Dissociative photoionisation of NO2 up to 26 eV

Chemical Physics 237 Ž1998. 139–148

Dissociative photoionisation of NO 2 up to 26 eV J.H.D. Eland a

a,)

, L. Karlsson

b

Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, UK b Uppsala UniÕersity, Department of Physics, Box 530, S-751 21 Uppsala, Sweden Received 4 May 1998

Abstract The dissociations of NOq 2 from the states populated in photoionisation at 30.4 and 58.4 nm have been examined by photoelectron–photoion coincidence spectroscopy. Ions in the a 3 B 2 state with Õ s 1,2,3 and in the a 1A 2 state with Õ s 0 are found to be metastable on a microsecond time scale; estimates of their lifetimes are given. Ions in the b 3A 2 state dissociate rapidly, with an anisotropic distribution of the product ions relative to the photoelectrons. The kinetic energy releases show little excitation of the diatomic products in dissociations of the low-lying NOq 2 states. Relevance of the results to atmospheric ion–molecule reactions is discussed. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction NOq 2

The dissociations of cations were first studied in detail by Chupka w1x, whose photoionisation yield measurements identified the four lowest vibrational levels of the a 3 B 2 state as metastable, with lifetimes of 5–155 ms. At the same time Chupka showed, from the widths of the metastable peaks in a sector mass spectrometer, that most of the available energy in the slow predissociations appears as translation. Despite the importance of NO 2 as an atmospheric molecule, including the role of NOq 2 as an intermediate in stratospheric ion–molecule reactions w2x, little further work on the ionic dissociations of this molecule was done until the threshold photoelectron–photoion coincidence ŽTPEPICO. study by Shibuya et al. w3x in 1997. The TPEPICO work confirmed the metastability of the a 3 B 2 vibrational levels, but gave much longer lifetimes. Shibuya et al. )

Corresponding author.

also found another metastable level, with very long lifetime at 17.34 eV, apparently in the c 3A 1 state, whose assignment is, however, not quite certain. Assignments depend on the photoelectron spectrum, which was studied sporadically in earlier decades w4–6x, and has recently been remeasured with much better resolution than before w7x. The new photoelectron spectrum w7x forms the backdrop to the present work, in which we have examined the dissociation of NOq 2 from all its accessible valence states by fixed wavelength photoelectron–photoion coincidence ŽPEPICO. spectroscopy. Because of the relatively low density of states, triatomic molecular ions are expected to show state-specific rather than statistical dissociation behaviour. Nevertheless, clear examples of vibrational or even electronic state specificity are relatively uncommon. We have found in this work that NOq 2 shows distinct electronic state specificity and provides an unusual example of vibrationally statespecific dissociation behaviour, in the formation of

0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 3 7 - 7

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J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

3 q Oq 2 q N from the B 2 state of NO 2 near 19 eV. Agreement with the previous results is not perfect, however, and reasons for the discrepancies, which may also be related to state-specific behaviour, are discussed in the text. For the ion-state nomenclature in this paper we follow the usage in the recent study of the photoelectron spectrum w7x. Individual letter names are not given to the higher states because calculations show that several related states in the same energy region are as yet undetected.

2. Experimental PEPICO spectra of NO 2 have been measured using an improved apparatus, described in detail elsewhere w8x. Briefly, ions are formed at the intersection of an effusive gas jet with a wavelengthselected light beam from an atomic discharge lamp. Electrons are collected under zero-field conditions by a hemispherical analyser of 125 mm mean radius fitted with a position-sensitive detector. When an electron is detected a pulsed extraction field is applied to the source, and ions are detected in a two-field Wiley–McLaren w9x time-of-flight mass spectrometer. This arrangement allows relatively high-mass resolution Ž MrD M f 200. while preserving an electron resolution dependent only on the analyser pass energy. For maximum sensitivity a pass energy of 30 eV is used giving 300 meV resolution; for best resolution a pass energy of 5 eV allows peak widths of 50 meV. A complication of the pulsing technique is that a delay is introduced, due to the electron flight time, between ion formation and extraction. The effects of this delay are accounted for in simulations of the experimental peak shapes. Kinetic energy release distributions ŽKERD. in ion dissociation are determined from the ion peak shapes either by forward simulation or by direct inversion using smoothing and iterative deconvolution. In both procedures, broadening due to the initial thermal velocities of the molecules is allowed for on the basis of the measured profile of the parent ion peak. In forward simulation, effects of anisotropic angular distributions can be included w10,11x, but in direct inversion we assume isotropic distribution of fragment ions.

Ž Parent NOq 2 ions which decay in flight metastable ions. are detected in three regions of the spectra. Those that dissociate during acceleration in the source region appear as a long-time tail on the daughter ion signal; those that decay in the field-free region appear around the parent ion Žboth before and after. because of the kinetic energy release in the decay. To estimate the lifetimes of metastable NOq 2 ions we have calculated fractional intensities in each area of the experimental coincidence mass spectrum for a range of pure monoexponential decays, using Monte Carlo simulations. Comparison with the experimental results is complicated by problems of photoelectron peak overlap, so the error limits on the final lifetimes are relatively wide. Multiexponentials may be present in reality, as discussed below, but cannot be distinguished in the present data. The experiments were run at background pressures below 3 = 10y6 Torr, under conditions where the fraction of accidental coincidences did not exceed 10% of the true coincidences. The accuracy of the subtraction can be judged directly from Fig. 1, where there is no visible cross-channel contamination such as would be produced by poor subtraction, particularly on the parent ion signal. The commercial sample of NO 2 used in this work contained a small amount of NO impurity, contributing spurious NOq and Nq signals. As the dissociation behaviour of NOq has already been studied w10x these artifacts could be recognised and have been removed in the data reduction.

3. Results Fig. 1 gives a low-resolution view of the photoelectron spectrum and major decay channels in the whole range accessible using HeIa Ž58.4 nm. radiation. Figs. 2 and 3 show composite spectra using both HeIIa and HeIa , together covering the whole energy region where distinct bands are present. Because of the long run times needed, only two sections have been examined at our best resolution of 50 meV Ž5 eV pass energy.; the remainder is at 300 meV resolution. The lower curves are photoelectron spectra in coincidence with each product ion. For each selected electron energy the coincident mass spectrum is obtained; two examples are shown

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

141

Fig. 1. Coincident photoelectron spectrum and spectra coincident with the main product channels taken using HeIa light at 300 meV resolution. The lowest panel shows the summed intensity of metastable ions from the three regions of the time-of-flight spectrum mentioned in the text. The error bars are two standard deviations long and include the small effect of accidental coincidence subtraction.

in Fig. 4. Within such mass spectra, the peak shape for each mass reflects the kinetic energies released in formation of the selected ion. Examples of peak shapes are shown in Fig. 5, and kinetic energy release distributions obtained from them by direct inversion are illustrated in Fig. 6. The energy releases can be compared with the available excess energies, which are derived from the thermodynamic onsets listed in Table 1. From the coincidence mass spectra, branching

ratios to all distinct products are obtained at each energy. The branching ratios are of interest where more than one product is formed; data for relevant cases are listed in Table 2, with other experimental information on the reactions. 3.1. Ground state of NO2q X 1Sgq The ion is stable in the main part of its ground state populated by one-photon ionisation. The finely

142

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

structured vibrational band w7x is very broad and its tail extends to the threshold for dissociation, but no fragment ions are detected at this point. 3.2. a 3B2 state Both the parent ion NOq 2 and the fragment ion NOq are detected in coincidence with the vibration levels Õ s 1,2,3 and 4 within this state, and metastable ions are detected in coincidence with Õ s 2 and Õ s 3. ŽFig. 1.. The thermodynamic limit for ground-state product formation ŽTable 1. at 12.38 eV is below the Õ s 0 level Ž12.85 eV., so ions in all the levels could dissociate. By comparing the observed intensities of the different ion signals with Monte Carlo simulations for monoexponential decays, we estimate lifetimes between 0.3 and 20 ms as listed in Table 2. The values are rather shorter than

those found by Chupka w1x and very much shorter than the lifetimes found by Shibuya et al. w3x. The coincident peak shapes for the metastable levels are distorted, but from their widths and those of the other peaks Že.g. Fig. 3. it is clear that the kinetic energy release has a narrow distribution and rises steadily through the band; the energy release is almost equal to the total excess energy available to the products. It follows that in dissociation from this state, NOq is formed with very little internal vibrational energy. 3.3. b 3A 2 state The high-resolution photoelectron spectrum w7x shows a marked broadening in the band for this state, interpreted as due to very fast dissociation. In agreement with this, the ions in this state dissociate com-

Fig. 2. Composite photoelectron spectrum of NO 2 from 9 to 16 eV excited by HeIa Ž21.22 eV. radiation Župpermost curve., together with q photoelectron spectra coincident with product ions. The lowest curve shows the intensity of metastable NOq 2 ions which dissociate to NO during acceleration in the source region. The section from 12.7 to 15 eV is at 50 meV resolution; the remainder is at 300 meV resolution. All the intensities are on a common scale and the photoelectron spectrum is the sum of the ion spectra.

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

143

Fig. 3. Overall photoelectron spectrum of NO 2 and coincident product ion spectra from 16 to 26 eV. HeI was used for the range below 20 eV, HeII Ž40.8 eV. radiation for the higher energy part.

Fig. 4. Mass spectra of NO 2 coincident with photoelectrons of particular selected energies near 19 eV, showing vibrationally selective production of Oq 2.

pletely to ground-state NOqq O products, with a high fraction of available energy going into kinetic energy. No metastable ions are detected. The NOq ion peak shape ŽFig. 4. is peculiar, showing forward–backward asymmetry, in clear contrast to all the other peak shapes for ion fragments from NO 2 . This sort of asymmetry has recently been observed in dissociations of NO and CO w10,11x; it is due to an anisotropic distribution of product ions ŽNOq. with respect to the direction of the departing photoelectron, not necessarily related to the direction of the electric vector. Dissociation faster than molecular rotation is necessary for observation of a strong effect, again in accordance with the photoelectron spectrum. The sense of the asymmetry is that the electron and the NOq ion depart in opposed directions Žangle ) 908.. From the weak excitation of the bending vibration in the photoelectron spectrum, it seems that the initial bond angle in NOq 2 in this state is similar to that in neutral NO 2 . The NOq and O must remain in the original molecular plane, so a possible interpretation could be that the electron is ejected preferentially in the plane of the molecule

144

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

Fig. 5. Samples of detailed NOq ion peak shapes in mass spectra coincident with selected electron energies. ŽA. a state, Õ 0 4; ŽB. b state, all levels; ŽC. B state, all levels; ŽD. 3 B 2 state Ž19 eV., all levels; ŽE. 3A 1 state Ž21.5 eV..

along the direction of the angle included between the two oxygen atoms. Where electron-fragment ion angular correlation has been found for diatomic molecules w10,11x, the photoelectron angular distributions relative to the polarisation vector have also been anisotropic Ž b / 0.. In contrast, when the b 3A 2 state of NOq is 2 formed at 21.2 eV photon energy, the photoelectron angular distribution is close to isotropic Ž b s 0., but varying strongly as a function of photon energy. The combination of isotropic photoelectron angular distribution with anisotropic photoelectron-fragment ion correlation must arise from the averaging over all molecular orientations, inherent in the measurement of b . However, we cannot yet give any detailed explanation.

3.4. a 1A 2 state q Both NOq ions are detected from the 2 and NO Õ s 0 level in this state, with a strong metastable signature. The lifetime is about 0.8 ms, measured here for the first time. Only the electronic ground states of the products are accessible, and the kinetic energy release is large. As the transition 1A 2 ™1A 1 is dipole forbidden, it is perhaps not surprising that no emission has been seen from this excited state of NOq 2 despite its long lifetime towards predissociation.

3.5. b 1B2 state An excited-state product channel, NOqŽ X 1 Sq . q OŽ2 D. opens at 14.36 eV, just below the onset of

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

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3.6. Composite c 3A 1 q d 3B1 band

Fig. 6. Kinetic energy release distributions derived from three of the peak shapes shown in Fig. 5 by direct inversion. The procedure involves smoothing and deconvolution followed by differentiation and conversion to an energy scale. The resolution is energy dependent. The two curves in each section of the figure correspond to independent analyses of the forward and backward portions of the peak shape, and give an indication of the reproducibility.

this state at 14.45 eV. About a quarter of the ions dissociate to the new channel with a small kinetic energy release, while the remainder form ground-state products. In both channels the energy release is a very large fraction of the available energy, so there is little vibrational excitation of the NOq. There is no significant count of metastable ions or parent ions; the statistical error limits on the intensities imply that the dissociations to both channels must have lifetimes of less than 70 ns.

q Two new product channels, Oq 2 q N and O q NO become energetically accessible just below the onset of this band at 17.067 eV. Only NOq is formed from the lower energy part of the band, however, and in the higher-energy part only a small fraction goes to Oq ŽTable 2.. In contrast to the results of Shibuya et al. w3x, no NOq 2 ions are detected in coincidence from any levels; the sensitive low-resolution scan shown in Fig. 1 demonstrates this point clearly. At q 17.3 eV any NOq 2 signal is 0 " 3% of the NO signal and no metastable ions are detected. At the energy where Oq is first formed, about 17.4 eV, it is produced with a kinetic energy release of 0.6 " 0.05 eV, almost equal to the available energy of 0.66 eV. The kinetic energy release in Oq formation remains constant as the ionisation energy increases within the band. These observations suggest the existence of a reverse activation barrier in the Oq channel. The onset of Oq is close to the onset ascribed to the d 3 B1 state at 17.26 eV, so there is a possibility that it is the d 3 B 1 state which produces Oq specifically. NOq may be formed from both electronic states in this band. The kinetic energy release in its formation is complex, with a main component at about 2 eV and minor contributions both at higher Ž4 eV. and lower Ž0.2 eV. energy. The distribution suggests that the major product channel is NOqŽ1 Sq, Õ . q OŽ1 Dg ., for which the available energy spans 2.6–4 eV, and Õ indicates a moderate vibrational excitation.

Table 1 Relevant dissociation limits of NOq 2 Products q

NO qO NOq qO NOq qO NOqOq NOqOq NqOq 2 NqOq 2 NqOq 2 Nq qO 2 Oq qOqN

States 1

q

Limit ŽeV. 3

S q Pg Sq q1 Dg 1 q S q1 S g 2 P q4 S u 2 P q2 Du 4 S u q2 P g 2 Du q2 P g 2 Pu q2 P g 3 Pg q3 Sy g 4 S u q3 Pg q4 S u 1

12.38 14.35 16.57 16.74 20.06 16.63 19.01 20.21 19.09 23.24

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

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Table 2 Dissociation characteristics of selected states and levels in NOq 2 State 1

Sq g

X a 3 B2

b 3A 2 A 1A 2 B 1 B2 3 A 1 q3 B1 3

B2

3 A1 CI band

Level

NOq 2

NOq

Oq

Oq 2

Lifetime

all 000

100 100

y

y

y

100

97 " 1

3"1

200

70 " 1

17.5 " 2

300

24 " 2

61 " 2

400 0 500 all 000 0 100

6"1 y

94 " 1 100 100 50 " 2 100 100 97 86 85 49 " 3 49 " 3 53 " 5 75 " 1 75 60

stable ) 150 ms b 1350 ms c 20 " 10 ms a 55 " 10 ms b 760 " 56r23 ms c 3.5 " 0.5 ms a 15 " 5 ms b 66 " 24r14 ms c 0.3 " 0.1 ms a - 5 ms b 42 " 9r7 ms c - 0.15 ms a

30 " 2

17.0–17.4 eV 17.4–18.1 eV 18.1–18.8 eV 000 100 200 all 23.3 eV 24 eV

; 10y13 s 0.8 " 0.3 ms a - 0.15 ms a 3"1 14 " 1 15 " 2 49 " 3 45 " 3 45 " 5 23 " 1 25 " 4 40 " 5

2"1 6"1 1"1 1.5 " 0.3

a

This work. Chupka w1x. c Shibuya et al. w3x. b

3.7. (3b2y 1)3B2 state (18.9–19.4 eV) This state dominates the HeI photoelectron spectrum by its great intensity. A channel giving Nqq O 2 opens within its energy range, but the products actually formed are NOq Ž48%., Oq Ž48%. and Oq 2 Ž4%.. There is no change in branching between the two major products with vibrational quantum numis formed ber in the main progression, but Oq 2 specifically from the vibrational levels between 18.95 and 19.05 eV. This energy coincides with the openŽ˜ 2 . Ž2 . ing of the excited channel Oq 2 X P g q N Du , q and the kinetic energy of the O 2 is indeed near zero, strongly suggesting that these products are being formed. In the NOq 2 ion, the symmetric stretch level Ž100. has the main population in this energy range,

but levels involving the bending vibration, which we do not resolve, are also populated weakly w7x. The kinetic energy release distribution in formation of NOq is broad and quasi-continuous; it seems certain that the O atom is formed in excited states as well as the ground state. There is also a broad kinetic energy release distribution in formation of Oq. 3.8. (4a1y 1)3A 1 state region (21–22 eV) Although the dominant photoelectron feature in this zone is the single strong broad peak at 21.25 eV assigned as 3A 1 , there is a weaker continuous band extending from 20.8 eV up to the main peak w7x. This lower energy band, which appears in both the HeI and HeII spectra, is presumably a satellite and has

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

been provisionally identified as a second 3A 1 state by comparison with two-holes–one-particle calculations by Schirmer et al. w12x. Above the main peak there is an even weaker continuous tail extending to about 22 eV. The main product is NOqq O, with a significant fraction of the atoms in excited states. Ž . The ions Oq and Oq 2 are also formed Table 2 , with branching ratios that remain constant across this energy region. 3.9. Satellite band (23–24 eV) In the range of this satellite band the main products are NOq and Oq; the branching ratio changes smoothly towards increasing production of Oq as the ionisation energy increases. This change may be related to the opening of the atomisation channel Oqq N q O ŽTable 1..

4. Discussion The ionic behaviour of NOq 2 is distinctly state specific. The ground-state surface seems to have a barrier to forming ground-state products. The first excited state dissociates slowly, the second extremely fast, the third slowly again, and the fourth relatively fast — all to the same ground-state products. The higher-energy states at 19 eV and above dissociate to two or more product channels with distinctly different branching ratios. These effects demonstrate that ions in each electronic state dissociate by individual dynamic pathways. The long-lived metastable states, and the very short-lived asymmetrically dissociating state are of special interest and call for detailed electronic structure calculations. Adiabatic correlations are unlikely to provide detailed interpretation, but the fact that the ground-state molecular ion correlates asymptotically to excited products may help explain why there is a barrier to dissociation at the first limit. The discrepancy between the lifetimes determined in this work and in the work of Chupka w1x and Shibuya et al. w3x is in the sense that where the lifetime zone sampled experimentally is longer, the derived lifetimes are longer too. Such variation is the hallmark of multiexponential lifetime distributions. Because none of the experiments are rotationally

147

resolved, many quantum states contribute at each selected energy in all the extant data. The lifetimes may be a strong function of rotational level, and if so, long lifetimes should dominate if the experiment is repeated with a rotationally cold molecular beam source. The discrepancy over the existence of a long-lived NOq 2 state at 17.3 eV is very puzzling, and we have no explanation for it except to note that the states populated at threshold are not necessarily the same as those populated by short wavelength photoionisation. The dissociations of NOq 2 ions formed by photoionisation may be related to the mechanisms of ion–molecule reactions of importance in planetary atmospheres. The reaction of ground-state Oq 2 with ground-state N atoms is a major source of NOq on earth, Mars and Venus, and the similar reaction of Nq ions with O 2 is a significant source of both NOq and Oq w13x. Both these reactions could proceed through an NOq 2 complex, whose behaviour might be similar to that observed here if angular momentum is not a major factor in the mechanism. The asymptotic energy of the Nqq O 2 channel at 19.09 eV is close to the energy of the vibrationless Ž3by1 .3 B 2 state, and matches low-lying vibration 2 Ž3 B 2 . was levels of that state almost exactly. If NOq 2 formed in the ion–molecule collision, the dissociation pattern might be expected to be the same as is observed in the PEPICO spectrum. In studies carried out by Smith et al. w14x and Howorka et al. w15x, the same products were indeed found as in the present study, but in different proportions. For NOq the yield was 43 " 5%, which is similar to our 48%, but Ž51 " 5%. and Oq Ž6 " 2%. the yields are for Oq 2 essentially reversed compared with those in Table 2. The high yield of Oq 2 in the ion–molecule collisions can probably be understood as a consequence of charge transfer, possibly at long range, since the reaction Nqq O 2 ™ N Ž 2 D . q Oq 2 is nearly resonant. The low yield of Oq relative to NOq, however, suggests that low vibrational levels of 3 B 2 are not formed in the collisions, since the PEPICO experiments show equal yields of these two ions. A much better match is found between the ion–molecule collisions and the unimolecular dissociations from the c 3A 1 and d 3 B 1 states in the 17–19

148

J.H.D. Eland, L. Karlssonr Chemical Physics 237 (1998) 139–148

eV region, where the OqrNOq ratio is in exact agreement. The kinetic energy releases found here in formation of NOq from the c 3A 1 and d 3 B 1 states are also in qualitative agreement with results of the collision studies of Langford et al. w16x and O’Keefe et al. w17x, which showed substantial energy release with the formation of both OŽ1 D. Ž70%. and OŽ3 P. Ž30%..

5. Conclusions The dissociations of ionised NO 2 show an unusually rich variety of dynamic behaviour, which calls for theoretical explanation, perhaps in terms of potential energy surfaces and non-adiabatic couplings. On the basis of several points of similarity it also seems reasonable to suggest that the unimolecular fragmentation processes seen in PEPICO may be partly related to some atmospheric ion–molecule reactions; it would be interesting to study the relationship by theoretical methods. We have begun a collaborative programme to address these points, and perhaps also to cast light on the discrepancies in existing experimental data. For deeper understanding it will be necessary to examine the dissociation behaviour of NOq 2 experimentally at higher-energy resolution, with different wavelengths of ionising light and perhaps to control its rotational temperature by molecular beam cooling.

References w1x W.A. Chupka, in: C. Sandorfy, P.J. Ausloos, M.B. Robin ŽEds.., Chemical Spectroscopy and Photochemistry in the VUV, Reidel, Dordrecht, The Netherlands, 1974. w2x R.P. Wayne, Chemistry of Atmospheres, Clarendon Press, Oxford, 1985. w3x K. Shibuya, S. Susuki, T. Imamura, I. Koyano, J. Phys. Chem. A 101 Ž1997. 685. w4x C.R. Brundle, D. Neumann, W.C. Price, D. Evans, A.W. Potts, D.G. Streets, J. Chem. Phys. 53 Ž1970. 705. ˚ w5x O. Edqvist, E. Lindholm, L.E. Selin, L. Asbrink, C.E. Kuyatt, S.R. Mielczarek, J.A. Simpson, I. Fischer-Hjalmars, Physica Scripta 1 Ž1970. 172. w6x T. Yamazaki, K. Kimura, Chem. Phys. Lett. 43 Ž1976. 502. w7x D.M.P. Holland, M.A. MacDonald, M.A. Hayes, P. Baltzer, L. Karlsson, B. Wannberg, J.H.D. Eland, in preparation. w8x T. Field, J.H.D. Eland, Meas. Sci. Technol. 9 Ž1998. 922. w9x W.C. Wiley, I.H. McLaren, Rev. Sci. Instrum. 26 Ž1955. 1150. w10x J.H.D. Eland, E. Duerr, Chem. Phys. 229 Ž1998. 1. w11x J.H.D. Eland, E. Duerr, Chem. Phys. 229 Ž1998. 13. w12x J. Schirmer, L.S. Cederbaum, W. von Niessen, Chem. Phys. 56 Ž1981. 285. w13x A. Dalgarno, J.L. Fox, in: C.-Y. Ng, T. Baer, I. Powis ŽEds.., Unimolecular and Bimolecular Ion–molecule Reaction Dynamics, Wiley, Chichester, 1994. w14x D. Smith, N.G. Adams, T.M. Miller, J. Chem. Phys. 69 Ž1978. 308. w15x F. Howorka, I. Dotan, F.C. Fehsenfeld, D.L. Albritton, J. Chem. Phys. 73 Ž1980. 758. w16x A.O. Langford, V.M. Bierbaum, S. Leone, J. Chem. Phys. 84 Ž1986. 2158. w17x A. O’Keefe, G. Mauclaire, D. Parent, M.T. Bowers, J. Chem. Phys. 84 Ž1986. 215.