This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/65/4/10.1063/1.433219

Hierl, Peter M.

Strattan, L. W.

Wyatt, J. R.

The Journal of Chemical Physics

English

KU ScholarWorks

Letters to the Editor 1593 Ser. B 268, 41 (1969). 3R. J. Kriegler and H. L. Welsh, Can. J. Phys. 46, 1181 (1968). 4J . De Remigis and H. L. Welsh, Can. J. Phys. 48, 1622 (1970). 5S. G. Thomas, Jr., and W. A. Guillory, J. Phys. Chern. 77, 2469 (1973). 6Note that 1% was the highest concentration used in Ref. 3 and 4. Those authors also did not report any change in structure with concentration. 7AIl gas phase data taken from B. P. Stoicheff, Can. J. Chern. 35, 730 (1957). 3D. W. Taylor and J. De Rernigis, Can. J. Phys. 51, 1075 (1973). 9A. J. Barnes, in Vibrational Spectroscopy of Trapped Species, edited by H. E. Hallam (Wiley, New York, 1973), p. 165. Dynamics of the ion-molecule reaction Kr+(H2,H)KrH+ J. R. Wyatt, * L. W. Strattan,t and P. M. Hierl Department of Chemistry, University of Kansas, Lawrence, Kansas 66045 (Received 4 March 1976) The ion-molecule reaction (1) has recently aroused considerable interest for several reasons. Although the reaction is exoergic by 0.3 eV (see below), an energy barrier of 0.44 to 1. 2 eV was predicted by Kuntz and Roach, 1 who employed a semi-empirical diatomics-in-molecules technique to calculate a potential energy surface for the ground state of linear KrH;. This prediction, at odds with the long estab-lished maxim that exoergic reactions of positive ions have no activation energy, 2 was seemingly supported by an analysis3 of the differential cross section for scat-tering of H ~ on Kr. However, the thermal energy rate coefficient for the reaction of Kr+eP 3/Z) with Hz has re-cently been measured by a variety of techniques. 4-6 The values obtained, although nearly an order of magnitude smaller than the thermal energy rate coefficient for the analogous reaction of Ar., have indicated that the activa-tion energy for Reaction (1) can be no larger than 0.05 eV. Ion beam studies 7 of the reaction Kr+(Dz, D) KrD+ gave the unexpected result that the product ion is isotropical-ly distributed about the center of mass (c. m. ) at 0.87 eV initial relative energy, although at higher energy (2.7 eV) the product distribution does more nearly re-semble what one would expect on the basis of the now familiar spectator stripping model. 8 A third unusual feature of this reaction is the ob-servation6• 9 that the zp 1/2 excited state of Kr+ reacts with Hz only half as rapidly as does the 2P3/2 ground state. This fact has been attributed6 to an avoided crOSSing which causes the 2PS/Z state to correlate with ground state products and the 2P1/2 state to correlate with excited state products. We report here the results of a study in which veloc-ity vector distributions and integral cross sectionS for Reaction (1) were measured over the range 0.036-3.0 eV initial relative energy. The beam instrument used in this study is a Single beam-collision chamber type of chemical accelerator with product velocity and angular analysis, 10.11 Because the reactant ions are produced by impact of 35 eV electrons, nearly all (98.5%) of the Kr+ ions are in the 2p state. 12 These ions are presumably distrib-uted statistically in a 2 : 1 ratio between the J = 3/2 and the J = 1/2 levels, 13 which differ in energy by 0.66 eV.14 The value of O. 3 e V for the exoergicity of Reaction (1) is calculated from the dissociation energy Do(Kr-H +) = 4.38 eV, derived from a flowing afterglow study of the equilibrium between KrH + and H; in Kr-H2 mix-tures. 15 This bond energy is in good agreement with values calculated from data on the elastic scattering of protons on krypton. 16.17 Product velocity vector distributions18 for Reaction (1) show that the KrH + ions are scattered in a nearly isotropic manner about the c. m. at relative collision energies less than about 1 eV. Such symmetry about the c. m. would necessarily be produced if the reaction proceeded via the formation of a collision complex whose lifetime exceeded several rotational periods; there is, however, no reason to suppose that the poten-tial energy hypersurface for Reaction (1) contains a basin of the depth necessary for the formation of such a complex. The most plausible explanation, therefore, for the observed isotropic product velocity vector dis-tributions is that, at these low energies, Reaction (1) occurs directly in rather intimate (1. e., small impact parameter) collisions which lead to hard-spherelike scattering of the ionic product and concomitant mo-mentum transfer to the freed atom. With increaSing collision energy, however, the peak in the KrH+ velocity vector distribution shifts to the forward direction, approaching the velocity expected on the basis of the spectator stripping model8 at collision energies of 2-3 eV (c. m.). These results, therefore, essentially confirm those previously reported by Hen-glein and co-workers 7 for the reaction Kr+(D2' D) KrD+. Integral reaction cross sections a R are calculated from the thin-target apprOXimation Ic(OO)Kc/KA a - -><";"'-7:--"-'-~ R- IA(OO)nBL (2) where Ic(OO) and IA(OO) are, respectively, the product and reactant ion intensities measured at the angle of The Journal of Chemical Physics, Vol. 65, No.4, 15 August 1976 Copyright © 1976 American Institute of Physics  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:129.237.46.100 On: Mon, 15 Sep 2014 13:21:591594 Letters to the Editor TABLE I. Integral cross sections for the reaction Kr+ (H2' H) KrIr. Most probable Collection Integral relative energy efficiency cross section E(eV) Kc/KA R(A2) 0.036 1. 32 9.77 0.056 2.29 10.5 0.058 2.16 9.62 0.113 1. 62 7.65 0.141 1. 42 6.36 0.174 1. 70 6.44 0.233 1. 51 5.27 0.280 1.17 3.23 0.282 1.47 4.07 0.316 1. 39 4.21 0.387 1. 41 3.81 0.458 1. 51 3.23 0.482 1. 21 3.33 0.581 1.01 1. 96 0.699 1.34 1. 82 0.940 1.27 0.73 1.17 1.15 0.98 1.19 1. 47 0.47 1.42 1. 21 0.50 2.13 1.10 0.59 2.37 1.20 0.56 2.96 1.16 0.49 The Kr+ reactant, produced by impact of 35 eV elec-trons, is presumably a 2: 1 mixture of Kr+(2P3/2) and Kr+ (2 P1/ 2)' maximum intensity, nB is the number density of H2 molecules in the collision chamber, and L is the col-lision path length. The factor Kc/KA' which accounts for differences in collection efficiency caused by dif-ferences in angular distributions, is obtained directly by appropriate integration of the measured angular distributions at each collision energy. 11 The values obtained and the integral cross sections for Reaction (1) are listed in Table I. The experimental uncertainty in aR is estimated to be ±50%. At the lowest collision energies, the cross sections reported here for Reaction (1) are only about 1/5 the size of those for the corresponding reaction of Ar+, in agreement with the thermal energy rate coefficients measured for Reaction (1).4-6 At higher collision en-ergies, the ratio is more nearly 1/10, as first re-ported by Henglein and co-workers. 7 Over the energy range 0.05-1. 0 eV (c. m.), the absolute values mea-sured in the present study are consistently smaller (by about 3cfYo) than the values determined by Henchman and co-workers, 19 although both sets of data indicate that the cross section varies with collision energy ap-proximately as E-1/2. A similar energy dependence. was observed in an ion cyclotron resonance study of this reaction from thermal energies up to 0.25 eV (c.m.).6 The magnitude of the cross sections reported here and the monotonic increase in a R with decreasing col-lision energy essentially confirm the conclusion reached in the earlier, thermal energy studies of this reaction; namely, there is no detectable energy barrier to Reaction (1). Additionally, the observation that the integral cross section for Reaction (1) is much smaller (approximately 1/10) than that predicted by the Langevin ion-induced dipole cross section is consistent with the conclusion, inferred from the isotropy of the product velocity vector distributions at low collision energies, that Reaction (1) only occurs in small impact parame-ter collisions. This research was supported by the Donors of the Petroleum Research Fund, administered by the Ameri-can Chemical SOCiety, the Research Corporation, and the University of Kansas General Research Fund. *Present address: Naval Research Laboratory, Washington, D. C. 20375. tPresent address: Department of Chemistry, Emory Univer-sity, Atlanta, GA. 30322. Ip. J. Kuntz and A. C. Roach, Trans. Faraday Soc. 11, 259 (1972). 2F • W. Lampe, J. L. Franklin, and F. H. Field, Prog. React. Kinet. 1, 84 (1961). 3A. Henglein, J. Phys. Chern. 76, 3883 (1972). 4K. R. Ryan and 1. G. Graham, J. Chern. Phys. 59, 4260 (1973). 5p. F. Fennelly, J. D. Payzant, R. S. Hemsworth, and D. K. Bohme, J. Chern. Phys. 60, 5115 (1974). GR. D. Smith, D. L. Smith, and J. H. Futrell (submitted to Chern. Phys. Lett.). , 7G. Bosse, A. Ding, and A. Henglein, 7:. Naturforsch. Teil A 26, 932 (1971). BA . Henglein, K. Lac mann, and G. Jacobs, Ber. Bunsenges. Phys. Chern. 69, 286, 292 (1965). 9W• A. Chupka, Interactions 13etween Ions and Molecules, edited by P. Ausloos, (Plenum, New York, 1975), p. 259. lOp. M. Hie rl , L. W. Strattan, and J. R. Wyatt, Int. J. Mass Spectrom. Ion Phys. 10, 385 (1973). l1J. R. Wyatt, L. W. Strattan, S. Chivalak, and P. M. Hierl, J. Chern. Phys. 63, 4582 (1975). 12H. D. Hagstrum, Phys. Rev. 104, 309 (1956). 13For a more detailed discussion of this problem, see A. J. Masson, Ph. D. Thesis, Brandeis University, 1970. 14C. B. Moore, Nat!. Bur. Stand. U. S. Circ. No. 467, Vol. II (1952). 15J. D. Payzant, H. I. Schiff, and D. K. Bohme, J. Chern. Phys. 63, 149 (1975). ISH. P. Weise, H. U. Mittman, A. Ding, and A. Henglein, Z. Naturforsch. Tei! A 26, 1122 (1971). 17W• G. Rich, S. M. Bobbio, R. L. Champion, and L. D. Doverspike, Phys. Rev. A 4, 2253 (1971). 1BL • W. Strattan, Ph. D. TheSiS, University of Kansas, 1973. 19p • Fennelly, Ph. D. Thesis, Brandeis University, 1972; P. Fennelly, M. J. Henchman, and A. S. Werner (unpublished results) . J. Chern. Phys., Vol. 65, No.4, 15 August 1976  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:129.237.46.100 On: Mon, 15 Sep 2014 13:21:59

Dynamics of the ion–molecule reaction Kr+(H2,H)KrH+

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Dynamics of the ion–molecule reaction Kr+(H2,H)KrH+

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