Unimolecular and collision-induced dissociation study of CS2+2 with Ar at high collision energy

Mass Spectrometry and Ion Processes

E L S E V I g R International Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62

Unimolecular and collision-induced dissociation study of CS 2+ with Ar at high collision energy

X.D. Zhou, A.K. Shukla, R.E. Tosh, J.H. Futrell*

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

Received 21 May 1996; accepted 1 July 1996

Abstract

Mass-analyzed kinetic energy spectroscopy (MIKES) has been used to study the unimolecular and collision-induced dissociation (CID) of CS 2+ ions in collision with argon at 6 keV ion energy. When analyzed in center-of-mass (CM) co-ordinates, MIKES spectra of CS + and S ÷ fragment ions are shown to originate from four dynamically distinguishable reaction pathways. They may be broadly classified as electron-capture-induced dissociation (ECID) and collision-induced charge separation dissociation (CICSD). Detailed analysis of the kinetic energy lost by CS ~÷ ions in forming product ions shows that S ÷ fragments originating from ECID are produced from ground state dications, whereas CS ÷ fragments originate predominantly from an excited state, probably the first or second electronically excited state of CS2 z÷. Two energetically distinct CICSD processes which result in pairwise formation of CS ÷ and S ÷ have been characterized, with recoil kinetic energies of 3.4 eV and 4.1 eV and nearly zero energy transfer from translation into internal modes. The lowest energy set has the same energy release as the unimolecular metastable decay process for electron-impact-generated CS~ + ions, suggesting that the activation step involves mainly those ions that are internally excited near the dissociation threshold prior to collisional activation. It is proposed that the low kinetic energy release mechanism is "re laxed", proceeding on the minimum energy pathway from reactants to products, whereas the higher kinetic energy release mechanism is a "sudden" dissociation in which decomposition occurs from a surface in which one of the bond distances is " f rozen" at or near its equilibrium position and decomposition occurs very rapidly. These experimental observations are supported by ab initio quantum chemical calculations which rationalize quantitatively the energetic consequences of following relaxed and sudden reaction paths. The minor products S~ and C ÷ are also formed by ECID and CICSD processes, but were in too low abundance to be fully characterized. © 1997 Elsevier Science B.V.

Keywords: MIKES; Reaction pathways

1. Introduction

Collision-induced dissociation (CID) of multiply charged ions in tandem mass spectrometers has assumed great importance recently as an ana- lytical technique for characterizing quasi-molecular ions of high molecular weight compounds generated by electrospray ionization [1-4]. In

* Corresponding author.

this "sof t" ionization technique there is very little initial fragmentation of molecular ions, and CID is commonly used to fragment these ions to generate the desired structural information. Although dissociation mechanisms are similar (e.g., formation of characteristic sequence ions from peptides [59, a coulombic barrier is expected to strongly affect the energetics and CID dynamics of multiply charged ions. As almost no CID dynamics information exists for

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50 X.D. Zhou et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62

multiply charged ions, we have initiated an experimental study of these phenomena. CS 2+ was chosen as a starting point, with the expecta- tion that it would be possible both to establish detailed mechanisms for CID of such a simple dication and to deduce principles which would apply to larger ions generated by electrospray ionization.

The dissociation of CSz z+ proceeds via two pathways: coulombic explosion, or charge separation dissociation (CSD), and electron- capture-induced dissociation (ECID) [6]. The major reaction products are S ÷ and CS ÷, which are formed by the following reactions

CS 2+ ---, CS ÷ + S + E = - 0 . 6 e V (1)

CS 2+ +Ar ---* CS ÷ +S + +Ar E-- -0 .6 eV

CS 2÷ +At ---, CS~* +Ar +

(2)

E = - I . 5 e V (3a)

reactions Eqs. (4)-(6).

CS 2++Ar---* S ] + C ÷ + A r E - - - 0 . 4 e V (4)

S~" + C + Ar E -- 5.5 eV (5)

"--* C + + S 2 + m r E -- 6 . 1 e V (6)

This report summarizes our experimental studies of these dissociation reactions for 6 keV CS 2+ ions colliding with Ar. The metastable reaction Eq. (1) provides baseline data for evaluating energy transfer in the bimolecular reactions Eqs. (2)-(6). The major CID reaction channels Eq. (2) and Eqs. (3) are analyzed in detail. A limited data set is reported for S~ ions formed in reactions Eq. (4) and Eq. (5); the cross section for this process is very small and the cross section for C ÷ formation, reaction Eq. (6), is too small for detailed analysis.

CS 2+ +Ar ---* S + + C S + A r + E- -3 .3eV

(3b)

CS 2+ +Ar ----, CS + + S + A r + E -- 4.3 eV

(3c)

Reaction Eq. (1) depicts the unimolecular CSD reaction observed as a metastable ion, and reaction Eq. (2) describes the collision-induced charge separation dissociation (CICSD). Reac- tions Eqs. (3a,b,c) describe the formation of CS~, S ÷ and CS ÷ by electron capture (EC) and ECID, respectively. A two-step reaction sequence is envisaged for ECID: charge transfer with Ar, followed by the dissociation of excited CS~ ions. Reaction Eq. (3) refers explicitly to formation of CS~ with too little internal energy to dissociate, and reactions Eq. (3) and Eq. (3) delineate those formed with internal energy exceeding the respective bond energies. Reaction energies are quoted for ground states of reactants and products [7].

The minor reaction products S~ and C ÷ are formed by the analogous CICSD and ECID

2. Exper imenta l

All results were obtained on a reversed-geometry, double focusing (VG ZAB-2F) mass spectrometer, using the technique of mass- analyzed ion kinetic energy spectrometry (MIKES) [8]. CSz z÷ ions were generated using 70 eV electron impact ionization and accelerated to 6 keV nominal ion energy for mass selection by the magnet. Reactant ions were focused into a 20 mm long collision cell, where Ar gas was introduced for collisional activation. Kinetic energies of the product ions were measured by scanning the electrostatic energy analyzer utiliz- ing a Digital Equipment Corporation's VAX 3100 workstation and VG's OPUS operating system. The collision gas was maintained at low pressure (corresponding to --~ 20% attenuation of the primary ion signal intensity) to ensure that CID occurred under single collision conditions. The collision cell was floated electrically in some cases to distinguish between collision-induced reactions occurring within the cell from those

X.D. Zhou et aL/International Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62 51

occurring in the field-free region (metastable and background gas CID).

It was necessary to calibrate the energy scale scanned by the computer at several points to achieve acceptable accuracy over a range which is approximately twice the normal range of MIKES scans. Three different methods were uti- lized for this purpose: (1) measurement of the kinetic energy of the two metastable fragment ions of the benzene molecular ion, C4H~ and C3H~, using the mass ratio relationship E~ = (m~ /mp ) x Ep to define their kinetic energies; (2) measurement of the energies of the electron capture peaks from C2+/I-Ie and Ar2+/Ar collisions, both of which have been very well characterized by several earlier studies [9,10]; and (3) using the metastable reaction Eq. (1) as a self calibrant. These calibration measurements were made at several acceleration voltages to ensure linearity and reproducibility of calibration constants.

To estimate total collision and CID cross sections, we examined charge transfer in the Ar+/Ar

system and used the known cross section for this reaction [11] to establish an effective pressure (number density times path length) for our collision cell. It was also used to calibrate and correct pressure readings from an ion gauge which is located just above the diffusion pump.

3. Ab initio molecular orbital calculations

All-electron SCF molecular orbital calculations of the potential energy surface of the ground electronic state of CS 2+, CS ÷, and S + were performed using the CAUSSlAN 94 program [12]. First calculations were performed by unrestricted Hartree-Fock (UHF) procedures using the D95"* (Dunning/Huzinaga full double zeta) basis set and adding the polarization function. These calculations were further extended using second-order M011er-Plesset correlation energy correction (MP2). Two types of energy surface scans were calculated: (1) a fixed potential surface, by fixing one C-S bond length at the

CM R e p r e s e n t a t i o n o f LAB D a t a

I I / ~ . ,

It t tt "' ',\\ t / /

\ - ~ / j " y

Fig. 1. CM representation of CID via coulombic explosion of CS~ ÷ at 6 keV laboratory energy. The circle marked ESC represents the spherical limit for the distribution of fragment ions when there is no energy transfer in the collisional activation step. Solid circles marked 3.4 eV and 4.1 eV represent the spherical distribution of fragment ions with CM 2 as the center when there is no energy transfer in the activation step and 3.4 and 4.1 eV energy is released from internal modes into the translational mode in the dissociation step. The small circles at the junction of the relative velocity vector and these spheres show the CID peak positions that would be observed in the experimental measurements. The dashed circle marked 4.0 eV corresponds to transfer of 20 eV kinetic energy into internal modes and release of 4 eV from internal to translational mode.

52 X.D. Zhou et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62

optimized minimum energy geometry and scanning the other, and (2) a relaxed potential surface which adjusted the non-dissociating C-S bond at every point of the calculation. Calculations for the first excited state of the CS 2+ dication were done using the same basis set and MP2 method.

4. Data analysis

In several publications, including a recent review article [13], we have discussed the value of presenting data from reaction dynamics inves- tigations of collision-induced ion dissociation processes in center-of-mass (CM) co-ordinates. This general procedure is extended in this article to include unimolecular and bimolecular reactions of multiply charged ions. Fig. 1 presents an example of collisional activation of an ion which subsequently decomposes in a "coulombic" explosion in which the strong repulsion of two fragments which are positively charged gen- erates spherical distributions of high energy fragment ions. Three hypothetical spherical explosions are depicted in the Fig. 1 plane as concentric and non-concentric circles to simulate the results anticipated by our experiments, which generally probe the experimental plane defined by the intersection of our reactant ion beam and a supersonic jet of neutral atoms. The present experiment involves high energy ions colliding with thermal Ar atoms in a collision cell with only zero scattering angle particles detected. The masses and velocities depicted in Fig. 1 were chosen to illustrate the collision of 6 keV CS ~+ ions with Ar as a framework for discussing experimental data for the present experiments. Because the velocities of particles before and after the collision event are much higher than we have typically used in our experiments, the full velocity contour diagram is mostly devoid of information. Accordingly, interrupted velocity vectors allow us to show the key reference points of such "Newton diagrams", together with the

features which will be analyzed to deduce energetics information for these processes.

The CM for the activating ion-neutral collision is shown at the left of the figure as "CMI" . To the right are shown " C M 2 " and "CM3", which are hypothetical post-collision velocities of the collisionally activated dication and the point of origin of the particles which separate from each other under the influence of Coulomb's law repulsion. The two CM values represent processes which have different activation energies and hence different values for conversion of translational energy into internal energy to promote the dissociation step. The two circles centered at CM 2 represent dissociations which have slightly different barrier heights but experimentally indistinguishable activation energies, or energy shifts; the example illustrated is collisional activation which converts 0 eV of translational energy into internal energy, followed by dissociations which are translationally exothermic by 3.4 eV and 4.1 eV, respectively. The circle with CM3 as center is translationally endothermic by 20 eV and releases 4 eV into recoil energy of the fragments. The first pair of mechanisms is shown as solid- line circles and the second as a dashed circle. The partial circle shown in Fig. 1 is the elastic scattering circle, or ESC, which we have described previously as the reference surface for deducing energy deposition (or energy release) in CID [131.

For MIKES experiments conducted at zero scattering angle with tightly apertured detector slits, a total of six peaks would be detected for the Fig. 1 hypothetical example; their maxima would match the points where the circles drawn in the figure cut through the relative velocity vector. The symmetric forward and backward scattered distributions of matching peaks for dissociations following the same mechanism would be diagnostic of the mechanisms illustrated in the figure. It is noteworthy that most of the scattered ions resulting from coulombic repulsion dissociation processes are not

X.D. Zhou et al.llnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62 53

detectable at any particular scattering angle, including zero. Only the "wings" of the distribution are detected; it is the limiting case example of a "dished top" CID peak. The origin is deduced from the symmetric spacing of forward and back scattered velocities about the CM2 and C M 3 origins by averaging the matching pairs of velocity distributions. It is also noted that the velocities of the post-collision-activated dication are nearly the same as the velocity of the reactant ion prior to collision. As usual, translational endothermicity is deduced from the shift of this origin from the CM velocity of the reactant ion prior to the collision. It should be noted that this figure does not include ECID mechanisms which would contribute additional peaks with a small

distribution of velocities offset from the initial velocity of the molecular ion by the translational endo- or exoergicity.

As discussed previously for singly charged cation CID [13], unimolecular (metastable) decay processes furnish an adventitious means for calibrating the energy scale. For the present experiments reaction Eq. (1), which is discussed in Section 5, provides just such a calibration procedure for CSD of CS 2+. The new feature in data reduction for such coulombic explosion unimolecular and CID processes is that the velocity of the metastable ion must also be deduced by averaging the forward and backward scattered product ion velocities. This defines the ESC in the kinetic energy range of the experiment, which is

LOOt

9si 9oJ 8sJ 8oi vs~ 70-

a Metastable c Charge Separation

Background ECID

.~ 6s.: b , j u l

60.

, I ~ 55.

5o- 4s-

> 40_" o ~

" 1 ¢~ 35 ~ 30

II~ 2 s

15

10

4'oh 'gi 'si~ :3s'oi~ :3s'so :~6'oD :~ds~ 3700 37s0 :~8'0i)' '

Energy (eV) Fig. 2. MIKES spectrum for the unimolecular dissociation of CS 2* to CS ÷ ions at 6.116 keV laboratory energy.

54 X.D. Zhou et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62

quite far removed from the energy scale in which the primary ion energy distribution is measured (this corresponds to an E/2E experiment as described in earlier literature [14]). As noted above, this is taken as the zero energy shift reference for deducing kinetic energy transfer into internal energy. Finally, we note that the forward and backward scattered peaks are symmetric in velocity, not in energy, reminding ourselves that we are doing vector analysis when we analyze experimental data in the CM framework shown in Fig. 1.

5. Results and discussion

5.1. Metastable and background CID

Fig. 2 shows a MIKES scan for the unimolecular dissociation of 6.116 keV CS 2+ ions to CS ÷ ions, reaction Eq. (1). The S ÷ ions give a very similar MIKES spectrum shifted in energy by the mass ratio. There are three peaks in this spectrum. The two side peaks marked a and c

are the forward and backward scattered CS ÷ ions from the CSD reaction Eq. (1) with no gas introduced into the collision cell. As anticipated, this coulombic explosion reaction gives strongly forward and backward scattered peaks. The center peak (b) was unanticipated. Its location near the centroid energy for reaction Eq. (1) suggests it is an ECID reaction product from collisions with background gases even though the measured base pressure in the flight tube is 8 × 10 -9 mbar. This is supported by two independent experiments: (1) plotting the intensity of these peaks as a function of flight tube pressure when Ar is added to the collision cell, as shown in Fig. 3, and (2) comparing intensity ratios for dissociation inside and outside the cell by applying a potential to the cell, as shown in Fig. 4.

The relative cross sections of reactions Eqs. (2) and (3) are given by the relative slopes of the two straight lines in Fig. 2. As shown in the figure, the large positive intercept and small slope demonstrate that reaction Eq. (1), the metastable decay process, accounts for at least 95% of the Fig. 2 intensity of peaks a and c when no collision gas is

1.4

¢=

..E .=

1.2

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

le-8 2e-8 3e-8 4e-8 5e-8 6e-8 7e-8

Pressure Reading (mbar)

Fig. 3. Pressure dependence of the relative intensities of the two CID process.

X.D. Zhou et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62 55

.=

I00~

9s]

85-

80- 7s- 70.

65.

a Metastable Charge Separation

C

Background ECID

::1 /" \ unshifted 5o shifted I, b /

20 a'

32 00

Energy (eV) Fig. 4. MIKES spectrum for the unimolecular dissociation of CS~ ÷ to CS ÷ at 6.116 keV laboratory energy when the collision cell is floated at 400 V.

added to the collision cell. Reaction Eq. (3) has a larger cross section than reaction Eq. (2) (by a factor of 4) and has a near zero intercept, indicat- ing that it is a bimolecular reaction with a background gas ECID component at the base pressure of 8 x 10 -9 mbar. This interpretation is further supported by the results in Fig. 4, which shows the effect of offsetting the collision cell from ground by 400 V. The centroid energy of CS ÷ ions produced from CS~ + ions undergoing unimolecular decay is

Ecs+ = (Mcs +/Mcs; )'Ecs~ (7)

Deceleration of doubly charged ions by the offset voltage reduces the energy of ions in the collision

cell by twice the applied potential (or 800 V for the example cited here). Hence, from Eq. (7), CS + ions leaving the cell have a centroid energy of 3078 eV and are accelerated by 400 V as they exit the cell. Their centroid location is 3478 eV, clearly separated from processes occurring in the long field-flee flight path and centered at 3541 eV.

Fig. 4 presents our results from the above experiment. Peaks marked a', b', c' represent the dissociation occurring in the 2 cm long collision cell, and a, b, c represent the dissociation occurring in the nearly 1 meter long field-flee region between the magnetic and the electric sector. The ratio of the intensities of a to a' and


c to c' is ~ 50:1, similar to their flight path ratio for out-of-cell to in-cell path length. Surprisingly, the ratio of the intensities of b and b' is of the order of unity, demonstrating that dissociation in a 2 cm long cell at baseline pressure contributes half of the total dissociation; as b' represents ECID induced by the background gases (N2, O2, H20) inside the cell and b represents ECID in the field-free region, the baseline pressure inside the cell is of the order of 50 times higher than that in the flight tube. Thus "zero" pressure in the collision cell is about 4 x 10 -7 mbar.

The symmetry requirement that the forward and backward scattered S ÷ peaks originating from reaction Eq. (1) have equal velocities with respect to the primary ion velocity defines the primary ion velocity in the Fig. 2 experiment as 123 639 m s -1. Kinetic energy release in this CSD process is the sum of the kinetic energies of S ÷ and CS ÷ with respect to the CM energy of CS 2÷.

A redundant determination of the kinetic energy release is obtained from the momentum conser- vation relationship m s ÷ Vs + = mcs + V c s ÷ and direct measurements of the laboratory energies of both products. Our measurements of kinetic energy release from the spontaneous dissociation reaction Eq. (1) is 3.4 eV, which is in excellent agreement with an earlier study by Cooks and coworkers [15].

Because electronic structures rearrange much more rapidly than nuclear motion can occur, the proper framework for analyzing kinetic energy release in coulombic explosions is the relevant potential energy surface connecting reactants and products. Although reaction Eq. (1) is exothermic by 0.6 eV, the CS~ + dication is strongly bound. The combination of the bound state electronic wave function with long range coulombic repulsion of S + and CS + results in a relatively high dissociation barrier for this

Potential energy surface of CS a dication

-831.9

-832.0

-832.1

c LLI

-832.2

-832.3

UHF (Relaxed Surface) MP2 (Relaxed Surface)

....... Coulomb repulsion

(a)

"'..., '-...

(b)

2 3 4 5 6 7

Bond distance SC - R1 - S (Angstrom)

Fig. 5. Potential energy surface for the ground electronic state of CS 2÷ when one C - S bond is stretched and the other C - S bond is optimized at every point of the calculation, using the D95 ** basis set (a) UHF method and (b) MP2. The dotted curve superimposed on (b) is the calculated energy curve for coulombic repulsion which has been adjusted to fit the potential energy surface at infinite separation of CS*-S ÷.

X.D. Zhou et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 49-62 57

exothermic reaction. The relevant potential surface for reaction Eq. (1) is the lowest energy relaxed path, and was calculated using the GAUSSIAN 94 quantum chemistry program as discussed earlier.

The results of our ab initio calculations of this potential energy surface are presented in Fig. 5. According to the UHF calculations (Fig. 5) the dication is located 2.25 eV above the infinite separation asymptote and is bound by 1.89 eV, in good agreement with earlier calculations by Mazumdar et al. [16]. Since ions crossing the coulombic barrier to dissociation recoil with kinetic energy defined essentially by the potential well and barrier height, the predicted kinetic energy release is 4.14 eV, compared to our experimental measurement of 3.4 eV. To improve our understanding of this decay process, we performed a correlation energy calculation

using second order Moller-Plesset perturbation theory and the D95"* basis set. The resulting potential energy curve is shown in Fig. 5, which gives 1.41 eV binding energy and 2.57 eV barrier height, predicting an energy release approximately 0.2 eV lower than that from the UHF calculation. The predicted kinetic energy release is reduced to 3.92 eV by applying zero-point energy correction.

A further correction is obtained by investigat- ing the possibility for tunneling through the potential energy barrier. The tunneling probability [17] is given by

exp[( - 2/h) Ip.dR] where p is the momentum of the particle, h is Planck's constant divided by 2~r and the

I0 o~

95.

90.

85.

7s-

70-

6s-

so l ow,ii ss

so I~I 45.

40_

35= ~J ~I~ 30

25.

2o-: is_-: I0.

s_: o:

CS:++ Ar EClD . CS;+ Ar +

L__.. + S + C S

E C I D

2+ CICSD CS 2 + A r " CS ++ S + + A r +

A K E = 4.1 e V

C I C S D i~ \ / \ \ : [

Fig. 6. MIKES spectrum for the CID of CS~ ÷ to S + ions in collision with Ar at 6.026 keV laboratory energy.


integration is done over the entire classical barrier length. The tunneling lifetime is calculated by dividing the appropriate vibrational per- iod by the tunneling probability. The analysis using the potential energy surface depicted in Fig. 5 indicates that all states with energies within 0.15 eV of the top of the barrier have life- times below 15 ~s. Hence the majority of observed metastable dissociations would exhibit recoil kinetic energies roughly 0.15 eV below the barrier height. The resulting theoretically predicted kinetic energy release is 3.75 eV, in reasonable agreement with our experimental value of 3.4 eV.

6. C I D of CS~ + to CS + and S +

6.1. CICSD reactions

Fig. 6 shows the zero degree scattering velocity spectrum for S ÷ ions generated by colliding 6.026 keV CS~ ÷ ions with Ar. Three distinguishable processes--two CICSD reactions and one ECID reaction--are observed and assigned following the metastable spectrum described above. The center peak in the spectrum, which results from ECID, increases in intensity by a factor of

100, whereas the side peaks resulting from CICSD--with characteristically high recoil velocities resulting from coulombic repulsion of the product ions--increase by a factor of ~ 6 compared with the unimolecular and background CID peaks. A second pair of side peaks, characterized by higher kinetic energy release and absent in Fig. 2, represents a new CID reaction channel previously unreported for this system. All these reactions were demonstrated to originate in the collision cell by changing its potential in the same manner as in Fig. 4. All five peaks shifted as a group, consistent with most ( > 95%) of the observed intensity originating in the collision cell.

Jonathan et al. [19] have suggested from their translational energy spectroscopy studies that the

first excited state, 1m-, which is 0.9 eV above the ground state, is long lived. As there is no information available on the potential energy surface for this state, we calculated this surface to obtain well depth and barrier height in order to under- stand and explain our results. The potential energy surface for this first excited state in Fig. 7 shows a 4.1 eV deep potential well. If this state is long lived, as suggested by Jonathan et al., it can be populated with very high vibrational excitation. The charge transfer collision obviously collapses this state into the CS~ mani- fold of electronic states, retaining most of the excess energy as internal energy above the asymptote giving CS + as a product.

The coulombic explosion peaks in Fig. 6 can be resolved into four peaks, one pair of which is in nearly the same position (within our measurement errors) as the unimolecular process discussed earlier. The CICSD reaction products are readily distinguished by floating the collision cell as in Fig. 4. The sideband peaks in Fig. 6 demonstrate that there are two energetically distinct CID processes associated with coulombic repulsion of the S + and CS ÷ product ions. Both are associated with very small translational energy transfer but different kinetic energy release on dissociation. By carefully and repeat- edly scanning the peak profiles, we can distinguish that the higher kinetic energy release peaks have a centroid -~ 0.5 eV more endothermic than the lower energy release mechanism which superimposes on the metastable decay peaks.

The energy release for the two CICSD processes corresponds to 3.4 eV and 4.1 eV for the inner and outer pairs of peaks, respectively. Low energy shift implies that reactant CS 2÷ ions have internal energies only slightly below the dissociation threshold and require little energy transfer to initiate the dissociation process. We have already discussed the unimolecular process for ground state 3~g ions shown in Fig. 2. The CICSD process is 0.6 eV exothermic, as calculated from known thermochemical data [7], and the overall


reaction shown in Fig. 4 is calculated to release 3.7 eV, compared with our experimental value of 3.4 eV.

A second ab initio calculation for the ground state in which one C - S bond distance is locked at its equilibrium value for the ground state is calculated and presented, together with the relaxed surface, in Fig. 7. Fig. 7 also shows the calculated coulombic repulsion curve which is the long range interaction force. The coulombic repulsion curve has been calculated for infinite separation and adjusted to join to the calculated potential energy surface at longer separation• We have also included the potential energy surface for the first excited state calculated using the same basis set.

The Fig. 7 calculations for the non-relaxed ground state surface suggest an explanation for our measured higher kinetic energy release CICSD process. The predicted kinetic energy release is 4.1 eV (barrier height 2.82 eV plus exothermicity 1.41 eV, zero-point energy

correction 0.06 eV), which coincides with our measured kinetic energy release of 4.1 eV. If we assert that the dissociation proceeds so fast that there is no redistribution of energy into internal modes other than the bond that dissociates, there is no tunneling correction to the energy barrier and reaction proceeds on the upper surface. In an earlier study of CID of CS~ we presented a compelling argument that a substantial fraction of dissociations occurs so rapidly that there is no equilibration of energy among internal modes [20].

6.2. ECID reactions

Energy loss measurements for the formation of CS ÷ and S ÷ ions are given in Table 1 as Q values for the respective reactions, and reaction exo- and endoergicities are given as AE. Energy lost by CS 2+ ions fragmenting to S ÷ ions in ECID reaction Eq. (3) is 1.0 +_ 0.5 eV and the kinetic energy release calculated from the peak width at

-832,10

-832.15

-832.20

UJ

-832.25

-832.30

Ground state surface (~xed) I .. . . . . (b ) ~ Ground state surface (relaxed)

. - • . . . . . Fimt excited state surface (fixed)

K

(a) 4•21eV

I I I I

1 2 3 4 5 6

Bond distance SC - R1 - S (Angstrom)

2+ Fig. 7. Potential energy surface for (a) the ground and (b) the excited states of CS 2, plotted as a function of C-S bond distance while keeping the other C-S bond fixed at the equilibrium distance for the minimum energy ground state. The relaxed surface for the ground state is also shown for

comparison.


Table 1

Energy information for CS~ ÷ CID with Ar

Q/ev a T/eV b ~ / e V c

S ÷ (CICSD) 0 -+ 2.0 3.4 - 0.6

0.5 -+ 2.0 4.1

CS ÷ (CICSD) 0 -+ 2.0 3.4 - 0.6 0.5 -+ 2.0 4.1

S + (ECID) 1.0 _+ 0.5 0.4 3.3

CS* (ECID) - 1.0 -+ 0.5 0.4 4.3

a Kinetic energy loss. b Kinetic energy release in dissociation

c ,5,E is the thermochemical threshold. step.

the baseline is of the order of 0.40 eV. As the measured energy shift is less than the endoergi- city of the reaction we conclude that the dissociation of CS 2+ to S + involves parent ions in their ground electronic state which are highly vibrationally excited to just below the potential barrier for dissociation. This is the same population of ions responsible for the observed strong metastable CSD reaction of CS 2+.

CID of CS2 2+ to CS + has a spectrum similar to that for S+; the dominant bimolecular reaction path is reaction Eq. (3). The measured energy loss is given in Table l a s Q = 1.0 _+ 0.5 eV; i.e., the process is slightly more exothermic than reaction Eq. (3) even though thermochemically it is endothermic by 4.3 eV. Evidently most CS~ +

Table 2

Energy levels (eV) for some low lying states of CS ~+

Designation Calculation/eV ~ DCT/eV b SEC/eV c TES/EV c

3~g 0 1A- 0.90 1.0

1Zg 1.39 1.4

1 ~ 2.58 2.7

3~ ~ 3.04 3A~ 3.14 3y~ 3.30

1A~ 4.18 3.9

1~+ 5.04 5.2 5t~ 5.49 3I~ 5.81

0

0.9 1.5 1.5

2.5 2.7

a Full SCF CI PSI calculations and photoion-photoion coinci- dence data [21].

b Double charge transfer mass spectrometry [21]. Translational energy spectroscopy [19].

ions have sufficient internal energy to drive this dissociation. This suggests that the dissociation of CS 2+ to CS ÷ proceeds from highly excited CS~ + ions that are generated in the ion source and do not dissociate during their transit time from the source to the collision cell, which is of the order of 15/zs. Moreover, the internal energy requirements exceed the well depth of ground state CS 2÷ ions shown in Fig. 2. The minimum internal energy these ions must have for this CID process is 5.3 _+ 0.5 eV, which corresponds to the 3Eg state of CS 2+ (Table 2). To our knowledge there are no suggestions that this or any other state in this energy range is long lived. A recent study by Mathur et al. [18] has suggested that there is a long lived excited state of CS22+; however, they did not identify it.

6.3. S~ ions The MIKES spectrum of S~ fragment ions is

similar to CS ÷ and S ÷ fragment ion spectra except only one pair of CICSD peaks was observed. The experimental energy shift reported in Table 1 for this ECID process is Q = 3.5 -+ 1.0 eV, whereas the thermochemistry requires 5.4 eV internal energy. Kinetic energy release for this dissociation process is approximately 3.4 eV. As the relaxed surface for CS 2+ dissociation in Fig. 4 and the related discussion implies that the reactant ion beam contains ions with nearly 2 eV vibrational energy, that can readily account for the observed energy shift. Alternatively, Jonathan et al. [19] have suggested that the first excited state of CS2 2+, lag, is long lived and this or other excited states may also contribute for the dissociation to S~ via the CICSD process.

6.4. CID cross sections

The experimental measurement of total CID cross section is difficult as such because fragment ions from the CICSD process are distributed in a very large sphere owing to large kinetic energy release and are discriminated in the z-direction. However, by using the pressure dependence of


the relative intensities of the two CID processes, we can get relative cross sections of ECID and CICSD processes quite accurately. The cross section for the ECID process is calculated to be

0 2 0.06 A , which is a factor of four larger than for the CICSD process, as discussed earlier. Hence the total CID cross section is 0.065 .~2. This is very small compared with cross sections measured for many CID processes of singly charged ions. We therefore measured the total collision cross section which includes elastic and inelastic scattering and also charge transfer processes. The total collision cross section is

0 2 measured as 10 A , which has the major contri- bution from the charge transfer process (reaction Eq. (3)) whose cross section is 0.4 A 2. AS all the product ions are collected in only a very small cone at zero degrees in our experiments, our measurements give only the lower limit of CID cross section.

7. Conclusions

Even at very low analyzer pressure, e.g. 8 x 10 .9 mbar, it is possible to observe CID reactions, particularly for sector instruments with long flight paths, in the field-free region between the electric and the magnetic sectors. This often results in incorrect identification of background gas CID as metastable peaks.

The two dissociation processes--collision- induced charge separation and electron-capture- induced dissociation--proceed via different mechanisms of energy transfer and dissociation. There are two energetically distinct mechanisms for the dissociation via charge separation, both involving ground state ions. However, the possibility of dissociation via an excited state can not be completely ruled out. CID to S ÷ fragment ions via ECID process proceeds from vibrationally excited ground state dications, and CS ÷ fragment ions are formed from excited CS 2÷ ions which are formed in the ion source and have a lifetime of up to 15/zs.

Acknowledgements

We thank the DuPont company for the donation of the mass spectrometer used in this research, John Dykins and members of our departmental instrument and machine shops for their help in setting up the instrument, and Dr. M. Krishnamurthy and Dr. Pam Seida for their help in performing ab initio calculations. Financial support from the National Science Foundation, grant no. CHE-9021014, is grate- fully acknowledged.

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Unimolecular and collision-induced dissociation study of CS2+2 with Ar at high collision energy