. In the latter case, one would expect an excitation line shape which would be the superposition of a narrow line and a broad line of equal area. The excitation curve, crude as it is, does not have this shape, and therefore, the two atoms do not have greatly different speeds. The reaction with D2 was observed but gave a rather weak signal which was not investigated further. Xe*(6p[*/$) + HD. Kinetic Energies and H/DRatio. Figure 5 shows plots of the FWHM of the Doppler profile of the H and D atoms produced from the reaction of Xe* with HD as functions of time delay between firing of the pump and probe lasers. The average kinetic energy of the H atoms at 200 ns is 65 12 kcal/mol. What is startling is the H/D ratio, which has the value 5.3 f 1.3 after 200 ns. Figure 6 shows this ratio as a function of time under various conditions. Figure 7 shows typical spectra of H and D atoms taken after 6 ps. A greatly improved signal over that shown in Figure 4 proves that the high H / D ratio is indisputable. The ratio does not reach the vicinity of unity until after tens of microseconds. We postpone a discussion of the time dependence of the H/D ratio until the end of this section.
*
7012 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992
It might be argued that there is a systematic error in the experimental values of the H/D concentration ratio. In the first place, the absorption coefficients of the two atoms are proportional to their dipole moment matrix elements squared, that is, to the inverse of the reduced mass squared. The ratio of the squares of the reduced masses in the H and D atoms differs from unity only by the quantity 1/1836. A far more serious potential source of error is the fact that H atoms, on average, move faster than D atoms and therefore may disappear from view, lowering the H/D ratio. However, this latter effect could only serve to increase the anomalously high ratio. What is the cause of the excess of H atoms over D atoms? The important initial reactions, which are assumed to be
-
+ HD Xe* + HD
Xe*
and Xe* + HD
+H+D XeD(A) + H
Xe('So)
-
XeH(A)
+D
+ HD
D + HD
+
+
high H O V C I I U ~
2 Delay/
ps
Figure 8. H/D ratio from Kr* + HD as a function of time. pKr= 0.9 Torr, pHD= 0.1 Torr.
(3) (4)
(5)
must have negligible importance. This kind of isotopeselectivereaction of an electronically excited atom with HD has been seen before. When Hg(3PI)reacts with HD, the ratio of HgD to HgH found by directly monitoring the ground-state HgH and HgD is about 6.1° Also, NeD has been found to radiate to a lower state, in contrast to NeH which predissociates. I We conclude that the major reaction, with a yield of about 85% is reaction 4, with reactions 3 and 5 contributing the remainder. What is the electronic state of XeD? There are arguments both theoretical and experimental against the formation of XeD in the ground state. Ab initio calculations on all the rare gas hydrides show that the ground states have only shallow van der Waals minima.l2,l3 For example, the minima of XeH and KrH are at 3.81 and 3.57 A with well depths of 7 and 6 meV, respectively. Therefore, if XeD were formed in the ground state, the H atom would have to take away all the available energy, which is 124 kcal/mol in contrast to the observed 65 f 12 kcal/mol. We conclude that the XeD which is formed in our experiments must be in an excited electronic state. Electronically excited XeH and XeD molecules are now fairly well-known spectroscopically, having been generated in discharges in mixtures of Xe and H2 or D2. Four bound 221/2 and two bound zlIlj2 states have been identified." A series of transitions between them terminate in a 221/2 state which we shall call A2Z1p. The states for KrH are similar.' In the absence of any findings to the contrary, we will assume that XeD is formed in this A state. Unfortunately neither we nor anyone else has been able to identify a continuous emission which is expected for the transition from the lowest excited 221/2 state to the ground state. We looked for fluorescence between 510 and 1000 nm, but the emission from Xe2* throughout this wavelength region prevented us from observing any possible emission from XeD or XeH. We thus remain uncertain about the energy gap between the first two electronic states of XeH. The equilibrium bond distances in the A states of XeD and KrD are 1.59 and 1.43 A, respectively. The A -. X fluorescence spectra would give us information about the shape of the repulsive X curves in the region below the minima of the A states. Relaxation of the H/D Ratio. XeD has a finite lifetime, and if one starts with pure HD, one might expect that the H/D ratio would reach unity fairly quickly. Instead, the relaxation is rather slow. What is the mechanism for the relaxation of the H/D ratio? Hints are furnished by the observations that addition of excess He preserves the high ratio, whereas addition of excess HD brings it down to unity rapidly as shown in Figure 6. The reactions H
Kawasaki et al.
Hz
+D
(6)
D2
+H
(7)
have rate coefficients which are energy dependent. For example,
---wf+o
0
Delay/
ps
Figure 9. Line width (FWHM)of H and D with time when generated from Kr* + HD. plcr= 0.9Torr, pHD= 0.1 Torr.
addition of excess He thermalizes the velocity distribution of the H atoms.14 Thus, when most of the hydrogen atoms have kinetic energy less than the 8-10 kcal/mol barrier for reactions 6 and 7, the H/D ratio becomes frozen. On the other hand, the HD molecules must react with the hydrogen atoms about as fast as they decelerate them. If nonreactive thermalization were dominant, HD would have had the same effect as He. Collisions with the heavy rare gas atoms would have relatively little effect on the hydrogen atoms' speeds. HD collisions must therefore be the major cause of the decrease with time of the H/D ratio. A detailed explanation of the variation of the H/D ratio with time is difficult because the following processes must be included in the analysis: reactive collisions of residual Xe*(6~[~/,] I ) with HD, radiative decay of X e * ( 6 ~ [ ~ / ~radiative ] ~ ) , and nonradiative decay of XeD, and reactions of H and D atoms with HD with rate coefficients which depend on the previous history of the H and D atoms. We think we have shown qualitatively that HD collisions are the major cause of the decrease of the H/D ratio with time. Kr*(SP['/&) HD. The unusual results obtained for the Xe* HD reaction prompted a similar study of the Kr* HD reaction. In general, signals were poorer than with xenon. Figure 8 shows the H/D ratio as a function of time. Again there is a high initial ratio, 3.5. Also, as shown in Figure 9, after 600 ns, the average H atom kinetic energy is only 6.7 f 1.1 kcal/mol. Kr* has an internal energy of 269 kcal/mol. If we subtract the HD bond energy of 103.3 kcal/mol, there is 166 kcal/mol of available energy. It is difficult to imagine that the relatively slow H atoms at 600 11s would at 100 ns have had a kinetic energy this large. We conclude as with XeD that KrD must have been formed in an excited electronic state and indeed with even more internal energy than with XeD. Ab initio calculations on the A%+ states of KrH and XeH have not yet been carried out and would be very desirable. The calculated predissociation rates of the A states of HeH, HeD, ArH, and ArD are so high that observation of A X fluorescence is prevented.I5J6 On the other hand, NeH and particularly NeD have relatively slow rates of predissociation. The calculated radiative lifetimes of the A states of HeH, NeH, and ArH are 44, 32, and 135 ns, re~pectively.'~.~~ Thus, lifetimes of the order of 500 ns for KrD and XeD are not out of the question.
+
+
+
-
Conclusion Electronically excited g-state Xe and Kr atoms have been prepared by two-photon transitions. They react with HD to produce a transient excess of H atoms over D atoms. This is interpreted in terms of the selective formation of relatively
J. Phys. Chem. 1992, 96, 7013-7018
7013
long-lived electronically excited rare gas deuterides, XeD(A2ZIl2+) and KrD(A2ZIl2+).
Acknowledgment. This work was supported by the US. Department of Energy and the donors of the Petroleum Research Fund, administered by the American Chemical Society, to R.B. and the Monbusho International Scientific Program and a Grant-in-Aid from the Ministry of Education, Science and Culture of Japan to M.K.
Appendix The two excited states in question with electron configurations 5ps6p and 5ps6s are labeled p and s, respectively, and have radiative decay rates kRpand k b and quenching rates k e and k,, respectively. An atom in the upper state p radiates to state s and thence to the ground state. If the populations of the two states are also labeled p and s, the rates of radiation from the two states are kRpP and kR$, respectively, the rate equations for the populations can be written as dp/dt = -(kQp(H2) + kRpb h / d t = -(kQs(H2)
+ kRsb + kRp
(AI) (A2)
The total fluorescent radiation I p from the p state is given by the integral kRpJomp(t) dt. If we divide the intensity in the absence of H2 by that in its presence, we obtain the equation Iop/Ip
= 1 + (kQp/kRp)(HS)
(A31
with a similar expression for the intensity ratios of the radiation emitted by the atom in state s:
The coefficients of (H2) in eqs A3 and A4 are nearly equal, and therefore, we have the inequality k ~ d k
