- Email: [email protected]
The CS2 dication
International Journal of Mass Spectrometry and Zon Processes, 86 (1988) 351-355 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
THE CS, DICATION S. MAZUMDAR,
351
*
V.R. MARATHE,
S.V.K. KUMAR and D. MATHUR
Tata Institute of Fundamental Research, Homi Bhabha Road Bombay 400 005 (India) (Received 30 June 1988)
ABSTRACT The energetics of CS,‘+ formation is investigated experimentally using a crossed electronbeam molecular-beam apparatus incorporating nearly monoenergetic electrons and a quadrupole mass spectrometer, as well as theoretically using an ab initio quantum-chemical method with Gaussian basis sets of double zeta plus polarisation quality.
INTRODUCTION
Doubly-charged molecular ions of CS, have attracted considerable interest from the viewpoints of molecular physics and quantum chemistry [l-3] as well as chemical dynamics of gas-phase ion/neutral reactions [2,4]. Thermochemical data on this di-cation have been reported from diverse experimental sources, such as Auger spectroscopy, double-charge transfer collision studies, photoionisation and conventional electron-impact mass spectrometry (the reader is referred to refs. 5 and 6 for compilations of such data; a comprehensive review of the properties of singly- and multiplycharged ions of CS, has also been recently prepared [7]). The results of a combined experimental and theoretical study of the energetics of CS,2+ formation are reported here. Experimental threshold ionisation efficiency functions have been obtained using nearly monoenergetic electrons in a crossed electron-beam molecular-beam apparatus. Theoretical calculations of the potential energy surface of the ground electronic state of CSi+ have been carried out using ab initio quantumchemical techniques. EXPERIMENTAL
AND THEORETICAL
DETAILS
A detailed description of the apparatus used in the present experiments has been recently published [8] so only the salient features are briefly * Dedicated to Professor John H. Beynon on the occasion of his 65th birthday. 0168-1176/88/$03.50
0 1988 Elsevier Science Publishers B.V.
352
described. A well-collimated electron beam, whose energy monochromaticity is ca. 100 meV (FWHM), crosses, at right angles, a molecular beam of CS, vapour from a double-distilled sample which has undergone several freezepump-thaw cycles. Ions produced in the interaction zone are extracted into a quadrupole mass analyzer and detected using a channel electron multiplier operating in the particle counting mode. Ion counts are monitored as a function of incident electron energy; the entire apparatus and data-handling is by means of an on-line microcomputer-controlled data acquisition system. Thresholds are determined using the vanishing current method. The energy scale is calibrated by mixing a small partial pressure of krypton or xenon in the molecular beam and determining the onset for Kr*+ or Xe*+ formation. Background gas pressures were in the region of 1 X lo-* torr whereas in the interaction zone the molecular beam density was equivalent to a gas pressure of the order of 10m4 torr. Experimental tests were performed to ensure “single-collision conditions” with respect to gas density as well as electron beam intensity [8]. All-electron SCF molecular orbital calculations of the potential energy surface of the ground electronic state of CSi+ were performed on a CDC Cyber 730 mainframe computer using the GAUSSIAN 76 series of programs. The basis sets used in these calculations had double zeta representation for valence orbitals, with 3d polarisation functions included. The basis consisted of 3s, 2p and Id functions for carbon, and 4s, 3p and Id functions for sulphur. These sets were obtained by appropriate splitting of the basis and by addition of polarisation functions to the original Gaussian basis sets of Huzinaga [9]. All calculations were performed using unrestricted Hartree-Fock (UHF) procedures. A detailed description of the theoretical procedures has been presented in a recent report on CO*+ ions [lo]. In the present calculations, geometry optimisation was performed for each of the ionic species studied and for each of its configurations. For both CS, and CSi+, the optimised geometry in the ground electronic state was found to be linear. RESULTS AND DISCUSSION
A typical double-ionisation efficiency curve for CSz+, obtained after a multiple-scanning data acquisition period of 12 h, is shown in Fig. 1. The onset for double ionisation is determined to be 27.22 &-0.08 eV. This compares well with the recent photoionisation value of 27.3 + 0.2 eV obtained using synchrotron radiation [ll] as well as with earlier non-monoenergetic electron impact data (see Table 1). It is noteworthy that the electron impact experiments of Cuthbert et al. [12] yielded an anomalously low double ionisation energy of 25.5 eV; this can be attributed to the fact that
353
0
.........
16
....
...”
. . . .
..A...
I
. . . . .
. . . . . . . . . . “..
20 ELECTRON
. . . . . . . . . . . . . .
24 ENERGY
. .
..A
28 (eU)
Fig. 1. Thresholddouble-ionisationefficiencyfunctionfor CS:+ obtainedusing an electron beam with FWHM of ca. 100 meV. The total data accumulationtime was 12 h. these experiments were conducted using extremely intense electron beams within the space-charge of which ion trapping could occur with significant efficiency. These experiments cannot, therefore, be regarded as pertaining to single electronic collisions with ground state neutral CS,. Table 1 also indicates that both the charge stripping as well as the double charge transfer results yield rather high values. The shape of the double ionisation efficiency function in the near-threshold region has already been discussed in a recent report [3] in terms of an indirect double ionisation mechanism interfering, in a quantum-mechanical sense, with the direct double electron ejection process. The electronic ground state of neutral CS, is ‘Xp’ with the electronic configuration
where sulphur Is, 2s and 2p electrons and carbon 1s electrons are included
TABLE 1 Experimentaldata on the doubleionisationenergyof CS, Method
Ref.
DoubleIE (ev)
Photon impact Electron impact Electron impact Electron impact Double charge transfer Charge stripping Monoenergetic e- impact
11 13 14 12 15 16 This work
27.3 ItO. 27.3 27 25.5 f0.3 28.3 28.1 27.22 f 0.08
354 0.1 i
s > 2
z d F
-0.05
-
E 6 a
-0.1
0
’ 0.2
.
’ 0.4
’ 0.6
’ 0.6
.
1
X Fig. 2. Potential energy surface for the ground electronic state of CSi+ plotted as a function of the reduced C-S internuclear distance X where X = (rI - r2)/( r, + r2) and r,,, are the two C-S bond distances.
in the core. Double ionisation occurs by removal of two electrons from the non-bonding lrra orbital to yield the X3x, ground electronic state. The same configuration also gives rise to ‘El and ‘Aa excited electronic states. The energy required for a vertical transition from the ground electronic state of CS, to the X32, state of the dication is calculated to be 24.3 eV. This value is considerably smaller than the experimental values shown in Table 1. Incorporation of configuration interaction in the calculation for neutral and ionised states is not likely to increase the theoretical value. It is noteworthy that the double ionisation energy value reported by Millie et al. [l] on the basis of full CIPSI calculations (24.5 eV) is also much less than the experimental values. The lowest-energy dissociation pathway on the potential energy surface for the ground electronic state of CSl+ is shown in Fig. 2; the two C-S bond distances are variables ri and r, and the minimum energy pathway is plotted as a function of normalised distance, X, where
X=Owhenr,=r,=1.6174~andX=lwhenr,=~andr,=1.5337~.A local potential minimum is observed at ri = r, = 1.6174 A, indicating that the ground electronic state is metastable. The dissociation limits are found to be CSf + S+. In order to obtain the dissociation pathway along the potential energy surface, the bond distances r, and r, were independently varied and rz was optimised for every value of r, in Fig. 2. The potential barrier towards dissociation to CS+ + S+ is determined to be 1.74 eV while the dissociation energy E(CS+) + E(S+) - E(CSz’) is calculated to be 2.28 eV.
355 ACKNOWLEDGEMENT
The authors are grateful to Dr. C. Badrinathan for invaluable assistance with the implementation of the computerised data-handling procedures. REFERENCES 1 P. Millie, I. Nenner, P. Archirel, P. Lablanquie, P. Foumier and J.H.D. Eland, J. Chem. Phys., 84 (1986) 1259. 2 P. Jonathan, M. Hamdan, A.G. Brenton and G.D. Willett, Chem. Phys., 119 (1988) 159. 3 S.V.K. Kumar and D. Mathur, Rapid Commun. Mass Spectrom., 2 (1988) 90. 4 D. Mathur, R.G. Kingston, F.M. Harris and J.H. Beynon, J. Phys. B, 19 (1986) L575. 5 H.M. Rosenstock, K. DraxI, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 (Suppl. 1) (1977) 420. 6 R.D. Levin and S.G. Lias, Ionization Potential and Appearance Potential Measurements, 1971-1981, NSRDS-NBS 71, National Bureau of Standards, Washington, DC, 1982, p. 328. 7 D. Mathur and F.M. Harris, Mass Spectrom. Rev., to be published. 8 D. Mathur and C. Badrinathan, Phys. Rev. A, 35 (1987) 1033. 9 S. Huzinaga, Gaussian Basis Sets for Molecular Calculations, Elsevier, Amsterdam, 1984. 10 S. Mazumdar, F.A. Rajgara, V.R. Marathe, C. Badrinathan and D. Mathur, J. Phys. B, 21 (1988) 2815. 11 P. Lablanquie, I. Nenner, P. Millie, P. Morin, J.H.D. Eland, M.J. Hubin-Franskin and J. Delwiche, J. Chem. Phys., 82 (1985) 2951. 12 J. Cuthbert, J.F. Farren, B.S. Prahallada Rao and E.R. Preece, Proc. Phys. Sot. A, 88 (1966) 91. 13 A.S. Newton and A.F. Schiamanna, J. Chem. Phys., 52 (1970) 329. 14 R.G. Cooks, D.T. Terwilliger and J.H. Beynon, J. Chem. Phys., 61 (1974) 1208. 15 P.G. Foumier, J. Foumier and H. Mousselman, unpublished data (quoted in ref. 1). 16 C.J. Porter, C.J. Proctor, T. Ast and J.H. Beynon, Croat. Chim. Acta, 54 (1981) 407.
THE CS, DICATION S. MAZUMDAR,
351
*
V.R. MARATHE,
S.V.K. KUMAR and D. MATHUR
Tata Institute of Fundamental Research, Homi Bhabha Road Bombay 400 005 (India) (Received 30 June 1988)
ABSTRACT The energetics of CS,‘+ formation is investigated experimentally using a crossed electronbeam molecular-beam apparatus incorporating nearly monoenergetic electrons and a quadrupole mass spectrometer, as well as theoretically using an ab initio quantum-chemical method with Gaussian basis sets of double zeta plus polarisation quality.
INTRODUCTION
Doubly-charged molecular ions of CS, have attracted considerable interest from the viewpoints of molecular physics and quantum chemistry [l-3] as well as chemical dynamics of gas-phase ion/neutral reactions [2,4]. Thermochemical data on this di-cation have been reported from diverse experimental sources, such as Auger spectroscopy, double-charge transfer collision studies, photoionisation and conventional electron-impact mass spectrometry (the reader is referred to refs. 5 and 6 for compilations of such data; a comprehensive review of the properties of singly- and multiplycharged ions of CS, has also been recently prepared [7]). The results of a combined experimental and theoretical study of the energetics of CS,2+ formation are reported here. Experimental threshold ionisation efficiency functions have been obtained using nearly monoenergetic electrons in a crossed electron-beam molecular-beam apparatus. Theoretical calculations of the potential energy surface of the ground electronic state of CSi+ have been carried out using ab initio quantumchemical techniques. EXPERIMENTAL
AND THEORETICAL
DETAILS
A detailed description of the apparatus used in the present experiments has been recently published [8] so only the salient features are briefly * Dedicated to Professor John H. Beynon on the occasion of his 65th birthday. 0168-1176/88/$03.50
0 1988 Elsevier Science Publishers B.V.
352
described. A well-collimated electron beam, whose energy monochromaticity is ca. 100 meV (FWHM), crosses, at right angles, a molecular beam of CS, vapour from a double-distilled sample which has undergone several freezepump-thaw cycles. Ions produced in the interaction zone are extracted into a quadrupole mass analyzer and detected using a channel electron multiplier operating in the particle counting mode. Ion counts are monitored as a function of incident electron energy; the entire apparatus and data-handling is by means of an on-line microcomputer-controlled data acquisition system. Thresholds are determined using the vanishing current method. The energy scale is calibrated by mixing a small partial pressure of krypton or xenon in the molecular beam and determining the onset for Kr*+ or Xe*+ formation. Background gas pressures were in the region of 1 X lo-* torr whereas in the interaction zone the molecular beam density was equivalent to a gas pressure of the order of 10m4 torr. Experimental tests were performed to ensure “single-collision conditions” with respect to gas density as well as electron beam intensity [8]. All-electron SCF molecular orbital calculations of the potential energy surface of the ground electronic state of CSi+ were performed on a CDC Cyber 730 mainframe computer using the GAUSSIAN 76 series of programs. The basis sets used in these calculations had double zeta representation for valence orbitals, with 3d polarisation functions included. The basis consisted of 3s, 2p and Id functions for carbon, and 4s, 3p and Id functions for sulphur. These sets were obtained by appropriate splitting of the basis and by addition of polarisation functions to the original Gaussian basis sets of Huzinaga [9]. All calculations were performed using unrestricted Hartree-Fock (UHF) procedures. A detailed description of the theoretical procedures has been presented in a recent report on CO*+ ions [lo]. In the present calculations, geometry optimisation was performed for each of the ionic species studied and for each of its configurations. For both CS, and CSi+, the optimised geometry in the ground electronic state was found to be linear. RESULTS AND DISCUSSION
A typical double-ionisation efficiency curve for CSz+, obtained after a multiple-scanning data acquisition period of 12 h, is shown in Fig. 1. The onset for double ionisation is determined to be 27.22 &-0.08 eV. This compares well with the recent photoionisation value of 27.3 + 0.2 eV obtained using synchrotron radiation [ll] as well as with earlier non-monoenergetic electron impact data (see Table 1). It is noteworthy that the electron impact experiments of Cuthbert et al. [12] yielded an anomalously low double ionisation energy of 25.5 eV; this can be attributed to the fact that
353
0
.........
16
....
...”
. . . .
..A...
I
. . . . .
. . . . . . . . . . “..
20 ELECTRON
. . . . . . . . . . . . . .
24 ENERGY
. .
..A
28 (eU)
Fig. 1. Thresholddouble-ionisationefficiencyfunctionfor CS:+ obtainedusing an electron beam with FWHM of ca. 100 meV. The total data accumulationtime was 12 h. these experiments were conducted using extremely intense electron beams within the space-charge of which ion trapping could occur with significant efficiency. These experiments cannot, therefore, be regarded as pertaining to single electronic collisions with ground state neutral CS,. Table 1 also indicates that both the charge stripping as well as the double charge transfer results yield rather high values. The shape of the double ionisation efficiency function in the near-threshold region has already been discussed in a recent report [3] in terms of an indirect double ionisation mechanism interfering, in a quantum-mechanical sense, with the direct double electron ejection process. The electronic ground state of neutral CS, is ‘Xp’ with the electronic configuration
where sulphur Is, 2s and 2p electrons and carbon 1s electrons are included
TABLE 1 Experimentaldata on the doubleionisationenergyof CS, Method
Ref.
DoubleIE (ev)
Photon impact Electron impact Electron impact Electron impact Double charge transfer Charge stripping Monoenergetic e- impact
11 13 14 12 15 16 This work
27.3 ItO. 27.3 27 25.5 f0.3 28.3 28.1 27.22 f 0.08
354 0.1 i
s > 2
z d F
-0.05
-
E 6 a
-0.1
0
’ 0.2
.
’ 0.4
’ 0.6
’ 0.6
.
1
X Fig. 2. Potential energy surface for the ground electronic state of CSi+ plotted as a function of the reduced C-S internuclear distance X where X = (rI - r2)/( r, + r2) and r,,, are the two C-S bond distances.
in the core. Double ionisation occurs by removal of two electrons from the non-bonding lrra orbital to yield the X3x, ground electronic state. The same configuration also gives rise to ‘El and ‘Aa excited electronic states. The energy required for a vertical transition from the ground electronic state of CS, to the X32, state of the dication is calculated to be 24.3 eV. This value is considerably smaller than the experimental values shown in Table 1. Incorporation of configuration interaction in the calculation for neutral and ionised states is not likely to increase the theoretical value. It is noteworthy that the double ionisation energy value reported by Millie et al. [l] on the basis of full CIPSI calculations (24.5 eV) is also much less than the experimental values. The lowest-energy dissociation pathway on the potential energy surface for the ground electronic state of CSl+ is shown in Fig. 2; the two C-S bond distances are variables ri and r, and the minimum energy pathway is plotted as a function of normalised distance, X, where
X=Owhenr,=r,=1.6174~andX=lwhenr,=~andr,=1.5337~.A local potential minimum is observed at ri = r, = 1.6174 A, indicating that the ground electronic state is metastable. The dissociation limits are found to be CSf + S+. In order to obtain the dissociation pathway along the potential energy surface, the bond distances r, and r, were independently varied and rz was optimised for every value of r, in Fig. 2. The potential barrier towards dissociation to CS+ + S+ is determined to be 1.74 eV while the dissociation energy E(CS+) + E(S+) - E(CSz’) is calculated to be 2.28 eV.
355 ACKNOWLEDGEMENT
The authors are grateful to Dr. C. Badrinathan for invaluable assistance with the implementation of the computerised data-handling procedures. REFERENCES 1 P. Millie, I. Nenner, P. Archirel, P. Lablanquie, P. Foumier and J.H.D. Eland, J. Chem. Phys., 84 (1986) 1259. 2 P. Jonathan, M. Hamdan, A.G. Brenton and G.D. Willett, Chem. Phys., 119 (1988) 159. 3 S.V.K. Kumar and D. Mathur, Rapid Commun. Mass Spectrom., 2 (1988) 90. 4 D. Mathur, R.G. Kingston, F.M. Harris and J.H. Beynon, J. Phys. B, 19 (1986) L575. 5 H.M. Rosenstock, K. DraxI, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 (Suppl. 1) (1977) 420. 6 R.D. Levin and S.G. Lias, Ionization Potential and Appearance Potential Measurements, 1971-1981, NSRDS-NBS 71, National Bureau of Standards, Washington, DC, 1982, p. 328. 7 D. Mathur and F.M. Harris, Mass Spectrom. Rev., to be published. 8 D. Mathur and C. Badrinathan, Phys. Rev. A, 35 (1987) 1033. 9 S. Huzinaga, Gaussian Basis Sets for Molecular Calculations, Elsevier, Amsterdam, 1984. 10 S. Mazumdar, F.A. Rajgara, V.R. Marathe, C. Badrinathan and D. Mathur, J. Phys. B, 21 (1988) 2815. 11 P. Lablanquie, I. Nenner, P. Millie, P. Morin, J.H.D. Eland, M.J. Hubin-Franskin and J. Delwiche, J. Chem. Phys., 82 (1985) 2951. 12 J. Cuthbert, J.F. Farren, B.S. Prahallada Rao and E.R. Preece, Proc. Phys. Sot. A, 88 (1966) 91. 13 A.S. Newton and A.F. Schiamanna, J. Chem. Phys., 52 (1970) 329. 14 R.G. Cooks, D.T. Terwilliger and J.H. Beynon, J. Chem. Phys., 61 (1974) 1208. 15 P.G. Foumier, J. Foumier and H. Mousselman, unpublished data (quoted in ref. 1). 16 C.J. Porter, C.J. Proctor, T. Ast and J.H. Beynon, Croat. Chim. Acta, 54 (1981) 407.