T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY 0 Copyright, 1980, by the American Chemical Society
Registered i n U.S. Patent Office
VOLUME 84, NUMBER 20
OCTOBER 2,1980
LETTERS Evidence for the Rare Gas-Rare Gas Halide Displacement Reaction H. C. Brashears, D. W. Setser,” and Y. C. Yu Chemlstty Department, Kansas State Unlversky, Manhattan, Kansas 66506 (Received: June 17, 1980)
The formation of KrF(B) and KrCl(B) has been observed from the reaction of Xe(3P1)with Fz or Clz in a Kr bath gas. Also ArF(B) is observed from the reaction of Kr(3P1)with F2in an Ar bath gas. The displacement of the heavy rare gas (Rg’) from vibrationally excited Rg’X* by the lighter rare gas (Rg) to give RgX(B) t.Rg’ is suggested to explain the observations. The exoergic displacement reaction of a lighter (Rg) by a heavier (Rg’) rare gas atom from a rare gas halide molecule has been included in the kinetic modeling of rare gas halide lasers (reaction 1).l However, there is little Rg’
+ Rg+X-*
-
Rg’+X-* + Rg
(1)
AHo % -(IP(Rg) - IP(Rg’))
direct evidence for these reactions and experimental identification of the primary product state distribution from (1) remains a ~hallenge.~”Such experiments are difficult because Rg+X-* molecules normally are generated by the rapid reaction of some excited state of Rg*, e.g. reaction 2. Since excitation transfer from the excited
Rg(3P2) + X2
-
Rg+X-* + X
(2) lighter rare gas atom to the heavier atom also is fast, a mixture of Rg* and Rg’* will be present in such experiments and each can give Rg+X-* and Rg’+X-* by (2). Additional possible complications are collisional transfer between RgX(B) and RgX(C) and two- and three-body quenching by Rg’. Thus, it is not so surprising that isolation and direct observation of (1) has been difficult. We report evidence for the reverse of reaction 1, the endoergic displacement reaction by a lighter rare gas atom.
We have observed the formation of KrF(B), KrCl(B), and ArF(B) under experimental conditions that suggest the mechanism to be displacement from highly vibrationally excited XeF(B,C) and XeCl(B,C) by Kr and KrF(B,C) by Ar. Vibrational excitation appears to be unusually effective in driving the endoergic reaction. The observations were made during studies of the relaxation of the Kr and Xe halides by lighter rare gas atoms? The experiment involves the steady-state absorption of Kr or Xe resonance radiation (generated by microwave powered rare gas lamp) by a mixture of -0.05 torr of Rg’ (Xe or Kr), 0.05 torr of C12 or F2, plus added Rg (Kr or Ar, respectively). The Rg pressure was less than 200 torr, and three-body formation of KrXe* or ArKr* is not serious. The XeF*, XeCl*, and KrF* are formed by (2) with either the 3P1or 3Pzstate atoms. Both states give the same rare gas halide product di~tribution,~ which has been characterized for C12and F2 with Xe(3Pz)5and for F2with Kr(3Pz).s In all cases the B/C ratio is -1.5 and the vibrational distributions are high with ( f v ) = 0.70. Upon addition of He or Ne to the sensitized systems, the expected C-B transfer and vibrational relaxation of Rg’X* is ~ b s e r v e d .However, ~ the addition of Kr to the XeC1* or XeF* systems gave KrCl(B) and KrF(B), respectively, and addition of Ar to the KrF* system gave
0022-3654/80/2084-2495$01 .OO/O 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 20, 7980
XeFiB1 K r = 4 0 Torr
Kr=lO Torr
11
Letters
1
A
/'I
B
i lxec"B'
"1/5
KrCI'
I
Kr=l Torr
NO K r ADDED
A
\
I
/
\ I
220nm
260
y-:
I
'300
'340
I
D
Figure 1. Emission spectra from sensitization of Xe/F, mixtures (0.05 torr of each) in various pressures of Kr. Note the growth of KrF(B) and the vibrational relaxation of XeF(B) with increasing Kr pressure. F
ArF(B). Displacement studies of other KrX* by Ar requires observation below 195 nm and have not yet been done. The formation of KrF(B) from sensitized Xe/F,/Kr mixtures is shown in Figure 1. The KrF(B) emission becomes evident at 1torr Kr, and the intensity increases linearly with increasing Kr pressure to -50 torr and then becomes constant with a KrF*/XeF* ratio of -0.35. The formation of KrCl(B) from sensitization of a Xe/Clz/Kr mixture and ArF(B) from sensitization of a Kr/Fz/Ar mixture is shown in Figure 2. An important point is that in each case the RgX(B-X) emission is characteristic of a vibrationally relaxed RgX(B) distribution. The RgX(C-A) emission band is very broad and identification of the KrF(C-A) or KrCl(C-A) band in the presence of the strong XeF* or XeC1* emission is not possible. As is evident from Figures 1 and 2, KrF(B) formation from XeF* is the most easily observed of the three cases. In fact, a small amount of KrF(B) was even observed when XeFz is photolyzed at 147 nm in the presence of Kr buffer because a small fraction of the photoproduct XeF(B) molecules are formed with vibrational energy above the threshold for the reverse of (1). Before proceeding further, it is necessary to prove that the lighter rare gas halide is not being formed via reaction 2 because of impurity radiation from the microwave resonance lamp. For the Xe sensitization (147.0 nm) experiments, a sapphire window was used which should be opaque to the Kr resonance line (123.6 nm). For the Kr sensitization experiments a MgFz window was used which should be opaque to the Ar resonance line (106.6 nm). Direct observation of the output of the lamps with these windows with a vacuum monochromator showed no evidence for the shorter wavelength emission. Sensitization (147 nm) experiments of F2/Kr and ClZ/Kr mixtures without added Xe gave no KrF* or KCl* emission. Similarily sensitization (123.6 nm) of Ar/F, mixtures gave no ArF* emission. The most compelling argument against (2) as the source of RgX* emission is that, if a halogen donor is selected that gives a vibrational distribution lying below the threshold for the reverse of (l),formation of Rg'X* is observed but RgX* is not generated upon addition of Rg. Such an example is shown by the sensitization of a
-
+--+,*+?+? 270
230nm
i&+(4wF 310
'
J'
't, 350
Flgure 2. Emission spectra from (A) sensitization of a KrlF, mixture with 15 torr of Ar; the apparent structure on the ArF" band is a consequence of absorption by atmospheric 02,(B) sensitization of a Xe/CI, mixture with 14 torr of added Kr, and (C and D) comparison of the XeF(B) emission from sensitization of a Xe/NF, mixture (0.03 torr Xe, 0.2 torr of NF3)with (40 torr) and without added Kr. Note that KrF(B) is not f o m d by senskization of Xe/NF, mixtures in contrast to the results shown in Figure 1 . Note also that the XeF(B-X) emission from Xe(3P,) -t NF, does not extend to the threshold for KrF(B) formation. The apparent band at 307 nm is scattered light from the resonance lamp.
Xe/NF3 mixture with 40 torr of added Kr in Figure 2. The XeF(B) vibrational distribution6from Xe* + NF3 is below the threshold for KrF(B) formation as can be qualitatively deduced from the fact that little of the KrF(B-X) emission is at wavelengths below the KrF* emission band. Also, if the source of the lighter rare gas halide emission was sensitization via (2), the KrF(B) and KrCl(B) emissions in Figures 1and 2 should show high vibrational excitation and this is not the case. We, thus, conclude that the reverse of (1)is responsible for formation of the lighter rare gas halide emission in Figures 1 and 2. Since the displacement product is observed for pressures as low as 1torr of Kr with XeF* and XeCl*, any contribution from reaction of XeKr* (formed by Xe* + 2Kr) with Fz or C12 can be neglected. The reaction of electronically excited halogens with rare gases also gives RgX*; however, the 147-nm sensitization experiments of Kr/F2 and Kr/C12 mixtures excludes this process from the present experiments. This conclusion is consistent with the finding from synchrotron experiments' that KrCl* formation from Kr/C12 mixtures requires X 5 143 nm. Potential curves for ArzF and KrzF are well established by t h e ~ r y . ~The , ~ three lowest potentials are covalent and repulsive. The next three are ionic and attractive relative to Rg,+ and F-. However, only the lowest Coulombic curve is bound relative to RgX* + Rg. The structure of RgzX* is expected to be an isosceles triangle. The situation for mixed rare gas halide trimers, (Rg'Rg)+X-*,will be similar except that the binding is considerably smaller and the isosceles triangle will be distorted. In fact we have observed'O emission from KrXeCl*, KrXeBr*, KrXeI*, ArKrF*, and ArKrCl*, which are formed a t high pressures
The Journal of Pbysical Cbemisfry, Vol. 84, No. 20, 1980 2497
Letters
pulsive (RgRg')+X- surface. After Rg approaches Rg'+ the transfer of F-to Rg+ from Rg'+ requires energy. Possibly vibrational excitation allows the smaller Rg atom to insert between Rg'+ and F- during the outward extension of the vibration, as in (3). After the insertion Rg' separates from Rg
I Rq'+-F-
Flgure 3. Diagramatic representation of potentials and energies for the displacement reactions of Kr with XeF and XeCI. The dotted lines denote the KrXeX potentials. The repulsive potential connecting Kr XeF(X) and the bound (KrXe)-F+ potential is shown because three-body KrXleF" formation is not observed." The XeF' and XeCl F2 and C12. distributions are drawn to correspond5to that from Xe' Only a Small fraction of the XeCI" distribution lies above the KrCI* formation threshold, whereas a large fraction of the XeF" distribution is above the KrF' threshold. The Xe-Kr-X configuration is not expected to be collinear, see text.
+
(2500 torr) from the combination reaction (2Rg + Rg'X*). Failure to observe Xe2F* and KrXeF* emission, despite prolonged experimental searches, is consistent with crossing of the bound ionic curve by one of the repulsive covalent curves. In addition to the bound (Rg'RgPXstates, two dissociative Coulombic states (relative to Rg'+X-* Rg) are expected. Can the formation of RgX(B) from vibrationally excited Rg'X* be rationalized via the reverse of (l)?Figure 3 shows that the endoergic process can occur only if a significant part of the Rg'X* vibrational distribution from (2) lies above the RgX* + Rg' exit channel. This absolute energy requirement and the much larger AHo for the Xe* + F2reaction, relative to Xe* C12,explains why the Kr displacement reaction is so much more prominent for Kr + XeF* than for Kr + XeCl*. For thermal collisions, displacement, of Rg by Rg' in reaction 1could be expected via either bound or repulsive (RgRg')+X-* potentials, providing the dynamical restrictions associated with interchanging Rg and Rg' is easily overcome. However, for the reverse (endoergic) direction the initial conditions may be much more critical for a successful encounter. The approach of Rg to the X end of Rg+'X- is expected to be strongly repulsive and vibrational excitation may not drive the displacement reaction from this configuration. However, no barrier is expected for the approach of Rg to the Rg' end of Rg'+X- (see Figure 7 of ref 8). Collisions of vibrationally excited Rg'X* and Rg on the bound (RgRgYX- potential should exhibit complex trajectories with escape from the bound region strongly favoring (although perhaps not to the full statistical extent) the low energy pathway.ll On the other hand, vibrational excitation may be efficient for driving the reaction on the re-
+
+
-
Rg
3-
Rg'+-F
Rg'Rg+-F-
-
Rg' t RgfF-
(3)
R+X-. According to the calculations,s~sthe bound Rg2+Xtrimer potential correlates to RgX(C) + Rg. One RgZ+Xrepulsive curve correlates to RgX(B) + Rg and a second one to RgX(D) + Rg. However, the correlation can be altered by the interaction strength of the long-range crossing between the two Rg2+X-curves and the ordering of the RgX(B) and RgX(C) states. In addition to the implications of the correlations, the different lifetimes of the B and C states of Rg'X suggest that the B and C states may play different roles in the displacement reactions. However, at the present time distinction between the B and C states of Rg'X* is not possible for the displacement by Rg. The experimental observation of RgX(B) formation upon sensitization of Rg'/X2 mixtures in the presence of lighter Rg bath gas appears to demonstrate the utilization of vibrational excitation to drive the endoergic displacement reactions. The main evidence against displacement via the bound (Rg'Rg)+X- potential is as follows: (i) The Kr XeF*(high u) displacement reaction is easily observed even though two-body quenching is found for Kr + XeF* (low u ) rather than three-body KrXeF* formation.l') (ii) Displacement on the bound potential should favor Rg'X* + Rg rather than RgX* + Rg'. Therefore, we suggest that the displacement of Rg' from vibrationally excited RgX* occurs via one of the repulsive (RgRg')+X- potential surfaces. The displacement reactions of alkali metal atoms (M) with alkali metal halides (M'X), which have been extensively studied by molecular beam technique, provide support for the postulated
+
Acknowledgment. This work was supported by the U.S. Department of Energy, DE-AC02-80ET33068.
References and Notes (1) Ch. A. Brau, "Toplcs in Applied Physics", Vol. 30, Ch. K. Rhodes, Ed., Sprlnger-Verlag, NY, 1979. (2) J. G. Eden, J . Appl. Pbys., 49, 5368 (1978). (3) H. C. Brashears and D. W. Setser, unpublished work involvlng photolysis of KrF, in Xe buffer gas. (4) H. C. Brashears, Jr., and D.W. Setser, J. Phys. Cbem., 84, 224 (1980). (5) J. H. Kolts, J. E. Velazco, and D. W. Setser, J. Cbem. Pbys., 71, 1247, 1264 (1979). (6) K. Tamagake and D. W. Setser, J. Cbem. Pbys., 67,4370 (1977). (7) M. C. Castex, J. Lecalve, D. Haaks, B. Jordon, and 0. Zimmerer, Chem. Pbys. Lett., 70, 106 (1980). (8) M. R. Wadt and P. J. Hay, J. Cbem. Pbys., 68, 3850 (1978). (9) D.L. Huestis and H. E. Schlotter, J. Cbem. Pbys., 69,3100 (1978). (10) H. C. Brashears, Jr., D.W. Setser, and Y. C. Yu. J. Chem. Phys., submitted for publication. (1 1) G. H. Kwei, B. P. Boffardi, and S. F. Sun, J. Cbem. Pbys., 56, 1723 (1973). (12) G. Aniansson, R. P. Creaser, W. D. Held, and C. Holmid, J . Cbem. Pbys., 61, 5361 (1974). (13) S. Solte, A. E. Proctor, and R. B. Bernstein, J . Chem. Pbys., 65, 4990 (1976); 62, 2506 (1975). (14) W. B. Miller, S. A. Safron, and D. R. Herschbach, Faraday Discuss. Chem. SOC.,44, 108 (1967).
