collision-induced dissociation of low energy benzene ions

E L,S EVl E R International Journal of Mass Spectrometry and Ion Processes 165/166 (1997) 237-247 and hm Rocesres

Collision-induced dissociation of low energy benzene ions’

Rahul Chawla, Anil Shukla, Jean Futrell*

Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA

Received 1 April 1997; accepted 29 May 1997

Abstract

The reaction dynamics of collision-induced dissociation of benzene molecular ions at low collision energy has been investigated using a tandem hybrid crossed-beam apparatus. The major primary dissociation reactions forming C,H: and C,H; have been investigated over the range of center-of-mass collision energies from 4.9 to 17 eV. The energy transfer mechanism is impulsive at all energies; side-scattering is observed at low energy and significant angular scattering occurs at the highest energy studied. At low collision energy the average translational endoergicity to form C,Hi ions is approximately equal to the appearance energy of this ion in the benzene mass spectrum. At 10 eV and higher collision energy the average energy transferred exceeds the lowest energy reaction mechanism by about 1 eV for both channels. A very broad energy deposition function is deduced, with energy transfer ranging from about 1 eV up to 10 eV, with most probable energy transfer of about 6 eV. The very high translational endoergicity and broad range of energy deposition in reactant ions which are highly vibrationally excited prior to collisional activation suggests non-ergodic decomposition pathways are favored. 0 1997 Elsevier Science B.V.

Keywords: Collision-induced dissociation; Energy deposition; Impulsive collisions; Reaction dynamics; Angular scattering

1. Introduction

Collision-induced dissociation (CID) of small molecular ions (viz. Hl, Dl, HeH+, CS,+, CO,+, etc.) has long been of interest to experimentalists

‘Dedicated to Professor Keith R. Jennings on the occasion of his 65th birthday. It has been a great pleasure for two of us (AS and JF) to have been associated with Keith at different times.

*Corresponding author.

and theoreticians attempting to understand energy transfer and dissociation phenomena [l-3]. However, it was not until Jennings [4] and McLafferty et al. [5] independently demonstrated that CID is uniquely valuable for analytical pur- poses that tandem mass spectrometry or mass spectrometry/mass spectrometry (MS/MS) emerged as one of the most important techniques of modem analytical chemistry [6]. It is note- worthy that the mechanism of decomposition of benzene cations was one of the problems investi-

016%1176/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO168-1176(97)00159-6

238 R. Chawla et al. /International Journal of Mass Spectrometry and Ion Processes l&5/166 (1997) 237-247

gated in Keith Jennings’ pioneering CID study [4]. the benzene system is that there are at least 10 We reproduce in Fig. 1 a summary from this and isomeric C,Hl isomeric structures accessible an earlier paper [7] on the metastable dissociation below the lowest energy dissociation limit for the reactions which he established as the principal benzene cation [lo]. This introduces the possibil- mechanisms thought to be responsible for the ity of many ‘hidden isomerization’ steps which mass spectrum of benzene. There are four pri- may be important in the dissociation of benzene mary reactions - breaking a C-H bond, elimina- cations. There are analogous structures and elec- tion of H,, concerted elimination of acetylene and a high energy reaction to form C,Hl - plus

tronic energy levels for the fragment ions and

a plethora of secondary reactions. The first of neutral dissociation products, leading to an ex-

these could not be studied in our particular tremely complex array of microscopically distinct

crossed beam apparatus for technical reasons; the reaction paths. The barrier height to the lowest

reaction dynamics of the latter two primary reac- decomposition channel is at least 3.88 eV, giving

tions are the subject of this paper. rise to a very large ‘kinetic shift’, of the order of 1.7 eV, for the rate to exceed lo6 s-r [ll].

One of the central questions in the unimolecular decay of excited benzene ions - intimately connected to the question whether the RRKM/QET theories [8,9] adequately account for its mass spectrum - is the participation of electronic states in the dissociation mechanism depicted in Fig. 1. Rosenstock et al. [lo] pointed out that vertical ionization of benzene leads to very little excitation of the ground electronic state of the benzene cation. Consequently, even for the lowest energy fragmentation processes, electronic excitation of higher states is the operative mode of energy deposition when benzene fragment ions are investigated. Another intriguing feature of

The richness of the ion chemistry of benzene has made it a controversial subject of many investigations, many of which are related to the special characteristics of the molecular ion described above. It now appears to be well-established - based primarily on photoelectron-photoion-coin- cidence (PEPICO) studies of Baer et al. [12], the multiphoton ionization study of Kuehlewind et al. [131, and the combined experimental and theoretical studies of Bowers et al. [14] and Klippenstein et al. 1111 - that all four primary dissociation reactions are in competition with each other and are well-described by statistical theories. Evi- dently the initial details of energy deposition into different electronic states is irrelevant to the unimolecular ion chemistry, which takes place on the ground state hypersurface. In their transition state switching treatment of benzene cation dissociation Bowers et al. [141 calculated the density of states of the reactant ions using a combination of the two most stable C,H,+ structures, benzene and fulvene. The other authors have considered only the ground electronic state of the benzene cation. The lowest energy structures for the product ions C,H: and C,HT are cyclical and theoretical calculations have focused on the low energy channels. Traditional tight transition state, the transition state switching model, phase space, a fully optimized variational phase space calcula- tion and the thermodynamic-based Arrhenius model of KIots have been applied to these reactions with considerable success [153. Details of activation entropies and threshold energies are

I f

Fig. 1. The dissociation pathways of the benzene molecular ion as determined from metastable and collision-induced dissociations. Adapted from [4] and [7].

R Chawla et al. /Intemational Joutnul of Mass Spectrometty and Ion Processes 165/ 166 (I 997) 237-247 239

model-based and different for each of these ap- proaches. However, all of these recent treatments are in good agreement with each other and are an adequate framework for discussing rates of CID reactions.

We have applied the crossed molecular beam method to the investigation of the dynamics of CID of the benzene molecular ion at low collision energies. We attempted to investigate the four primary dissociation pathways

C,H,f +X-+C,H: +H+X

+ C,H,f + H,(or 2H) +X

--$ C,H,f + C,H, +X

+ C,H; + C,H, +X

(1)

(2)

(3)

(4)

However, severe interference by the wings of intensity of primary ion beam and metastable dissociations (broadened peaks due to kinetic energy release require increased voltage ramp for energy analysis which overlaps with the kinetic energy of the parent ions) prevented us from characterizing the relatively low cross-section reactions (1) and (2). We have therefore concen- trated our efforts on the ring opening reactions (3) and (4), leading to C,H,f and C,Hf fragment ions, respectively.

2. Experimental

The crossed-beam tandem mass spectrometer used for the present study has been described in detail elsewhere [16] and only the salient features are given here. Benzene molecular ions produced by 70 eV electron ionization are accelerated to 3 keV for energy and mass analysis and decelerated to desired laboratory energy by a series of cylin- drical and rectangular tube lenses. The low energy benzene ions collide with a vertically moving supersonic molecular beam of neat argon or helium generated by expansion through a 100 mi- cron pin-hole nozzle and passing the central core through a l-mm diameter aperture skimmer and a collimating chamber. After collision, the fragment ions are decelerated/accelerated by a linear-field lens for transmission and energy analysis

by a hemispherical energy analyzer and mass analysis by a quadrupole mass filter. Ions are detected by a Channeltron electron multiplier operated in pulse counting mode. The detector assembly of linear-field lens, energy analyzer, quadrupole mass filter and the electron multiplier is mounted on a rotator platform to rotate it with respect to the collision center to determine intensity and energy distributions of fragment ions as a function of laboratory scattering angle. The supersonic molecular beam is chopped by a tuning fork chopper for signal averaging and subtracting the contributions from metastable and back- ground CID contributions.

The kinetic energy distributions measured at a series of laboratory scattering angles are converted into velocity distributions using the rela- tionship E = l/2 mr~*. These velocity distributions in polar coordinates are transformed into Cartesian coordinates using appropriate Ja- cobians which have been described elsewhere [17,18]. The transformed distributions are plotted at each angle and drawn into contour plots by joining points of equal intensity. A Newton diagram with ion and neutral velocity vectors and appropriate center-of-mass (CM) is superimposed on this contour plot for discussing the underlying mechanism of energy transfer and scattering behavior. Since these plots do not represent all the scattered intensity due to limited detector accep- tance as scattering angle increases, it is necessary to integrate the intensity over all scattering angles using the sinx factor to obtain relative cross-section especially when more than one pathway contributes to the total reaction. Relative probability of energy deposition (product relative translational energy distribution) is obtained using the rela- tionship 1191

P(T) = 2mn,(m,M)-’ UJPc(U,,U2,U3) sinx.dx (5)

where U is the magnitude of the product velocity vector with respect to the CM, x is the CM scattering angle and Pc(Ul,U2,U3) is the probability density of finding the velocity of the product

240 R Chawla et al. /International Journal of Mass Spectrometry and Ion Processes 165/ 166 (1997) 237-247

ions defined by Cartesian coordinates infinitesi- mally close to the point defined by the CM velocity vector U,,U,,lJ,, M is the total mass of the two reactants, m, is the mass of the fragment ion and md is the mass of the neutral fragment. This also provides us with a distribution of energy transfer probability and the most probable energy transfer from translational into internal modes which is drawn as a circle in the CM velocity contour plots discussed here.

3. Results and discussion

3.1. C,H,+ formation

Fig. 2 presents as a velocity contour diagram the differential cross-section for forming the C,H: product ion at a relative collision energy of 4.9 eV in the center-of-mass reference frame. This energy is approximately equal to the appearance energy of this product ion in the mass spectrum of benzene. In this presentation the laboratory velocities of reactants and products are sup- pressed and only the relative velocity vector is displayed. The zero velocity point is the tip of the CM velocity vector for the collision of benzene ions with helium. Displaying contours of constant relative intensity of products in this velocity framework implicitly assumes that energy transfer involves inelastic scattering of benzene cations in the collision. Using the measured intensity of the product ion, C,H:, to map the intensity of scattered and internally excited benzene ions in such a diagram implicitly assumes that the velocity of the product ion is nearly the same as its benzene ion precursor. Provided the kinetic energy of fragment ions in the dissociation step is small (or that the mass of the ion is much greater than its corresponding neutral fragment) this kind of representation of CID scattering data is a reliable measure of the dynamics of collisional activation of the ion. The usual assumptions of two-body scattering mechanics described in detail elsewhere [18] is also assumed.

We use as a reference for discussing energy deposition in these contour plots the elastic scattering circle for the benzene cation, labeled ESC in Fig. 2. Positive velocity denotes the initial

4.9 eV COHN + He + [CSHi]* + He

[CgH;]* --+ C4H+4 + CZHZ

-1000 -500 0 500 1000 m/set CM

Fig. 2. CM velocity contour plot for the CID of benzene molecular ions to C,H: in collision with helium at 4.9 eV collision energy. The circles marked ESC and AT= -4.5 eV correspond to the elastic scattering circle and energy transfer of 4.5 eV into internal modes of the benzene ions, respectively. The numbers marked on contours represent relative intensities of each contour.

velocity of the benzene cation, about 770 m/s in the present experiment. Full back-scattering of the reactant ion with no conversion of translational energy into internal energy of the benzene ion and with translational acceleration of the He atom (forward-scattering of the neutral) would appear as a velocity of N 770 m/s. The ESC defines the full elastic scattering circle in the plane of the collision. The contours of constant relative intensity of product ion velocity vectors are plotted, with inelastic scattering which de- composes the benzene ion falling generally within the ESC. The maximum intensity is bounded by the contour labeled 0.9, representing the normal- ized 90% relative intensity contour line. Contours are shown for the indicated relative intensities, with the 10% line defining the region where the signal is barely distinguishable above noise. Since relative velocity is readily converted into kinetic energy of the scattered benzene ion, and since the change in translational energy of the reactant ion and neutral in the scattering process must equal the internal energy deposited in the benzene ion by the scattering process, these contour plots provide a quantitative measure of the probability of depositing a given amount of internal energy at each scattering angle. The ordinate of Fig. 2 is a symmetry axis for scattering processes

R Chawla et al. /International Journal of Mass Spectrometly and Ion Ptvcesses 16S/ I66 (1997) 237-247 241

and the dynamics are fully determined by map- ping the scattering hemisphere depicted in Fig. 2.

From these considerations and the Fig. 2 contour diagram we can describe the dynamics of collisional activation and dissociation of benzene ions at a relative collision energy of 4.9 eV as exhibiting a most probable scattering angle of the order of 100 f 20” with a most probable energy deposition of about 4.5 eV. This is indicated in Fig. 2 by the inner circle in the contour diagram which is marked AT = - 4.5 eV. At this collision energy a rather broad distribution is observed, with significant intensity in both the forward- and backward-scattered sectors of the diagram. The energy deposition is approximately equal to the threshold energy of the reaction, 4.6 eV and is slightly less than the expected appearance energy - the sum of threshold energy and kinetic shift to achieve a rate of the order of lo6 s-l. The latter value corresponds to the rate of ion dissociation which is required for excited ions to decom- pose in the field-free collision region in our apparatus between the - 2-mm diameter neutral beam and the entrance plate to the linear field acceleration lens which injects ions into the energy analyzer and product ion mass filter. Only those ions dissociating in this region are observable as product ions in our experiments. The maximum scattering around 90” is expected for hard sphere collisions and we may therefore conclude from Fig. 2 that the energy transfer mechanism is impulsive.

The data in Fig. 2 illustrate both the distin- guishing features of the crossed-beam method for investigating the mechanism of CID reactions and the merits of plotting data in CM coordinates. If it were tacitly assumed that only forward-scattering occurs, all the back-scattered intensity would be interpreted as highly endothermic reactions, perhaps as reactions proceeding on isolated electronic state hypersurfaces. It would also be difficult to make any statements about scattering angles in a collision chamber experiment and equally difficult to resolve the collision-induced component from the underlying metastable ion intensity.

We have carried out similar experiments at collision energies of 6.8, 10.2 and 17 eV. The

most probable energy deposition at 6.8 eV collision energy is AT = -5.6 eV and the most probable scattering angle shifts forward to about 69 &- 10”. As collision energy increases to 10.2 eV, the translational endothermicity shifts to AT = -6 eV and the most probable scattering angle closes to about 50 + 5”. The 17 eV experiment is plotted in Fig. 3. No further shift in translational endoergicity is noted. The most probable scattering angle has shifted to about 17” and the peak of intensity is now fairly well defined in the forward scattering region. As in Fig. 2, the ESC defines the elastic scattering velocity vector space of the benzene reactant ion. At 17 eV collision energy (N 50 eV LAB) the dynamics of benzene ion dissociation to C,H: is well described as gener- ating a cone of intensity which is scattered about the original velocity vector of the ion with a most probable CM scattering angle of 17 + 3” and an energy shift of 6 eV. Significant angular scattering is characteristic of impulsive mechanisms, with the momentum-exchanging collision governed primarily by the repulsive interactions of the benzene cation with He or Ar neutrals. The gradual closure of scattering angle with increasing collision energy indicates that larger impact parame- ter collisions provide sufficient energy to drive the reaction. A line-of-centers model is capable of rationalizing this result [20].

The differential scattering presentation of Fig.

17 ev C6H+6 + Ar W [CsHi]’ + Ar

[C&l*- C4Hq+ + CZHZ

CM 1000 2000 3000 4000 m/set

Fig. 3. CM velocity contour plot for the CID of benzene molecular ions to C,H: in collision with argon at 17 eV collision energy.

242 R Chawla et al. /IntemationalJoumd of Mass Spectrometty and Ion Processes 165/166 (1997) 237-247

2 and Fig. 3 provides a good description of the reaction dynamics in the plane defined by the intersecting ion and neutral beam containing the relative velocity vector. It is a somewhat distorted representation of the relative importance of different scattering angles insofar as the total cross- section for reaction is concerned and may provide a distorted view of energy deposition. A first-order correction is made by integrating the contour diagram over scattering angles using Eq. (5). Since the leading term of this equation is sinx, where x is the CM scattering angle in the Cartesian velocity contour diagram, the impulsive character of benzene cation collisional activation is immedi- ately apparent. Especially for the lowest collision energy experiment in Fig. 2, the 100” scattering, pseudo-hard sphere, excitation mechanism is clearly dominant.

The P(T) diagram corresponding to the 17 eV scattering experiment depicted in Fig. 3 is shown as Fig. 4. Here we have integrated over scattering angles and velocities to obtain an overview of energy deposition function in CID reaction (3). The zero energy on this scale is the kinetic energy of the reactant ion and translational endoergicity is shown as a negative energy shift in this diagram. Integration over all scattering angles has shifted the apparent most probable energy deposition to about - 6.5 eV, almost the same value deduced in Fig. 3. The distribution is rather broad, extending from an energy shift of about - 1 eV to - 11 eV (at the 20% relative intensity levels). This distribution is the convolution of energy spread in the reactant ion beams, the distribution of energy deposition responsible for the activation of the ion above its dissociation limit, and kinetic energy release in the recoil of C,H: and acetylene neutral from their common CM, the energized benzene cation. Because of the multi- plying effect of the velocity of the recoiling cation, the latter factor often contributes significant broadening to P(T) diagrams for polyatomic ion CID reactions.

If we assume that the kinetic energy release in the CID reaction is similar to the kinetic energy release of the lowest energy unimolecular dissociation measured for metastable ions we can further analyze the energy deposition function given

C4H: (17 eV)

1.00 1 73

/\

1 \ 2.7eV

AE(eV)

Fig. 4. Relative translational energy distribution, P(T), of C,H: ions from CID of benzene molecular ions at 17 eV collision energy. The horizontal bar marked 2.7 eV shows the width of the distribution that is due to the kinetic energy release of 28 meV measured from metastable dissociation.

in Fig. 4. The kinetic energy release (FWHM, full width at half maximum) measured for metastable reaction (3) is 28 meV [21], which in the transla- tionally-excited benzene CM coordinate frame accounts for 2.7 eV width in the Fig. 4 P(T) diagram. This is shown as the horizontal line in Fig. 4. Combining this with the energy width of the primary ion beam rationalizes about 3 eV of the observed distribution and leads to the conclusion that the energy deposition function has a FWHM of about 5 eV and a total width of about 7 eV.

It follows from this analysis that a moderate range of impact parameters and a relatively broad range of energy deposition characterize the CID of this reaction of the benzene ion. This conclusion is entirely consistent with the generally accepted description of the unimolecular dissociation mechanism of this ion, in particular, the PEPICO studies of Baer et al. 1121 of benzene

R Chawla et al. /International Journal of Mass Spectrometry and Ion Processes 165 / 166 (1997) 237-247 243

and several of its structural isomers. At low pho- ton energy, up to about 15.5 eV (corresponding to internal energy of 6.25 eV in the parent benzene ions), competitive dissociation of the primary reaction channels l-4 was observed. At higher energy a new mechanism for C,HT formation was noted, plausibly interpreted as a reaction which generated linear rather than cyclic C,H4+. Dif- ferent behavior of different C,H,f structures was also noted and was interpreted as the onset of dissociation prior to intramolecular vibrational relaxation to the ground electronic state. When the system is limited in internal energy, dissociation consistent with statistical theories is observed, with all dissociation occurring from the ground state hypersurface. As energy content is increased the richness of the electronic and structural possi- bilities for benzene cations opens additional channels for dissociation. The features of our experiments which are consistent with this description is the lower energy shift observed in the Fig. 2 experiments, the shift to higher average energy transfer as collision energy is increased and the broad range of energy deposition at all collision energies.

The width of the energy deposition function can be understood from a consideration of internal energy content in the reactant ions and the breakdown graph of the benzene cation [22]. The internal energy of benzene ions formed by electron impact is limited by the dissociation energy of the lowest energy decomposition path, reaction (1) and by th e 1 ength of time it takes the ion to reach the crossing point of our molecular beam apparatus. This ‘aging’ of the ion beam takes about 45 ~LS and one can estimate from Fig. 8 of reference [ll] that the internal energy of the reactant benzene cations extends from zero to 5.1 eV. From Fig. 6 of reference [13] the rate of reaction (3) exceeds lo6 s- ’ for benzene ions with 5.6 eV internal energy. The minimum energy shift expected for our experiments is therefore predicted to be about 0.5 eV for the highest internal energy benzene ions in our reactant ion beam. The maximum energy shift to access the lowest energy (cyclic C,H:) pathway would be 5.6 eV for ions which are not vibrationally excited. In our lowest energy experiment the energy

shift is from less than 1 eV up to the full collision energy of 4.9 eV. As energy is increased, additional channels open and the maximum energy transfer measured increases (e.g. to 8-9 eV in Fig. 4). We can describe the CID energy transfer mechanism as having a most probable energy deposition of 6.5 eV with a very broad range of energy deposition. Taking into account the addi- tivity of internal energy already present in the reactant ion beam a range of internal energies in fragmenting benzene cations ranges from 6-14 eV.

3.2. C, H3+ formation

As in our study of reaction (31, we attempted to investigate reaction (4) at a CM collision energy approximately equal to the appearance energy for formation of this ion in the mass spectrum of benzene. However, the cross-section for this process was too small for us to acquire meaning- ful data for the reaction dynamics at this low energy. The lowest energy experiment for which we could construct a contour plot was for 10.2 eV CM collision energy. The results of this experiment are shown in Fig. 5. As before, we utilize the ESC as the reference for translational endoergicity and establish that the most probable energy transfer into benzene ion to drive the endothermic reaction (4) is 5.5 eV at this collision energy. This is indicated in Fig. 5 by the inner circle labeled AT = - 5.5 eV. The most probable scattering angle is about 25”, corresponding to an impulsive collision mechanism. The width of the distribution is slightly broader than we observed for C,H: formation at the same collision energy.

Both the small CID cross-section for this channel at low energy and measurement of an energy transfer of about 1 eV greater than the appearance energy for the lowest energy reaction channel are consistent with the Dawson et al. study of the same reaction utilizing a triple quadrupole mass spectrometer [23]. These workers reported translational endoergicity which exceeds the thermochemical value by 0.9 eV for their experiments over a range of CM energy up to about 20 eV. They utilized a retarding potential method to measure the kinetic energies of reactant and

244 R Chawh et al. /International Journal of Mass Spectmmmy and Ion Processes 165 /I66 (1997) 237-247

+ 10.2eV CgH6 + Ar w [CsH6]+*+ Ar

[CsHs]+k C3H; + C3H3

AT= - 5.5eV \

0 CM 1000 2000 3000 m/set

Fig. 5. CM velocity contour plot for the CID of benzene molecular ions to C,Hf at 10.2 eV collision energy.

product ions and deduced energy transfer in the CM reference frame. They investigated several collision gases, ranging from monatomics through small polyatomics and found the energy shift exceeded the expected thermochemical value by about 1 eV for all collision gases investigated. They concluded that the linear C,Hl isomer, the propargyl cation, which is about 1 eV less stable thermodynamically than the cyclic cyclopropenyl cation, is the most easily accessible CID product. They further suggest that there is a barrier to the formation of the lowest energy isomer but no barrier to the formation of the linear isomer, which requires only ring opening and fragmentation of the benzene ring for its formation.

Figure 6 reports our experimental results for forming C,Hl at 17 eV collision energy. The most probable energy transfer remains at -5.5 eV, unchanged from our 10.2 eV experiment. The most probable scattering angle has shifted to 13”, slightly less than for reaction (3). The smaller scattering angles correlate with smaller energy shifts if the excitation mechanism is impulsive and the two reactions are competing decomposition pathways. These characteristics mirror related features in their unimolecular decay proper- ties, in which cyclic ions are produced at threshold but linear structures are formed at about 1 eV higher. We infer that CID reactions, which must occur with rates exceeding lo6 s-l to be observed as such in our scattering apparatus, se-

17eV C6Hfg + Ar + [C6H$]* + Ar

[C&l* - C3H3+ + C3f$

ESC

CM 1000 2000 3000 4000 m/set

Fig. 6. CM velocity contour plot for the CID of benzene molecular ions to C,H; at 17 eV collision energy.

lectively sample the higher energy, linear structures, dissociation paths.

As described above, velocity contour diagrams are invaluable for deducing energy transfer mechanisms but intrinsically distort the relative contributions of forward and sidewards-scattering to the overall reaction dynamics. Energy transfer, in particular, can best be interpreted through the use of Eq. (5) to integrate contour diagrams over all scattering angles and product velocities. Fig. 7 reports the P(T) diagram corresponding to the Fig. 6 contour diagram for CID of the benzene cation at 17 eV collision energy. As before, we illustrate in Fig. 7 the width of the distribution which is attributable to recoil kinetic energy of the fragments, assuming that the kinetic energy measured for metastable ion dissociation, 30 meV [18], can be used to estimate this contribution to the rather broad P(T) distribution shown in Fig. 7. Because of the mass effect in the CM/LAB conversion, the contribution of a very similar kinetic energy release to the Fig. 7 P(T) diagram is larger than in Fig. 4. However, the contribution of the energy spread in the primary ion beam is slightly less, leading to a similar contribution of about 3.9 eV of these factors to the width of the distribution in the Fig. 7 P(T) diagram. This leads to the conclusion that the average energy deposition is 5.5 eV at this energy but the width of the distribution extends from 1 to 10 eV.

Within our experimental error the P(T) diagrams in Figs. 4 and 7 are essentially identical,

R Chawla et al. /InternationalJournal of Mass Spectmmetty and Ion Processes 165/ 166 (1997) 237-247 245

C3Hi (17 eV)

1.0 -

i= z

0.0 I 010 I I I I I

-20.00 -15.00 -10.00 -5.00 0.00

AE(eV)

Fig. 7. Relative translational energy distribution, P(T), of C,Hl ions from CID of benzene molecular ions in collision with argon at 17 eV collision energy. The horizontal bar marked 3.9 eV corresponds to the energy broadening due to kinetic energy release of 30 meV in the metastable dissociation process.

consistent with the similarities noted in the contour diagrams and their energy dependence. Con- sequently the same remarks apply about the good correspondence between our CID experiments and the most recent statistical theory compar- isons with high resolution photoionization experiments. Our experiments are carried out with benzene cations formed by electron impact in the ion source with a broad range of internal energies, up to 5.1 eV. The threshold energy for their CID excitation above the dissociation limits for reactions (3) and (4) is about 1 eV, consistent with RRKM calculations and experimental k(E) curves which demonstrate that benzene ions with internal energies greater than 5.7 and 5.9 eV, have unimolecular rates exceeding lo5 s-i for reactions (3) and (4), respectively. Thus threshold energy shifts for our CID experiments are predicted to be 0.6 and 0.8 eV, corresponding quite well to the - 1 eV minimum energy deposition deduced from our P(T) diagrams. The average energy shift - perhaps coincidentally - corresponds to activation of a population of relatively

low internal energy ions to the dissociation cont- inuum.

The cutoff in the P(T) diagram at about 10 eV corresponds to a very large energy shift; consider- ing that benzene molecular ions in our reactant ion beam contain as much as 5.1 eV internal energy, the excitation in dissociating benzene cations must range from 6-16 eV. This range of internal energy is difficult to rationalize within a statistical model if only the lowest energy dissociation mechanisms are considered. Carry over of internal energy into the ion product would be sufficient to dissociate it further, according to the Fig. 1 mechanisms. The richness of the ion chemistry of benzene pointed out in Sec. 1 leads to the plausible interpretation that the number of dissociation mechanisms accessible to benzene cations increases dramatically above 6 eV internal energy. Several papers have reported that additional mechanisms, which cannot presently be rational- ized within the framework of statistical theories, open at higher energies [10,12,24]. These new channels can be ascribed to dissociation occurring more rapidly than energy randomization to the ground state or to dissociation occurring on elec- tronically excited hypersurfaces. Except for se- mantics these may be equivalent statements, al- though the observation of most probable energy shifts of the order of 6 eV and preferred scattering angles could plausibly be interpreted as evi- dence for electronic excitation in the collision process. In any case, the rapid increase in phase space accessible to dissociation of highly excited benzene rationalizes the large energy shifts and broad distribution of energy shifts measured by our experiments.

4. Conclusions

Our crossed-beams investigations of the dynamics of CID of the benzene cation have established that the energy transfer mechanism is impulsive over the range of collision energies investigated, which range from relative energies of 4.9 eV to 17 eV. At 4.9 eV, product ions are predominantly sidewards-scattered, with the most probable scattering angle of the order of 90”. The

246 R Chawla et al. /International Journal of Mass Spectmmetry and Ion Processes 165/166 (1997) 237-247

distribution of velocity vectors is very broad, ranging from forward-scattered to backward-scattered and with velocity shifts corresponding to translational endoergicities ranging from an eV or less up to the full collision energy. With increasing collision energy the preferred scattering angle moves forward, consistent with the notion that an increasing range of impact parameters can now deposit sufficient energy to cause the ion to dissociate. At the highest energy investigated, 17 eV CM, the most probable scattering angles remain quite large, 17 + 3 and 13 + 2” for C,Hi and C,H:, respectively. Simplistic models which cap- ture these concepts are ‘hard sphere’ scattering at low energy and ‘line-of-centers’ energy deposition [20]. However, the efficient transfer of large amounts of energy and substantial angular scattering which is observed imply very strong interactions of translational energy with internal modes of the polyatomic cation - e.g. ‘soft sphere’ and ‘sticky’ collisions are better descriptors of the energy transfer mechanism.

The amount of energy transferred is very large and only roughly correlated with the ther- mochemistry of the lowest dissociation energy channels. At 4.9 eV the energy shift to generate C,H: approximately equals the appearance energy of this fragment ion in the mass spectrum of benzene. However, as soon as the collision energy is increased the energy deposition increases by the order of 1 eV. For C,Hl formation the cross-section was too small for us to investigate its dynamics under conditions where the collision energy provided a restriction to energy transfer, forcing the reaction to explore the lowest energy product ion channel. Possibly because of this arti- ficial constraint on our experiments, the average energy deposition exceeded the thermochemical appearance energy expectation at all energies where measurements could be made.

For both reaction channels the most probable energy transfer in the collisional activation step corresponded to the population of known higher energy reaction pathways in the unimolecular reaction ion chemistry of benzene cation - namely, to ring-opening and formation of linear reaction products. This result was unexpected, since our reactant ions have internal energies ranging from

zero to about 5.1 eV. However, the range of energy deposition in our experiments was shown to be very broad. Energy transfer thresholds were estimated to be of the order of 1 eV for both reaction channels, corresponding within experimental error to RRKM theory predictions for the energy required to cause the highest internal energy components of the reactant ion beam to have dissociation rates exceeding lo6 s-l. The average energy transfer is understood as adding about 5 eV to vibrationally excited ions, which rapidly dissociate via reaction mechanisms other than the lowest energy product ion channels. These rates become too rapid for energy relaxation to the ground state and may be described as mainly non-ergodic reactions. We speculate that the highest energy transfer suggested by our analysis of the data, AT = 10 eV, results in the population of high-lying electronic states of the benzene cation. The weak kinetic energy dependence of the most probable energy depositions and relatively well-defined most probable angular scattering at each collision energy also support the hy- pothesis that energy transfer predominantly involves electronic excitation.

Acknowledgements

The continuous support of our dynamics studies of CID by the National Science Foundation (Grant No. CHE-9021014) is gratefully ac- knowledged. We also acknowledge with particular pleasure various helpful and stimulating discus- sions on CID phenomena with Keith Jennings over the years.

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collision-induced dissociation of low energy benzene ions

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