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Electron Attachment to Hydrogen Halide Clusters E Subscriber access provided by Johns Hopkins Libraries Electron attachment to hydrogen halide clusters E. L. Quitevis, K. H. Bowen, G. W. Liesegang, and D. R. Herschbach J. Phys. Chem., 1983, 87 (12), 2076-2079• DOI: 10.1021/j100235a011 • Publication Date (Web): 01 May 2002 Downloaded from http://pubs.acs.org on May 5, 2009 More About This Article The permalink http://dx.doi.org/10.1021/j100235a011 provides access to: • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article The Journal of Physical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 2076 J. Phys. Chem. 1983, 87, 2076-2079 Electron Attachment to Hydrogen Halide Clusters E. L. Oultevls,+ K. H. Bowen,$ 0. W. Liesegang,' and D. R. Herschbach' mpaftment of Chemlstty, Harvard Unlverslty, Cambrkige, Massachusetts 02 138 (Recelved: October 25, 1982; In Fhal Form: February 15, 1983) Negative ion production by endoergic charge transfer from fast alkali atoms to a crossed molecular beam of hydrogen halide clusters, (HX),, has been studied for HF, HC1, HBr, HI, and mixed HF, HBr clusters. Cluster ions corresponding to n up to the order of 10 are readily detected, although the unit mass resolution required for full identification is only feasible for small clusters (masses 1100 amu). Dissociative attachment to form the "solvated" halide or bihalide anions, [X(HX)],-,-, is the predominant process. No evidence is found for any intact (HX),,- ions. Introduction observable products. "Solvated" halide ions, Cl-(HCl), Often a single molecule cannot bind an excess electron, with n = 1-4, have been observed in the multiple collision whereas a weakly bound cluster of the same molecules evironment of drift tubes."J2 Yet results of swarm-beam might be able to do so. Prototypical cases are the gas- experiments have been interpreted as excluding the pos- phase water' and carbon dioxide2 clusters, (H20),- and sibility that XHX- or X2- ions originate from (HX), (C02);. The hydrogen halides may also exemplify such spe~ies.~This view contradicts the mechanism postulated systems. All attempts to attach electrons to HX monomers for radiolysis experiments,69' in which (HCl), subsequently have resulted in dissociative attachment to form the halide reacts to form the Cl-(HCl), species.13J4 ion, X-, with a cross section that decreases with increasing This paper reports attempts to observe intact negative electronegativity of the halogen atom.3 Electronic cluster ions (HX),- with X = F, C1, Br, and I. Our method structure calculation^^?^ suggest that HX- (,E+) is nomi- employs endoergic electron transfer from a fast alkali atom nally stable in some cases, but if so its autodetachment beam to a target beam of hydrogen halide clusters. The lifetime appears to be extremely short. Experimental target beam contains a range of clusters, with n = 2-10 or evidence for relatively stable, negative dimer ions, (HX),-, more. This is an advantage since dissociative attachment has been inferred from radiation chemistry. In discussing to the larger clusters offers another route to small (HX),- mechanisms for thermal electron capture by gaseous HCl cluster ions (m< n),whether or not direct formation of + HBr, Armstrong and Nagra6 suggest that a previously such ions occurs. proposed (HX)f* intermediate3*7+sis produced chiefly by electron attachment to (HX), dimer molecules: Experimental Procedure e- + (HX), G (HX)g* (L-1) The apparatus and techniques have been described in (HX),-* H XHX- detail else~here.'~Fast rubidium atoms are generated by - + (2) seeded supersonic expansion from a nozzle oven, using He (HX),-* + M - (HX),- + M (3) or H2 as the diluent gas. The neutral molecular cluster In this mechanism, reaction 2 is the major exit channel for beam is likewise produced by a supersonic expansion. HBr but reactions -1 and 3 are dominant for HC1. The Typical source conditions were as follows: stagnation generation of negative dimer ions from the ambient neutral pressure of hydrogen halide -700 torr, nozzle diameter dimers and the subsequent collisional deactivation of these -0.008 cm, nozzle-to-skimmer distance -0.5 cm. The ions distinguish this mechanism from earlier proposals. various negative ions formed in single collisions at the Electronic structure calculations by Jordan and Wendo- intersection of the beams are detected and mass analyzed loski indicate that (HF),- is table.^ However, they caution that their calculations were restricted to linear configu- (1) Armbruster, M.; Haberlund, H.; Schindler, H. G. Phys. Rev. Lett. rations of both the neutral dimer (known experimentally 1981, 47, 323. to have a zig-zag geometry'O) and the dimer negative ion, (2) Klota, C. E.; Compton, R. N. J. Chem. Phys. 1978,69,1636,1644. and their method probably overestimated the electron (3) Christophorou, L. G.; Compton, R. N.; Dickson, H. W. J. Chem. affinity of (HF),. Phys. 1968,48,1949. Christophorou, L. G.;Stockdale, J. A. D. Ibid. 1968, 48, 1956. This tentative evidence is consistent with chemical in- (4) Crawford, 0. H.; Koch, B. J. D. J. Chem. Phys. 1974, 60,4512. tuition, which suggests that hydrogen halides involving the (5) Griffing, K. M.; Kenney, J.; Simons, J.; Jordan, K. D. J. Chem. more electronegative halogens will be more likely to form Phys. 1975, 63, 4073. (6) Armstrong, D. A,; Nagra, S. S. J. Phys. Chem. 1975, 79, 2875. stable negative dimer ions. It appears that both (HF),- Nagra, S. S.; Armstrong, D. A. Can. J. Chem. 1976,54, 3580. and (HC1)f may be stable while (HBr),- and presumably (7) Wilson, D. E.; Armstrong, D. A. Radiat. Res. Reu. 1970, 2, 297. are not. In all cases, both dissociation of dimer ions (8) Johnson, G. R. A.; Redpath, J. L. Trans. Faraday SOC.1970, 66, 861. and concurrent reaction processes are expected (when (9) Jordan, K. D.; Wendoloski, J. J. Chem. Phys. 1977, 21, 145. thermochemically allowed) to yield halide ions, X- or Xz-, (10) Dyke, T. R.; Howard, B. J.; Klemperer, W. J. Chem. Phys. 1972, and especially the quite stable bihalide ion (XHX)- as 56, 2442. (11) Corbin, R. J.; Nygaard, K. J.; Snow, W. R.; Schemer, L. D. J. Chem. Phys. 1978,68, 4373. Deoartment of Chemistry.-. University of Toronto. Toronto, (12) Yamdagni, R.; Kebarle, P. Can. J. Chem. 1974,52, 2449. Canada M5S 1Al. (13) Armstrong, D. A,; Back, R. A. Can. J. Chem. 1967, 45, 3079. * Department of Chemistry, The Johns Hopkins University, Bal- (14) Wilson, D. E.; Armstrong, D. A. Can. J. Chem. 1970, 48, 598. timore, MD 21218. (15) Bowen, K. H.; Liesegang, G. W.; Sanders, B. S.; Herschbach, D. .L National Institutes of Health, Bethesda, MD 20205. R. J. Phys. Chem. 1983,87, 557. 0022-3654/83/2087-2076$01.50lO 0 1983 American Chemical Society Electron Attachment to Hydrogen Halide Clusters The Journal of Physical Chemistry, Vol. 87, No. 12, 1983 2077 2 TABLE I: Estimates of Threshold Energies (eV) Rb tiHF1n B process HF HCl HBr HI ref 101 Rb + Rb+ + e- 4.18 4.18 4.18 4.18 16 HX+HtX 5.87 4.33 3.76 3.30 17 X- --f X e- 3.40 3.61 3.36 3.06 + 18 L1llill1I (XHX)- + X- + HX 1.31 1.03 0.78 <0.5 19 24 68 (HX), --f 2HX 0.26 0.10 <0.1 <0.1 20 POLYMER NUMBER n Rb + fHX): +. Rb' + (HX),' -4.0 -4.1 >4.2 >4.2 6, 9 Rb+ + H + (XHX)- 5.6 4.0 <3.9 <4.0 Rb+ + H + HX + X- 6.9 5.0 <4.7 <4.5 max Erel(2) 3.1 4.2 6.3 7.2 by a quadrupole mass filter. Ion flight times are typically I"'"~'"'I'~''1'"'1"'I"''l"''l'"'l""I'~''l'"'i'~ s. Signal pulses from the electron multiplier are 20 40 60 80 100 I20 capacitively coupled to pulse-counting electronics and MASS NUMBER signal averaging is employed necessary. reliable as A and Figure 1. (a) Low- and (b) hlgh-resolutlon negatlve Ion mass spectra accurate internal mass calibration is obtained from high- of products observed in the reactlon between fast Rb atoms and (HF), resolution mass spectra of (SO2),- cluster ions in con- clusters. In (a), the letter "B" labels surfacsbom background peaks. junction with a digital sweep generator. Unit mass reso- In (b), unlabeled features are due both to lmpurlties (probably SO2 from lution is essential because of the possibility of confusing calibratbn run) in the beam gas and to surfaceborn background. The the species [X(HX),,]- and (HX),-. In these experiments collision energies correspond to E,, = 8.1 eV. this was achieved for the HF and HC1 systems although (FHF)- from n 1 7, and F- from n L 11. For the other the largest cluster ion detectable with unit resolution de- hydrogen halide systems such kinematic constraints do not pends on the available ion intensities and feasible aver- appear except for production of C1-, which requires n 1 aging times (-1 h). The only positive ion species observed 3. with appreciable intensity was Rb+, as in our previous Hydrogen Fluoride. Figure 1 presents typical mass work.15 spectra of the negative ions formed from Rb + (HF),.This The relative translational energy in collisions between system yielded the most definitive results, by virture of Rb and an (HX), cluster is given to a good approximation high intensities and lack of isotopic distributions. The only by observed mass resolved ions from crossed beam collisions Erel(n) [nmB/(mA + nmB)@hb are the [F(HF),-,]- species, with m = 1-6. As noted, kinematic Constraints require m < n. Unit mass resolution Here mA and mB denote the masses of Rb and HX, re- was maintained up through mass 120, but observed peaks spectively.
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