Double-ionization energies to ground and electronically excited singlet and triplet states of CCl2+4, CCl3F2+, CCl2F2+2 and CClF2+3

International

Journal

of Mass Spectrometry

and Ion Processes,

I 12 (1992) 285-302

285

Elsevier Science Publishers B.V., Amsterdam

Double-ionization energies to ground and electronically excited singlet and triplet states of Ccl:+, CC13F2+, CC12 F2+ and CClF2+ 2 3 M.L. Langford’ Mass Spectrometry

and F.M. Harris’ Research Unit, University College of Swansea. Singleton Park,

Swansea SA2 8PP (UK)

C.J. Reid and J.A. Ballantine SERC

Mass Spectrometry

Service Centre, University

College

of Swansea,Singleton

Park,

Swansea SA2 8PP (UK)

D.E. Parry Department

of Chemistry,

University

College of Swansea,

Singleton

Park, Swansea SA2 8PP (UK)

(First received 23 August 1991; in final form 10 October 1991)

ABSTRACT Double-charge-transfer spectroscopy has been used to measure singlet and triplet electronic-state energies of Ccl:+, CCl,F ** , CC12Fz* and CCIF:+ ions. Because of spin conservation, singlet states were populated when using H+ as the projectile ion and triplet states when using F+. Also, double-ionization energies to ground singlet and triplet states were calculated with second-order Moller-Plesset perturbation theory. The calculated energies are in reasonable to good agreement with those measured. Some of the energies have been measured previously, and the corresponding data from the present investigation are in good agreement with them. The singlet-state energies for the last three title ions have been measured for the first time in the present investigation, and the higher-lying triplet-state energies of all the ions have been measured more accurately using the F+ projectile ion. Keywords: perhalogenomethanes;

double-charge-transfer

spectroscopy.

INTRODUCTION

The perhalogenomethanes Ccl,, CCl,F, CC&F, and CClF, are a series of molecules which are chemically inert and non-toxic. These properties have led to the widespread use of Ccl, F, and Ccl, F gases as aerosol propellants and as working fluids in refrigeration and air-conditioning units. The high stability of these molecules, however, has resulted in their accumulation in the ’ Present address: The Chemical Laboratory, The University of Kent at Canterbury, bury, Kent CT2 7NH, UK. ’ Author to whom correspondence should be addressed. 0168-l 176/92/$0X00

0

1992 Elsevier Science Publishers

B.V. All rights reserved.

Canter-

286 TABLE

1

Double-ionization energies (eV) of the perhalogenomethanes measured previously DCT spectroscopy” Proton projectile CCL

AE spectroscopyb CCI, Singlets

Ccl,,

DCT spectroscopy’

CCI, F, CCI, F, and CCIF,

OH+ projectile

ion

CCI,

CCll F

Ccl, F,

CCIF,

29.3 + 0.5

30.0 + 0.5 32.2 + 0.5 z 36

31.3 f 0.5 zz 33

35.1 + 0.5 40-4 1

Triplets

Singlets 29.8 30.5 f 0.2

30.5

33.0 f 0.2

32.9

31.4 33.7 34.5 35.8 f 0.2

z34

36.1 36.5

44.6 f 0.6 a Taken from ref. 7. ’ Taken from ref. 7. Estimated ’ Taken from refs. 5 and 6.

error is f 0.1 eV.

lower atmosphere and eventual diffusion into the stratosphere where photolytic reactions lead to dissociation processes, some of which result in the production of the chlorine radical which may be of prime importance in ozone destruction cycles [I]. A feature of these molecules is the instability of the singly charged molecular cation. Mass spectra, generated via 70eV electron impact ionization, show that stable molecular ions form less than 0.5% of the total number of ions generated. Ccl,+ proved particularly elusive but evidence for its existence on a metastable (lifetime 2 10v6 s) time scale has been obtained [2]. It is likely, therefore, that photoionization followed by rapid unimolecular dissociation of unstable singly charged ions results in the formation of the destructive chlorine radicals. The dications Ccl:+, Ccl, F2+, CClzFi+ and CClF:+ have not been observed although indirect evidence has been obtained [3,4] for the existence of metastable Ccl:‘. The double-ionization energies to form the dications have been measured [5,6] by double-charge-transfer (DCT) spectroscopy, an experimental method which does not require the dication to be stable or metastable. A detailed investigation of double-ionization energies to singlet and triplet states of Ccl:+ has recently been carried out by both DCT spectroscopy and Auger electron (AE) spectroscopy [7]. The results of that investigation and the earlier DCT spectroscopy investigations are listed in Table 1. A recent study of a “reaction-window” effect in double-electron-capture

287

(DEC) reactions [8] has shown that the choice of projectile ion in DCT spectroscopy is important in that it governs the cross-section for the population of the states in the dication. This study demonstrated that the population of certain states could be enhanced or suppressed by changing the projectile ion which effectively changes the endoergicity of the reaction. In general, a projectile ion formed from an atom or molecule having a low ionization energy, such as S, causes the population of lower-energy states in the resultant dication, whereas the use of a projectile ion, such as F+, which has a high ground-state energy, suppresses the population of the lower-energy states but enhances the population of higher-energy states. This effect has been utilized in the study of a number of dications [9-121 in order to measure an extended range of double-ionization energies. Although each molecule will have associated with it its own particular window, it is useful to note that for CH,Br this lies within the approximate limits of endoergicity of 8 eV and 22 eV. A further consideration is the multiplicity of the states accessed. DEC reactions of protons with molecular targets have been shown to obey the Wigner spin-conservation rule, even for a molecular target as heavy as Ccl, [7]. Thus, only singlet dicationic states of singlet-state target molecules are expected to be accessed. Similarly, OH+ and F+ projectile ions involved in DEC reactions have also been shown to obey the spin-conservation rule [9,10,12]. Thus, the use of these ions should lead to the population of triplet configured states of the dication following DEC reactions with singlet-state target molecules. With the increased knowledge acquired over the last two years on the effect of the type of projectile ion on DCT spectra, we decided to re-examine the four title ions. The DCT spectroscopy investigation described here had three main purposes. (i) To measure double-ionization energies to higher triplet electronic states of the dications by using the F+ projectile ion. (ii) To measure singlet-state energies by using H+ as the projectile ion. (iii) To compare the spectra produced using both these projectile ions in order to determine whether spin is conserved in DEC reactions with the four perhalogenomethane molecules. In addition, theoretical predictions of double-ionization energies to singlet and triplet ground states of the dications have been made in this investigation. METHODS

Experimental The experimental

results were obtained on a ZAB-E reversed geometry,

288 double-focussing mass spectrometer [13] located at the SERC Mass Spectrometry Service Centre. The projectile ions F + and H + were generated by 100 eV electron impact ionization of C H F 3 and H2 respectively, in a combined EI-CI source, and accelerated to a translational energy of 3 keV. The field of the magnet was set to transmit the ions which converged at an intermediate focal point in the second field-free region of the spectrometer. A collision-gas cell in this region contained the molecules under investigation. The negative ions resulting from DEC reactions were transmitted to the detector by reversing the polarities of the voltages normally applied to the electric sector, and their energy spectrum obtained by scanning these voltages. The transfer of two electrons from the target molecule, denoted by M, to the projectile ion, denoted by A +, in a single collision can be represented by A + + M - - + A - + M 2+

(1)

The energy required to form M 2+ is greater than the energy released in the charge inversion of the projectile ion so that the excess energy required for the reaction is taken from the translational energy of the projectile ion. As only forward-scattered anions are detected, the translational energy loss, AEt, is also the reaction endoergicity, and is given by AE, = I2(M) - E(A + --* A - )

(2)

where/2 (M) is a double-ionization energy of M and E(A + ~ A - ) is the energy released when A + captures two electrons to become A - . The transfer of two electrons can also take place in two sequential singleelectron-capture (SEC) reactions represented by A + +M~A+M

+(x)

(3a)

A + M --+ A - + M + (y)

(3b)

where M + (x) and M + (y) represent the singly-charged ion in generally different electronic states x and y. In this case, the loss of translational energy by A + in converting to A - is given by AE 2 = Ilx(M) + I,y(M) -- E(A + ~ A - )

(4)

where Ilx (M) and I l y ( M ) a r e the single-ionization energies to the states x and y of M +. The two different electron transfer processes can be easily distinguished since double-electron transfer in a single collision has a linear dependence on collision-gas pressure whereas the negative-ion yield resulting from the double-collision process has a quadratic pressure dependence. Calibration of the energy loss scale was achieved in all cases by repeating

289

the experiments using argon as the collision gas. Argon has well known singleand double-ionization energies, and exhibits a large cross-section for DEC reactions when using F+ and H+ projectile ions. Unless stated otherwise, the uncertainties in the double-ionization energies determined in this study are + 0.5 eV. Theoretical A theoretical prediction of the double-ionization energy to each dication ground state was obtained by calculating the difference in ground-state energies of the neutral molecule and the dication. The dication geometry was taken to be the same as that of the neutral molecule because transitions in DCT spectroscopy are vertical [14]. The calculations were performed with the GAUSSIAN 86 ab initio molecular orbital program package [15] at the Manchester Computer Centre. The basis set used was the split valence 3-21G(*’ set [16] in which the chlorine atoms were assigned d polarization functions. This combines the flexibility required to describe changes in the valence shell on ionization with computational economy. Hartree-Fock calculations (unrestricted for the dications) of total energies were approximately corrected for correlation effects by Moller-Plesset (frozen core) second-order perturbation theory, a procedure we have found to be effective in providing double-ionization energies in good agreement with experimental values [lo-121. RESULTS

AND DISCUSSION

ccl,

Figure 1 shows typical spectra generated by reactions of H+ (Fig. la) and F’ (Fig. lb) with Ccl, molecules. The spectrum of Fig. la is similar to, but not as well resolved as, the H+/CCl, spectrum obtained by Fournier et al. [7]. Spectra obtained at a number of different target-gas pressures showed that the heights of the peaks marked 1 and 2 (Fig. la) varied quadratically with Ccl, pressure and were thus due to the production of H- ions in two sequential SEC reactions. The photoelectron spectrum of Ccl, [17] can be used in the identification of the Ccl,+ states populated in the DCT spectroscopy experiment. Peak 1 is a broad feature extending on the translational-energy-loss scale from 22.6 eV to 27 eV at half height. This probably corresponds to the population of all the combinations of the three lowest states of Ccl,+, i.e. 2t,, 7t, and 2e which are calculated to result in energy losses of 23.4-26.9 eV in the resulting H- ions. Peak 2 is probably due to interactions involving the formation of Ccl,+ in the 6t, state. A listing of the possible Ccl,+ state

Translational

Fig. I. Double-charge-transfer collide with Ccl, molecules.

energy

spectra obtained

(eV)

when (a) H+ and (b) F+ 3 keV projectile

ions

assignments, and the resulting values of Z,,(CCl,) and Z,,(CC14), are shown in Table 2. Peaks A, B and C of Fig. la are due to H- ions formed in single-collision DEC reactions (reaction 1). The energies of the states of Ccl:’ populated in these reactions were determined from the positions of the peaks to be 30.3 + 0.5, 33.1 + 0.5 and 35.7 + 0.5eV. These values are in excellent agree-

291 TABLE

2

Assignments Peak”

1

2

and energies Assigned configurationh

(in eV) for the electronic Calculated [I,,(CC&)

states of Ccl,’

energiesb +

~,,(CCl4)1

Measured energies” DCT spectroscopy H+ projectile

F+ projectile

2t, 7t, 2e 7t, 2e 2e

23.38 24.31 25.13 25.24 26.06 26.88

25.1

_

2t, + 6tz 7t, + 6t, i 2e + 6tz

28.27 29.20 30.02

29.4

28.8

2t, + 2t, + 2t, + 7t* + 7tz + : 2e +

a Present investigation. ’ From ref. 17.

ment with those from a previous DCT spectroscopy experiment [7], i.e. 30.5 f 0.2, 33.0 f 0.2 and 35.8 f 0.2eV. A comparison made by Fournier et al. [7] of these previous DCT spectroscopy results with AE spectroscopy data showed that, whereas only three state energies were measured in the DCT spectroscopy experiments, many more were measured in the AE spectroscopy investigation. The authors concluded that the three states of Ccl:+ populated ‘in the H+ DCT spectroscopy experiments were singlet states. The other energies were assigned to triplet sets of the dication. The double-ionization energy to the ground singlet state calculated in the present investigation (using the method described in the Theoretical section and a C-Cl bond length of 1.77 A) is 28.00eV, over 2eV lower than the present and previously [7] measured values. Possible reasons for this difference are discussed later. Measured peak heights from the F+ /Ccl, spectra of Fig. 1b showed that peak 2 was due to a double-collision reaction whilst peaks D, E, F, G, H and I were due to single-collision reactions 1. Consider the two largest peaks (D and G) first. Peak D corresponds to a double-ionization energy of 29.7 + 0.5eV and is in good agreement with the value of 29.3 &-0.5 eV measured with the OH+ projectile in a previous DCT spectroscopy experiment [5]. It also agrees with the value 29.8eV from an AE spectroscopy experiment [7] which the authors suggest is due to the population of a triplet state. Since there is considerable evidence [9-121 that OH+ (3C-) and F+ (3P2) projectile ions tend to populate triplet states in reactions with ground singletstate molecules, the value 29.7 f 0.5eV for peak D is concluded to be the

292

double-ionization energy to the ground triplet state of Ccl:+. The value for the ground triplet-state energy of Ccl:’ calculated in the present investigation is 29.02 eV which is close to the present experimental value, and in agreement with that measured previously [5]. Consider peak G now. From its position, the associated double-ionization energy is 34.2 f 0.5 eV. In the previous OH+ /Ccl, DCT spectrometry experiment [5], a low-intensity broad peak was observed corresponding to a doubleionization energy of about 34eV. Thus, the use of F+ as a projectile ion has enhanced the population of the higher-energy state and allowed its energy to be measured much more precisely. It is likely that this too is a triplet state and it is relevant to note that triplet states at 33.7 and 34.5 eV were determined in the previous AE spectroscopy experiment [7]. Thus, all the evidence points to the double-ionization energies of 29.7 + 0.5eV and 34.2 ) 0.5eV being associated with population of triplet states of Ccl;+. The energy loss positions of the two minor, but nevertheless distinct, peaks E and F correspond to double-ionization energies of 30.5eV and 32.8 eV. These are very close to the two lowest singlet-state energies of 30.3eV and 33.1 eV. It is possible, therefore, that spin conservation is not strictly obeyed in this case. It is known [18] that H+ ions reacting with Xe atoms populate singlet and triplet states with the singlet-state peaks dominating. However, if spin is not conserved in reactions of H’ with Ccl, it would be logical to have evidence of triplet-state population in Fig. la, which there is not. This does not preclude, however, that spin is not conserved in reactions of F+ with Ccl, but it seems unlikely since reactions of OH+ with Ccl, have been shown to be spin-conserved [7]. An alternative explanation for the presence of peaks E and F in the DCT spectrum of Ccl, is that the two peaks are due to reactions involving F+ ions in the ‘D, state rather than in the ‘P2 state. It is known [19] that a beam of F+ ions generated in the manner of the present experiment will have within it a proportion of excited-state ‘D, ions which are sufficiently long-lived to reach the collision-gas cell, and which could, therefore, react with the Ccl, molecules. Since the ‘D, state is 2.59eV higher in energy than the 3P, state, and F+ ions in the ‘D, state will populate singlet states, it is anticipated, on the basis of the present singlet-state energies of Ccl:+ ions, that Ff (‘D2) ions would give rise to peaks in the spectrum of Fig. 1b with apparent double-ionization energies of 27.7eV, 30.5eV and 33.1 eV. The last two values correspond well with those deduced from peaks E and F on the basis of reactions of ‘P,-state Ff ions. There is no evidence of a peak corresponding to 27.7 eV but the endoergicity of this reaction is low and may be so close to the edge of the reaction window as to make the cross-section for the reaction very small. Although the presence in the beam of a small proportion of excited-state Ff ions seems well able to account for peaks E and F, it was not possible to

293

confirm such an assignment by, for example, reducing the electron energy. At the high resolutions used in the present study, the Ff ion beam intensities became unworkably low when generated at ionizing electron energies below 40eV. It is possible, of course, that these minor peaks correspond to triplet states of the dication with energies essentially the same as those of the singlet states discussed above and that they are populated by spin-conserving reactions of F+ (3P2) ions, It is quite possible for such triplet surfaces to “overlap” the singlet surfaces especially as the dications are dissociatively unstable in the present case, i.e. the surfaces are repulsive in the Franck-Condon region. If this is the case, the peaks E and F should be associated with triplet states. The low-intensity features H and I correspond to double-ionization energies of 35.1 eV and 35.9 eV, these values being based on the assumption of reactions of 3P,-state F’ ions. In summary, it is probable from the H+/CCl, spectra that (within the Franck-Condon region) singlet states (or possibly groups of singlet states) of Ccl:’ exist at energies of 30.3 eV, 33.1 eV and 35.7eV. Strong evidence exists for associating energies of 29.7 eV and 34.2 eV with triplet states. The multiplicity of states corresponding to double-ionization energies of 30.5, 32.8,35.1 and 35.9 eV determined from the small peaks in the F+ /Ccl, spectra is uncertain. CC, F

Typical spectra generated as a result of collisions of H+ and Ff ions with Ccl, F molecules are shown in Fig. 2a and Fig. 2b respectively. The variation of peak heights with pressure showed 1 and 2 to have a quadratic dependency and A, B, C and D to have a linear dependency. Both 1 and 2, resulting from double-collision processes, are broad and ill-defined. Peak 1 has [I,,(CCl, F) + I,,(CCl,F)] values from 24 to 28 eV at half height and peak 2 from 29 to 3 1.5 eV. The lowest possible equivalent energy for double-collision reactions, calculated using photoelectron spectrum information [ 171 for Ccl, F, is 23.8 eV which corresponds approximately to the low-energy onset of peak 1. Numerous combinations of Ccl, F+ state energies are possible in the energy range 24-3 1.5 eV so further assignments have not been attempted here. Peaks A and B of Fig. 2a have maxima which correspond to double-ionization energies of 30.9 eV and 35.2 eV. Presumably, these are singlet states (or groups of singlet states) since H+ was the projectile ion used. The value calculated in the present investigation for the ground singlet state of Ccl, F2+ (using C-Cl and C-F bond lengths of 1.76 A and 1.408, respectively) is 29.03 eV. As in the case of Ccl:+, this is significantly lower than the value measured.

294

h y &.&A.. “V--

Y

0

2940

29'60

2950

Translational

Fig. 2. Double-charge-transfer collide with CC&F molecules.

29'80

2970

energy

spectra obtained

)I*7

h _,& -

29'90

(eV)

when (a) H+ and (b) Ff 3 keV projectile

ions

Figure 2b has two distinct peaks, C and D, which correspond to doubleionization energies of 32.2 eV and 35.7 eV. In the previous OH+ /Ccl, F DCT spectroscopy study [6], the lowest state was measured at an energy of 30.0 f 0.5 eV which, if it is present in the F+/CC13F spectrum, coincides with double-collision reaction peak 2 which was prominent even at low pressures. The ground triplet-state energy calculated in this work is 29.71 eV, i.e. in agreement with that measured previously [6]. Peaks C and D probably corre-

295

spond to the peaks observed in the OH+ /Ccl, F spectrum [6] at double-ionization energies of 32.2 + 0.5eV and zs 36eV. It is of interest to see where peaks generated by reactions of F+ (ID*), which would populate the two singlet states of energies 30.9eV and 35.2eV, would fall if they had taken place. One would be at a higher F- ion translational energy than that of peak 2. No peak is seen there but it is likely to be small because the endoergicity is only 7.7eV, i.e. probably close to the lower extremity of the reaction window. The second would have an apparent double-ionization energy of 32.6 eV and would thus be overlapped by peak C. The double-ionization energy measured from peak C is in agreement with the previous measurement of 32.2 f 0.5 eV [6] and so it is unlikely that this peak is principally due to a reaction involving F+ (‘D, ). Peak C is likely to obscure any peak due to such a reaction. For Ccl, F2+, therefore, the present investigation leads to the following energies for states (or groups of states): singlet 30.9 f 0.5 eV and 35.2 f 0.5 eV; triplet 32.2 f 0.5 eV and 35.7 + 0.5 eV. The ground triplet state is probably at 30.0 f 0.5eV [6].

Typical spectra obtained when H+ and F+ ions interact with CCl,F, molecules are shown in Fig. 3a and Fig. 3b respectively. By varying the CC&F, pressure it was established that peaks 1 and 2 were due to negative ions formed in double-collision processes whilst peaks A-F resulted from single-collision reactions. Peak 1 in Fig. 3a has associated with it values of [I,,(CCl,F,) + I,,(CCl,F,)] which extend from about 25 eV to about 29 eV, the maximum of the peak being at 25.7eV. The onset is probably due to reactions 3a and 3b in which two ground-state CC&F,+ ions are formed. Using photoelectron spectroscopy data [20] for CCl,F,, this onset is predicted to be at 25.2eV. Higher-energy components of peak 1 and peak 2, which was measured at 29.2eV, are caused by the population of higher-energy cationic states. Thus, peaks 1 and 2 are formed in reactions in which combinations of CCl,F: states are populated. No detailed assignments are attempted here. Peak A of the Hf/CC12F2 spectrum (Fig. 3a) is sharp and corresponds to a double-ionization energy of 31.6 eV, probably that of the ground singlet state. The double-ionization energy to this state has been calculated using the procedure described in the Theoretical section and with C-Cl and C-F bond lengths of 1.78 A and 1.34A respectively; the value obtained is 31.59 eV which is in excellent agreement with that measured. Peak B is much broader and may result from the population of a group of singlet states centred at 35.1 eV. Peak C of Fig. 3b corresponds to a double-ionization energy of 31.7 eV which agrees with that measured by Griffiths and Harris [6] (31.3 f 0.5eV)

296

7i C

5 ? m L

.z fl ti

;

50-

E ; 0 .-5 I I 01

, 2940

2950

1 2960

I 2980

2970

LL

I 2990

2

, 2970

t 2990

2980

Translational

energy

Fig. 3. Double-charge-transfer spectra obtained collide with CCI,F, molecules.

(ev)

when (a) Hf and (b) F+ 3 keV projectile

ions

using OH+ as a projectile ion. Thus, this probably corresponds to the energy of the ground triplet state. The double-ionization energy to this state calculated in the present investigation is 30.98eV which agrees with the previously measured value [6] but is marginally lower than the present value. Peaks D and E correspond to double-ionization energies of 33.8eV and 35.0 eV and probably match the much weaker broad peak, measured from the

297

DCT spectrum obtained with OH+, at about 33.3 eV [6]. Peak F corresponds to a double-ionization energy of 38.2eV. The possible reactions of F+(‘D,) with CC&F, would lead to the population of the singlet states at energies measured at 31.6eV and 35.1 eV from the H+ /CC&F, spectrum. This would give rise to two peaks in the F+/CCl, F, spectrum. One should be close to peak 2, and is not evident; the other would be roughly 1 eV higher in translational energy than the position of peak D. No such peak is evident but it could be contributing to the broadness of peak D. Since the signal strength of peak D is large, it suggests it is very likely to be due to the population of a triplet state. For Ccl, Fi+, it is thus established that singlet states (or groups of states) exist at 3 1.6 &- 0.5 eV and 35.1 f 0.5 eV, and triplet states (or groups of states) at 31.7 f 0.5eV, 33.8 f 0.5eV, 35.0 f 0.5eV and 38.2 & 0.5eV. CClF, Typical spectra obtained in the present DCT spectroscopy study of CClF, are shown in Fig. 4. By varying the CClF, pressure it was established that the peaks marked 1, 2 and 3 were due to negative ions generated in doublecollision reactions. The values of [I,,(CClF,) + I,,(CClF,)] corresponding to the three peaks are 26eV, 27.3-30.1 eV and 30.7 eV. From photoelectron spectroscopy data [17], it can be established that for peak 1 two ground-state CClF: ions were generated, and for peaks 2 and 3 reactions 3a and 3b gave one ion in the ground state and the other in one of a broad band of states between 14 and 18eV. The dominant feature of Fig. 4a is peak A which increased in height linearly with pressure and was thus due to the DEC reaction 1. Again, since H+ was the projectile ion, it is reasonable to assume that a singlet state (or group of states) is populated in the reaction. The double-ionization energy determined from the position of A is 35.4 f 0.5 eV. Unfortunately, the calculations carried out in the present investigation to determine the ground singlet-state energy did not converge and thus no theoretical value is available to compare with that measured. It can be seen from Fig. 4a that no other single-collision peaks were observed with the H+ projectile ion. This may indicate a strong concentration of singlet states in the region around 35.4eV, or that a narrow reaction window exists for CClF,. The spectrum of Fig. 4b has three peaks (B, C and D) which, from a pressure variation study, were found to be due to the single-collision DEC reaction 1. Consider the large peak C first. From its position it corresponds to a double-ionization energy of 35.7 _I 0.5 eV. This agrees, within combined experimental uncertainties, with the value 35.1 Ifr 0.5 eV determined for the lowest triplet state in a previous DCT spectroscopy study [6] using OH+ as the

30

I

2940

I

2950

Translational

Fig. 4. Double-charge-transfer collide with CCIF, molecules.

I

2960

energy

spectra obtained

29'70

29'80

1

2990

(eV)

when (a) H+ and (b) F’ 3 keV projectile

ions

projectile ion. It seems likely, therefore, that peak C is due to reaction 1 in which the ground triplet state of CClF:+ is populated. The calculation for the ground triplet-state energy carried out in this investigation converged, in contrast to that for the ground singlet state. Using C-Cl and C-F bond lengths of 1.75 8, and 1.33 8, respectively, a value of 34.61 eV was obtained. This agrees with the value 35.1 f 0.5 eV determined previously [6] but is

299

outside the range 35.7 f 0.5eV measured in the present work with the Ff projectile ion. The small peak D corresponds to a double-ionization energy of 40.6eV which is also assigned to a triplet state. Peak B apparently corresponds to a double-ionization energy of 32.9 eV which is 2.8 eV below the energy of the ground triplet state discussed above. This peak has no counterpart in the OH+ generated spectrum [6] nor does spectrum (a) of Fig. 4 show any evidence of a singlet state at this energy. The most likely explanation for peak B, therefore, is the reaction of metastable F+ (‘D2) ions leading to the population of a singlet state. It is clear from Fig. 4a that there is probably’ a strong concentration of singlet states at 35.4eV. Reactions of the F+ (‘D2) ions with CClF, would give a peak with an apparent double-ionization energy of 32.8eV, i.e. close to that calculated for peak B. Because peaks B and C are well separated, it was feasible to perform two additional experiments designed to check if peak B was due to a reaction involving F+ (‘D) ions. (i) In order to try to reduce the concentration of F+ (ID*) ions in the beam, the energy of the electrons used to ionized CHF, in the source was systematically reduced. By reducing the resolution slightly, an acceptable signal-tonoise ratio was maintained down to an electron energy of 30 eV. At this lower limit, the intensity of peak B was reduced relative to that of A by about 10%. This is not significant, however, in view of the overlap of the peaks. (ii) Spectra were obtained by reacting O+ ions with CClF,. The O+ projectile ion was chosen because it is known [21] that about 30% of the ions in an O+ beam formed by 100 eV electron impact ionization of 0, molecules are in the metastable 2D state. Thus, reduction of the DCT spectral peak arising from this large metastable component may be more noticeable with O+ than with F+ when the energy of the ionizing electrons is reduced. Three spectra, obtained when O+ ions formed by dissociative ionization of O2 reacted with CClF, at the same collision-cell pressure as that used for the spectrum of Fig. 4b, are shown in Fig. 5. Again, because the currents reduced quite markedly as the electron energy was reduced below 30eV, the experiment was carried out at a resolution considerably lower than that used to obtain the spectra of Figs. l-4. The heights of peaks marked I and II in Fig. 5 were found to vary linearly with CClF, pressure and were thus due to DEC reactions in single collisions, i.e. analogous to reaction 1. Peak II corresponds to a double-ionization energy of 35.3 eV and is thus equivalent to peak C of Fig. 4b. It can be seen from Fig. 5 that when the ionizing electron energy was reduced from IOOeV to 40eV peak I was reduced in height relative to that of II, and, at 24eV, it vanished. Its position corresponds to a reaction of the *D-state ions with CClF, to populate the ground singlet state of CClF:+. As the *D-state component of the O+ beam is reduced by reducing the electron energy, a value is reached when it is virtually zero and the reaction is not observed. Only peak

300

Lo

.Z

II

1 oo-

5 ? ?

.Z D

2 z E ; 0

50-

E .'0

O-l 29'70

Translational

2980

energy

2d90

(ev)

Fig. 5. Double-charge-transfer spectra obtained when 3 keV O+ ions collide with CCIF, molecules. The O+ ions were formed by dissociative ionization of 0, using electrons of (a) 100 eV (b) 40 eV and (c) 24 eV energy.

301

I remains at the lowest electron energy and this must result from population of the lowest triplet state of CClF:+. Since the double-ionization energy of 35.3 eV derived from its position agrees, within experimental uncertainties, with the 35.7 eV derived from the position of peak C (Fig. 4b), the latter value is assigned in the present work to the lowest triplet-state energy. As mentioned earlier, this value is in agreement with that of 35.1 + 0.5 eV determined previously [6] for the energy of that state. In summary, for CClF:’ , the lowest triplet state (or group of triplet states) is at 35.7 eV whilst the equivalent energy for the lowest singlet state is 35.4 eV. Also, a triplet state exists at 40.6eV. Accuracy

of the theoretical predictions

While the present theoretical predictions for the double-ionization energies to the lowest triplet states of the dications are acceptable, agreeing in the main with the experimental values within the uncertainty estimates of the latter, predictions for the lowest singlet states are, in contrast, erratic. There is no definite explanation for this at present. The calculated total energies of the neutral molecule and dications are much larger than their difference and, especially for large molecules, this provides a severe test for the theory. All the ground singlet dication states proved to have zero-spin-optimised HartreeFock single determinant wave functions which were then used as reference wave functions in the Moller-Plesset perturbation calculations, so the problem cannot be attributed to difficulties with construction of a good spin function (which will occur for dications having open-shell singlet states). What can affect the accuracy of the Moller-Plesset perturbation estimate of the dication correlation is the number of low-lying excited states having the same symmetry as the ground state of interest. For Ccl:+ three groups of low-lying states have been observed in the present theoretical investigation but only two groups of triplet states; two singlet groups of states for CC13F2+ ; and two groups of singlet states for CCl?F:+ in contrast to four triplet groups of states. REFERENCES 1 T.M. Sugden and T.F. West (Eds.), Chlorofluorocarbons in the Environment. The Aerosol Controversy, Horwood, Chichester, 1980. 2 T. Drewello, T. Weiske and H. Schwarz, Angew. Chem., Int. Ed. Engl., 24 (1985) 869. 3 T. Drewello, W. Koch, T. Weiske, H. Schwarz and D. Stahl, Int. J. Mass Spectrom. Ion Processes, 72 (1986) 313. 4 C. Guenat, F. Maquin, D. Stahl, W. Koch and H. Schwarz, Int. J. Mass Spectrom. Ion Processes, 63 (1985) 265. 5 W.J. GrifIiths and F.M. Harris, Rapid Commun. Mass Spectrom., 2 (1988) 28. 6 W.J. Griffiths and F.M. Harris, Int. J. Mass Spectrom. Ion Processes, 86 (1988) 341.

302 7 P.G. Fournier, G. Comtet, J. Fournier, S. Svensson, K. Karlsson, M.P. Keane and A. Naves de Brito, Phys. Rev. A, 40 (1989) 163. 8 M.L. Langford and F.M. Harris, Rapid Commun. Mass Spectrom., 4 (1990) 125. 9 M.L. Langford, F.M. Harris, P.G. Fournier and J. Fournier, J. Chem. Phys. B, submitted. 10 M.L. Langford, F.M. Harris, C.J. Reid, J.A. Ballantine and D.E. Parry, Chem. Phys., 149 (1991) 445. 11 W.J. Griffiths, F.M. Harris and D.E. Parry, J. Chem. Sot., Faraday Trans., 86 (1990) 2801. 12 F.M. Harris, C.J. Reid, J.A. Ballantine and D.E. Parry, J. Chem. Sot., Faraday Trans., 87 (1991) 1681. 13 Manufactured by VG Analytical Ltd., Wynthenshawe, Manchester M23 9LE, UK. 14 J. Appell, in R.G. Cooks (Ed.), Collision Spectroscopy, Elsevier, Amsterdam, 1978, p. 227. 15 M.L. Frisch, J.S. Binkley, H.B. Schlegel, K. Raghavachari, CF. Melius, R.L. Martin, J.J.P. Stewart, F.W. Bobrowicz, C.M. Rohlfmg, L.R. Kahn, D.J. DeFrees, R. Seeger, R.A. Whiteside, D.J. Fox, F.M. Fleuder and J.A. Pople, GAUSSIAN 86, Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. 16 J.S. Binkley, J.A. Pople and W.J. Hehre, J. Am. Chem. Sot., 102 (1980) 939. M.S. Gordon, J.S. Binkley, J.A. Pople, W.J. Pietro and W.J. Hehre, J. Am. Chem. Sot., 104 (1982) 2797. 17 K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of He1 Photoelectron Spectra of Fundamental Organic Molecules, Japan Scientific Societies Press, Tokyo, 1981. 18 P. Fournier, C. Benoit, J. Durup and R.E. March, C.R. Acad. Sci., Ser. B, 279 (1974) 1039. 19 C.S. Enos and A.G. Brenton, personal communication, 1991. 20 J. Doucet, P. Sauvageau and C. Sandorfy, J. Chem. Phys., 58 (1973) 3708. 21 B.R. Turner, J.A. Rutherford and D.M.J. Compton, J. Chem. Phys., 48 (1968) 1602.