Excitation functions of cation formation in collisions of diatomic molecules with an ionic bond: CsCl + RbI

Vyacheslav M. Akimov, Vladimir M. Azriel, Lev Yu. Rush and Mikhail B. Sevryuk* Institute of Energy Problems of Chemical Physics, Leninskii prospect 38, Bldg. 2, Moscow I 17829, Russia

The excitation functions for the formation of cations Cs+, Rb', CsRbCl' and CsRbI+ in collisions of alkali-metal halide mol- ecules CsCl and RbI have been measured in crossed molecular beams for the collision energy ranging from 3 to 10 eV. Trajectory simulation of dissociative processes in the CsCl + RbI system has been performed on a potential-energy surface chosen to be the sum of six pairwise interaction potentials. The calculated excitation functions agree well with the experimental data. Simulation of the CsCl + RbI reaction by a hard-sphere model has also been carried out.

In spite of very intensive experimental' and theoretical2 studies of elementary chemical processes involving four atoms, the dynamics of four-centre reactions has still been much more poorly understood than the dynamics of atom + diatom reactions. Among the most important difficulties in investiga- tions of four-atom systems, is the fact that there are a large number of reaction channels and a rather slender experience of constructing sufficiently reliable potential-energy surfaces (PESs). It is therefore of great importance to consider four- atom processes that are interesting from both theoretical and practical viewpoints and, at the same time, simple enough to enable one to construct easily an adequate PES. Such pro- cesses are exemplified by collision-induced dissociation (CID) of two diatomic molecules and, in particular, dissociation of two molecules with an ionic bond. In the latter case, all the four particles involved are closed-shell ions, which simplifies significantly their interaction, reduces greatly the number of electronic states to be taken into account, and facilitates there- fore the construction of interaction potentials. On the other hand, the CID dynamics is of interest from the viewpoint of studying the dynamics of the corresponding recombination processes which have been explored rather p ~ o r l y . ~ - ~

There is a very rich body of literature devoted to both experimental and theoretical investigations of the dissociation of diatomic molecules MX with an ionic bond (mainly alkali- metal or thallium halides) in collisions with chemically inert atoms A (e .g . rare gas or mercury atoms) see e.g. ref. 5-21 and references therein. On the other hand, studies of collisions MX + NY of two alkali-metal halide molecules have been mainly confined to crossed molecular beam investigations of the reactions KCl + CsCl and KI + CsCl (ref. 22) and trajec- tory simulations of the reaction NaBr + KCl (ref. 23) for low collision energies Ecol ( ~ 0 . 1 7 eV22 or 0.25-2.5 eV23). For these values of Ecol, the only possible reactive channel is the neutral exchange MX + NY -, MY + NX. The main observ- ation of ref. 22 and 23 is the high probability of the formation of a long-lived four-particle complex, the ratio of reactive to non-reactive decay of this complex deviating significantly from the statistical theory predictions. We have carried out trajec- tory calculations for the reaction KI + CsCl at the collision energy of 0.165 eV,24 and the results obtained agree well with the experimental data.22

Recently, we also performed trajectory simulation of the reaction CsBr + CsBr for Ecol ranging from 0.1 to 15 eV25-27 and that of the reactions RbBr + RbBr, CsCl + RbI and CsI + RbCl for Ecol ranging from 1 to 15 eV.27

In the present paper, we report the results (the excitation

functions for the cation formation) of the first experimental studies of the CID processes in collisions of molecules CsCl and RbI, the collision energy ranging from 3 to 10 eV. For these values of Eco, , dissociation of one or both reagents becomes possible. Owing to a large crossing radius, R,, of the ionic and covalent terms and a very small splitting AV(R,) between the corresponding adiabatic potential-energy curves at the internuclear distance R = Rc,28*29 the caesium and rubidium halides exhibit ionic dissociation only (however, RbI is known to be able to dissociate partly into neutral atoms28). In fact, R , is equal to 51.4 8, for CsCl and 12.9 A for RbI,28 whereas AV(R,) is equal to eV for RbI.28929 Neglecting dissociation into neutral atoms compared with that into the alkali-metal cation and halide anion, the total number of the reaction channels in CsCl + RbI collisions for the collision energy range in ques-

eV for CsCl and 8 x

tion is equal to 12:

CsCl + RbI -,

CsCl + RbI

Cs' + Cl- + Rb' + I-

Cs' + Cl- + RbI

Rb' + I - + CsCl

Rb' + C1- + CsI

Cs' + I - + RbCl

CsI + RbCl

CsRbCl' + I-

CsClI- + Rb+

CsRbI' + C1-

RbClI- + Cs+

CsClRbI

This scheme includes neutral (1, 7, 12) and ionic (2-6, 8-11) channels. Our experimental apparatus enabled us to record positive ions only, so we have measured the cross-sections for the formation of two atomic cations Cs+ (formed in channels 2, 3, 6 and 11) and Rb+ (channels 2, 4, 5 and 9) and two three-centre molecular complexes CsRbCl + (channel 8) and CsRbI+ (channel 10). In fact, channel 2 (complete dissociation) is energetically inaccessible, because its threshold is equal to 9.28 eV (the sum of the CsCl and RbI ionic dissociation energies) and one should expect an observable cross-section for this channel at Ecol 2 10 eV only. Neither channels 3, 6, 11

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nor channels 4, 5, 9 could be distinguished by our experimen- tal technique.

Experimental The experiment was performed with a supersonic seeded CsCl beam and an effusive RbI beam crossed at 90". We used the apparatus exploited earlier by our group for CID studies in systems Xe + CsBr and Hg + CSB~"'~-' ' and described in detail el~ewhere. '~. '~ For the present study, however, we modified the supersonic beam source, scattering chamber and some other parts.

To obtain the supersonic seeded CsCl beam, we used a two- chamber oven, the chambers being heated separately by tanta- lum wires. The salt (4 cm3) was placed in the first chamber, heated up to 600°C which enabled us to achieve the vapour pressure Pcsc, M 0.033 Torr. The second (nozzle) chamber was kept at a higher temperature (650-700°C) to prevent conden- sation of the salt vapour on the nozzle orifice. The nozzle diameter was equal to 0.06 mm. The beam was collimated by a skimmer with a length of 20 mm and the exit orifice diam- eter of 1.2 mm. The work pressure in the nozzle/skimmer region was ca. Torr and the optimal distance between the nozzle and the skimmer was chosen by varying the nozzle location to achieve the maximal signal intensity. To prevent salt condensation on the skimmer edge, it was heated up to a temperature of 400-500 "C.

To obtain the desired beam energy, we varied the stagna- tion pressure, P o , of the carrier gas (hydrogen or helium). The beam energy, ECsCl, increases with P o , and as the latter varies in the range 1-5 atm, the energy ECsCl has been measured to range from 5 to 7.5 eV (for He) or from 9 to 16 eV (for H2). The characteristics of the CsCl beam were monitored by the time-of-flight method with the flight base equal to 94 cm, a monopole mass spectrometer being used to control the beam intensity stability. The same mass spectrometer was exploited as the beam detector in the time-of-flight measurements of the reaction cross-sections.

The quantitative characteristics of a supersonic seeded Xe beam (with H, or He as the carrier gas) obtained by a similar ~ o u r c e ~ ~ * ' ~ almost coincide with the theoretical predictions. On the other hand, using diatomic molecules of alkali-metal halides as the heavy component of the beam makes the experi- ment much more difficult because, in particular, it becomes hard to keep constant the partial pressure of the salt in the zone where the latter is mixed with the carrier gas. We did, therefore, not interrupt the monitoring of the CsCl beam during the experiment. The beam intensity we could work with was less than the maximal a priori possible one owing to the difficulties in maintaining sufficiently high temperature in the apparatus and achieving higher salt partial pressure in the mixture zone.

The source of the effusive RbI beam was a two-chamber oven, the chambers being also heated separately. The tem- perature of the first chamber containing the salt could vary in the range 500-700°C which corresponded to the vapour pres- sure P R b I ranging from to 1.5 Torr. The salt molecules effused through a 0.3 mm diameter orifice at the top of the second chamber and then the beam was collimated by a 0.5 mm x 8 mm slit. The second chamber was kept at a tem- perature 50-100" higher than the first one, and the colli- mating slit also had an autonomous heater to prevent condensation of the vapour. The whole oven was surrounded by a radiation screen and an outer water-cooled screen. The oven and the radiation screen were kept at a small positive voltage to eliminate the background ions signal arising from surface interaction of RbI molecules with the hot parts of the oven and the collimating slit. A surface ionization detector with a tungsten filament was used for the monitoring of the RbI beam.

To measure the total interaction cross-sections, we placed the beam intersection zone between a repelling plate and the entrance to the mass spectrometer drift tube. The ions formed in the reaction were separated in the drift tube according to their times of flight determined by the mass-to-charge ratios. This scheme of total cross-section measurements enabled us to avoid any significant loss of ions. The ions were then recorded by a channeltron, and finally one could observe the whole mass spectrum on the screen of a multichannel analyser. With a typical channel width of the analyser equal to 200 ns, we were able to resolve completely the cations Cs+, Rb+, CsRbC1' and CsRbI+.

The apparatus was governed by an AT-386 minicomputer connected with a CAMAC node. The signal accumulation time was determined by achieving a sufficiently small sta- tistical error of the measurement.

Experimental results The measured excitation functions (cross-sections as functions of the collision energy Eco,) for all the four cations under con- sideration are shown in Fig. 1. We emphasize again that, for each of the atomic cations Cs+ and Rb', the corresponding excitation function is the sum of the excitation functions for three separate reaction channels (3, 6, 11 for Cs+ and 4, 5, 9 for Rb'). The collision energy was determined by the expression6.9-1 1,13,16

Ecol = ; ( VZ.sCl + M,,, 3kTRb?

where ,u denotes the reduced mass of the CsCl-RbI system, VcscI is the velocity of projectile molecules CsCl in the labor- atory frame, TRbI is the temperature of the effusive orifice in the RbI beam source, k f R b I is the mass of target molecules RbI, and k denotes the Boltzmann constant. The error in mea- suring Ecol was no worse than 3% whereas the error in the cross-section measurements was ca. 8-1 2% (except for CsRbI' in which case the signal turned out to be especially small and the error was ca. 25-30%).

In general, the cross-sections of the ion formation in the CsCl + RbI reaction have proven to be much smaller than those in the triatomic processes (e.g. Xe + C S B ~ ' ~ . ' ~ , ' ~ ) . The cross-sections of the formation of atomic cations Cs' and Rb+ increase with the collision energy Ecol (Fig. 1) but more slowly

1 .o

h

.g 0.8

d c 13

(3.6 v

C 0

8 0.4 .- c.

3 v)

0 2 0.2

0.0

than, e.g. the cross-section of-- three-centre CID

3 4 5 6 7 8 9 1 0

collision energy/eV

Fig. 1 Experimental excitation functions for cation formation in CsCl + RbI collisions. The label Rb+ ( x 5 ) means that the actual values of the Rb+ formation cross-section are five times smaller than those shown in the figure. The labels CsRbCl+ ( x 25) and CsRbI+ ( x 105), as well as similar labels in the subsequent figures, have an analogous meaning. The error bars on this and the subsequent figures correspond to 67% confidence level.

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Xe + CsCl + Xe + Cs' + Cl-.6 Moreover, the excitation functions for the Cs' and Rb' formation in the CsCl + RbI

Cs', C1-, Rb', I - can proceed by all the reactive channels 2-12.

system shown in Fig. 1 can hardly be approximated, even in the threshold region, by the expression o = A,(E,,, - Eo)"/Ecol (E,,, = Ecol + E$ denoting the total energy of the system, Ett the initial internal energy of the reagents and E , the threshold), in contrast to the CID excitation functions for tri- atomic systems A + MX.6*9,10 The possible reason is that channel 2 of the formation of four atomic ions is not realized for the collision energies probed in our experiment (and espe- cially for the Ecol values in the threshold region). The effective thresholds for the formation of cations Cs+ and Rb' (ca. 3 and ca. 4 eV, respectively) are significantly smaller than the dissociation energies Dcscl and D,,, of the molecules CsCl and RbI into ions (4.86 and 4.42 eV, respectively2'). Note that the cross-section of Cs+ formation is ca. 5-15 times larger than that of Rb' formation (for the same value of Ecol), although D,,, is smaller than Dcscl .

The cross-sections of the formation of molecular cations CsRbCl+ and CsRbI' grow rapidly with Ecol in the threshold region and decrease steeply as the collision energy increases further (Fig. 1). The maxima of both excitation functions are located in the region Eco, x 4-4.5 eV. The cross-section of CsRbC1' formation is ca. 4-6 times larger than that of CsRbI' formation for these values of Ecol. Since the signal from the CsRbI' cations was very small, and the statistical error was therefore large (25-30%), we will not take into account the oscillations in the CsRbI ' excitation function.

Trajectory simulation and discussion Details of trajectory simulation of the CsCl + RbI reaction are described in ref. 27 (cf. also ref. 26). Here, we mention only that the PES for the CsCl-RbI system we exploited was chosen to be the sum of six pairwise potentials of the trun- cated Rittner (T-Rittner) form3'

a a1 + a2 c6 U ( R ) = A exp(-R/p) f - - - - -

R 2 ~ 4 ~6

where a1 and a2 are the polarizabilities of the two ions involved, the Coulomb terms + a/R and -a/R correspond, respectively, to ions of like sign and ions of opposite sign, and a = 1 in atomic units. The values of the Born-Mayer param- eters A , p and that of the dispersion constant C, for all the six pairwise potentials involved are reported in ref. 27. For each value of the collision energy (ranging from 1 to 15 eV), 3 x lo4 trajectories were run. The trajectory simulation has confirmed that the rearrangement processes for the four ions

The calculated excitation functions for all the ionic channels except channel 2 (i.e. channels 3-6 and 8-11) are presented and compared with the experimental excitation functions in Fig. 2-5. One sees a rather good agreement between the

1

0 3 4 5 6 7 8 9 1 0

collision energylev

Fig. 3 Calculated excitation functions for various channels of Rb+ formation. The labels 4, 5 , 9 ( x lo), 4 + 5 and 4 + 5 + 9 of the curves indicate the corresponding channels. The hollow squares represent the experimental cross-sections of Rb+ formation (in arbitrary units).

0.0 0 .5 I 3 4 5 6 7 8 9 1 0

collision energy/eV

Fig. 4 Calculated excitation function for CsRbCl+ formation (channel 8). The filled circles represent the experimental cross-sections (in arbitrary units).

16 I I I I I I I

1.00 , 1 I I I I I I

4

0 3 4 5 6 7 8 9 1 0

collision energy/eV

Fig. 2 Calculated excitation functions for various channels of Cs' formation. The labels 3, 6, 11, 3 + 6 and 3 + 6 + 11 of the curves indicate the corresponding channels. The filled squares represent the experimental cross-sections of Cs + formation (in arbitrary units).

0.75 (\I

5 C 0 .- c

0.50

0.25

0.00 3 4 5 6 7 8 9 10

collision energylev Fig. 5 Calculated excitation function for CsRbI+ formation (channel 10). The hollow circles represent the experimental cross-sections (in arbitrary units).

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experimental and calculated data. This indicates that the simple additive PES for the CsC1-RbI system we used in ref. 27 and in the present study is quite adequate which, in turn, can be attributed to the fact that the system involves closed- shell particles only. One may also conjecture that dissociation of the RbI molecule into neutral atoms (which is possible according to ref. 28) is not of great importance for the course of the reaction CsCl + RbI. Neutral dissociation of RbI can, however, affect the cross-sections of the formation of cations Rb', CsRbC1' and CsRbI+. The neutral dissociation channel of RbI is probably responsible for the fact that the experimen- tal ratios of the cross sections of Cs' formation and Rb' for- mation are larger than the trajectory ratios (Fig. 1-3).

Our calculations have confirmed that the cross-section of channel 2 (complete dissociation) is rather small and the con- tribution of this channel to formation of atomic ions is there- fore negligible for Eco, < 10 eV. The cross-sections of channels 3-6 (formation of two atomic ions) increase monotonically with the collision energy (Fig. 2 and 3). In agreement with the experimental data, our trajectory simulation indicates that the cross-sections of Cs' formation in channels 3, 6, 11 are con- siderably larger than those of Rb' formation in channels 4, 5, 9, respectively (Fig. 2 and 3), while the cross-section of CsRbC1' formation (channel 8) is significantly larger than that of CsRbI' formation (channel lo), see Fig. 4 and 5. The trajectory results suggest that in the low-energy region, the formation of cations Cs' and Rb+ is associated mainly with channels 11 and 9, respectively. On the other hand, for both atomic cations Cs' and Rb', the agreement between the tra- jectory and experimental excitation functions turn out to be slightly better, in general, in the case where one does not take into account the molecular ion formation channels in the tra- jectory cross-sections (cf: the curves 3 + 6 and 3 + 6 + 11 in Fig. 2 and the curves 4 + 5 and 4 + 5 + 9 in Fig. 3). Our cal- culations thus overestimate the contribution of channels 9 and 11 in the atomic cation formation.

The cross-sections of channels 8-1 1 (formation of an atomic ion and molecular ionic complex) exhibit ' bell-like' behaviour with a well pronounced maximum (Fig. 1, 4 and 5). The exis- tence of this maximum is due to the fact that raising the colli- sion energy above ca. 4.5 eV leads to strong instability of the nascent three-centre complex and considerable probability of its decay. Note, that the experimental excitation function for cation CsRbCl' drops off quickly as Ecol grows from ca. 4.1 to ca. 7.5 eV (Fig. 4) whereas the trajectory excitation function decreases much more slowly. A detailed comparison of the experimental and trajectory excitation functions for cation CsRbI+ is hardly possible because of a large statistical uncer- tainty in the former.

The triatomic product formation in channels 8-11 is of par- ticular interest. Molecular ions CsRbCl', CsRbI', CsClI- and RbClI- do not resemble weakly bound two-centre com- plexes CsA + observed in dissociation of caesium halides induced by collisions with rare gas or mercury atoms A.6,9,10*' Complexes CsA' are characterized by low bond energies (0.1-0.3 eV),20 and their formation is therefore pos- sible only under the special kinematic conditions of the colli- sion. On the other hand, the three-centre complexes formed in the reaction CsCl + RbI seem to possess large bond energies, and their formation should be attributed to an entirely differ- ent collision dynamics. As an evidence of considerable bond strength in cations CsRbCl' and CsRbI' whose excitation functions have been measured directly (Fig. 1, 4 and 5), one points out the fact that the thresholds of the CsRbCl' and CsRbI' formation are less than 3 eV (more than 1.5 eV smaller than the ionic dissociation energies of the salt mol- ecules RbI and CsCl). One can therefore conjecture that chan- nels 8 and 10 of molecular cation formation do not possess their own activation energy, which seems to agree well with the microscopic reversibility principle (according to the latter,

an ion recombination reaction should have zero activation en erg^).^ If this is the case, the threshold of each of these channels is equal to the difference between the dissociation energy of the corresponding reagent and the strength of the new bond. Unfortunately, we have not found any information in the literature on the structure of ionic complexes consti- tuted by two alkali-metal cations and one halide anion. The new bond strength in molecular cations CsRbCl' and CsRbI' has been estimated using the PES exploited in trajec- tory calculations. These estimates give values ranging from 1.2 to 2.5 eV (depending on the postulated internuclear distances in the triatomic cation in question).

The excitation functions for channels 9 and 11 of molecular anion formation have not been measured directly. However, our trajectory simulation suggests that the low-energy 'tails' of the Rb' (Fig. 3) and Cs' (Fig. 2) excitation functions are attributed, respectively, to these channels. The trajectory exci- tation functions for channels 9 and 11 enable one to conclude that the thresholds for the CsClI - and RbClI- formation are also considerably smaller than 3 eV, and the above discussion can be applied to these complexes as well.

We have also studied the influence of the initial internal energy of the reagents CsCl and RbI on the reaction cross- sections. To this purpose, the postulated temperatures Tcscl and TRbl of the beams in trajectory calculations were varied in the range 300-1000 K (the main part of the calculations was carried out for Tcscl = TRb, = K). It has been found that raising the internal energy of any of the two reagents by AE has almost the same effect on the total cross-sections of all the reactive channels as raising the collision energy by AE.

Conclusion The most interesting finding of the present work is that the cross-sections for Cs' formation in CsCl + RbI collisions are approximately an order of magnitude larger than those for Rb' formation, in spite of the fact that the CsCl dissociation energy is 0.44 eV larger than the RbI dissociation energy. This effect is observed in both the experiment and (to a lesser extent) the trajectory simulation and seems to be associated primarily with the masses of the ions involved, but it is prob- ably impossible to provide an explanation as simple as those typical for triatomic CID.S910,'1 The large Csf/Rb' forma- tion ratio cannot be explained just on the basis of impulsive considerations of the Ecol-Eint energy transfer. Indeed, if parti- cle A strikes diatomic molecule BC with zero initial internal energy then the amount of the collision energy, K , transferred into the internal degrees of freedom of the target BC for a head-on A-B hit is determined in the hard-sphere limit by the well known expression"

where m,, m2 and m3 denote, respectively, the masses of par- ticles A, By C (this expression holds provided that the process does not involve multiple encounters between A and B). Treating one of the molecules CsCl and RbI as a single pro- jectile and calculating the corresponding K values by this formula, we obtain the quantities listed in Table 1. This table

Table 1 Amount of the collision energy, K , transferred to the target internal degrees of freedom in the impulsive limit

projectile target K

CsCl CsCl RbI RbI

Rb-I I-Rb cs-Cl CI-cs

0.96 0.71 0.45 0.88

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would suggest that channel 4 of RbI dissociation has the maximal cross-section which strongly contradicts both the experiment and the trajectory calculations.

Note, also, that while channels 3 and 4 in our reaction resemble the three-body dissociation A + MX + A + M f + X-, channels 5 and 6 (the ion formation accompanied by an exchange process) have no analogues in triatomic systems. It would be interesting to measure and analyse the differential and double-differential cross-sections for various channels of the reaction CsCl + RbI to comprehend dynamical factors governing this reaction. The excitation functions alone cannot determine the reaction dynamics uniquely. The corresponding experimental work is in progress.

Besides the formation of three-centre molecular cations CsRbCl' (channel 8) and CsRbI+ (channel lo), our trajectory calculations predict the formation of three-centre molecular anions CsClI - (channel 9) and RbClI - (channel 1 l), see Fig. 2 and 3. In triatomic CID processes A + MX, molecular anions XA- were observed experimentally only in the reaction Xe + CsI.' Our experimental technique has not enabled us to find out whether anions CsClI- and RbClI- are indeed formed in the CsC1-RbI interaction. However, one is able to conclude that the formation of molecular ions in the reaction in question cannot be explained within the framework of the simple replacement model" or even the more sophisticated kinematic model,' which are applicable only for the case where complexes with very weak bonds are formed. In the reaction CsCl + RbI, the three-centre molecular products seem to be characterized by relatively strong bonds of 1.2-2.5 eV. To explore the dynamics of the molecular ion formation in the reaction under consideration, further experimental and theoretical studies are necessary.

The excitation functions for the ionic channels 2-6 and 8-11 in the reaction CsCl + RbI can also be obtained within the framework of the hard-sphere (impulsive) model which treats the ions involved as hard balls exchanging their energies and momenta in the collisions according to the elastic encounter laws.' 1 3 1 8 * 3 1 This model has been successfully applied to simulate CID in triatomic processes A + MX (both three-body dissociation and molecular ion formation).6,8-1 1,13,15.17-2 1 Th e particular version of the hard-sphere model we had especially developed for processes of the MX + NY family is described in detail in ref. 26 and 27 and applied to reactions CsBr + CsBr, RbBr + RbBr, CsCl + RbI and CsI + RbCl. Here, we confine ourselves to the remark that this version is close to the particular case of the hard-sphere model applied in ref. 32 to studies of the inter- action of T and F atoms with the molecules CH,, CF,, CH,F, . Within the hard-sphere approximation, the analysis of a collision is 2.5-3 orders of magnitude less time consuming (depending on the collision energy) than that for exact trajec- tory simulation. The hard-sphere excitation functions for channels 3-6 and 8-11 of the reaction CsCl + RbI are pre- sented in Fig. 6-8 (for each value of Ecol ranging from 1 to 15 eV, 5 x lo4 collisions were simulated).

One sees that the hard-sphere model reproduces satisfacto- rily the trajectory excitation functions for channels 3-6 (the formation of two atomic ions) and the experimental excitation functions for the atomic cation formation, cf: Fig. 2 and 3. On the other hand, the trajectory excitation functions for channels 8-11 (the formation of an atomic ion and a three-centre molecular ion) and the experimental excitation functions for the molecular cation formation are not described adequately by the hard-sphere model, cf: Fig. 2-5 (see ref. 27 for a much more detailed discussion without, however, the experimental results). The probable reason is that the molecular ion forma- tion is a rather complicated process which can involve an intermediate four-centre complex (which decay leads to atomic and triatomic products) whereas the hard-sphere model we used does not take into account the motion of the

I I.

15 cu 5 5 12

8 1 9

.- c

cn g 6

3

0 3 4 5 6 7 8 9 10

collision energy/eV

Fig. 6 Hard-sphere model excitation functions for various channels of Csf formation. The labels 3, 6, 11 and 3 + 6 + 11 of the curves indicate the corresponding channels. The filled squares represent experimental cross-sections of Cs+ formation (in arbitrary units).

10 , I I I I 1 I 1

8 i 0 0

the

3 4 5 6 7 0 9 10

collision energy/eV Fig. 7 Hard-sphere model excitation functions for various channels of Rb' formation. The labels 4, 5 ( x 5), 9 and 4 + 5 + 9 of the curves indicate the corresponding channels. The hollow squares represent the experimental cross-sections of the Rb+ formation (in arbitrary units).

3.5 , I I I I I I I

3 .0 - 2.5

t 9 2.0 0 Q, 1 1.5

5 c.

v)

b 1.0

0.5

0.0 3 4 5 6 7 8 9 10

collision energylev Fig. 8 Hard-sphere model excitation functions for the formation of molecular cations CsRbCl+ (channel 8) and CsRbI+ (channel 10). The circles represent the experimental cross-sections (in arbitrary units).

particles after the first impulsive exchange of energies and momenta.

The authors are grateful to C. Nyeland of the H. C. 0rsted Institute, Copenhagen, for discussions on the interaction potentials between halide and alkali-metal ions. The research described in this paper was made possible in part by Grant

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No. MKY300 from the International Science Foundation and Grant No. 94-02-03429 of the Russian Academy of Sciences Fundamental Research Foundation.

References 1

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13

14

M. Alagia, N. Balucani, P. Casavecchia, D. Stranges and G. G. Volpi, J. Chem. Soc., Faraday Trans., 1995, 91, 575, and refer- ences therein. D. C. Clary, J. Phys. Chem., 1994, 98, 10678, and references therein. L. S. Polak, Non-equilibrium Chemical Kinetics and Its Applica- tions, Nauka, Moscow, 1979 (in Russian), and references therein. I. M. Littlewood and R. E. Pyle, Phys. Rev. A , 1990,41, 1071, and references therein. L. Yu. Rusin, J. Chem. Biochem. Kinetics, 1991,1,205. S. H. Sheen, G. Dimoplon, E. K. Parks and S. Wexler, J. Chem. Phys., 1978,68,4950. S. Wexler and E. K. Parks, Annu. Rev. Phys. Chem., 1979,30,179. F. P. Tully, N. H. Cheung, H. Haberland and Y. T. Lee, J. Chem. Phys., 1980,73,4460. E. K. Parks, M. Inoue and S. Wexler, J. Chem. Phys., 1982, 76, 1357. E. K. Parks, L. G. Pobo and S. Wexler, J. Chem. Phys., 1984, 80, 5003. A. I. Maergoiz, E. E. Nikitin and L. Yu. Rusin, in Plasma Chem- istry, ed. B. M. Smirnov, Energoatomizdat, Moscow, 1985, vol. 12, p. 3 (in Russian). V. M. Akimov, V. M. Azriel and L. Yu. Rusin, XII International Symposium on Molecular Beams, Perugia, Italy, 1989, Book of Abstracts, p. 41. V. M. Akimov, A. I. Maergoiz and L. Yu. Rusin, Soviet J. Chem. Phys., 1990,5, 2828. L. V. Lenin and L. Yu. Rusin, Chem. Phys. Lett., 1990,170,502.

15

16

17

18

19

20

21 22

23

24

25

26

27

28

29 30 31

32

L. V. Lenin and L. Yu. Rusin, Soviet J. Chem. Phys., 1991, 8, 1507. V. M. Azriel, V. M. Akimov and L. Yu. Rusin, Soviet J. Chem. Phys., 1991,8, 2069. V. M. Akimov, L. V. Lenin and L. Yu. Rusin, Chem. Phys. Lett., 1991,180,541. L. V. Lenin, L. Yu. Rusin and M. B. Sevryuk, preprint deposited at VINITI 10.12.91, N 4561-V91, Moscow, 1991 (in Russian). L. Yu. Rusin and M. B. Sevryuk, XV International Symposium on Molecular Beams, Berlin, 1993, Book of Abstracts, D.22. L. Yu. Rusin and M. B. Sevryuk, Soviet J. Chem. Phys., 1994, 12, 1247. L. Yu. Rusin, J. Phys. Chem., 1995,99, 15502. W. B. Miller, S. A. Safron and D. R. Herschbach, Discuss. Faraday SOC., 1967,44,292; J . Chem. Phys., 1972,56,3581. P. Brumer and M. Karplus, Faraday Discuss. Chem. SOC., 1973, 55, 80. V. M. Azriel and L. Yu. Rusin, ZX European Conference on Dynamics of Molecular Collisions (MOLEC IX) , Prague, 1992, Book of Abstracts, B 35, p. 141. V. M. Azriel and L. Yu. Rusin, XV International Symposium on Molecular Beams, Berlin, 1993, Book of Abstracts, D.21. V. M. Azriel, L. Yu. Rusin and M. B. Sevryuk, Khim. Fizika, 1995, 14(7), 28 (in Russian). V. M. Azriel, L. Yu. Rusin and M. B. Sevryuk, Chem. Phys., 1995, 199,195. J. J. Ewing, R. Milstein and R. S. Berry, J. Chem. Phys., 1971, 54, 1752. R. Grice and D. R. Herschbach, Mol. Phys., 1974,27, 159. P. Brumer and M. Karplus, J. Chem. Phys., 1973,58,3903. J.-B. Song, E. A. Gislason and M. Sizun, J. Chem. Phys., 1995, 102,4885. D. J. Malcolme-Lawes, J. Chem. Soc., Faraday Trans. I I , 1974, 70, 1942.

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