J . Phys. Chem. 1985, 89, 4613-4617
4613
High-Temperature Photoelectron Spectroscopy: The Vanadium Monoxide Molecule J. M. Dyke,* B. W. J. Gravenor, M. P. Hastings, and A. Morris Department of Chemistry, The University, Southampton, SO9 5NH, U.K. (Received: April 23, 1985)
-
-
The first two bands in the UV photoelectron spectrum of VO(X4Z) have been recorded. These are assigned to the ionizations VOf(X3Z-) VO(X4Z-) and VO+(A'A) VO(X4Z-) on the basis of their experimental relative intensities and from the results of ab initio and Hartree-Fock-Slater calculations. Only the first band showed vibrational structure and, from the vibrational separations and band envelope, values of We = 1060 h 40 cm-I and re = 1.54 0.01 8, in the VOf X3Z- state were obtained. Also, the measured first adiabatic ionization energy, 7.25 0.01 eV, was used to derive a value for Do in VOc(X3Z-) of 5.98 f 0.10 eV. The reason for the discrepancy between the first adiabatic IP of VO measured in this work and that measured previously by electron impact mass spectrometry is discussed.
*
Introduction The study of a transition-metal oxide in the gas phase by photoelectron spectroscopy (PES) offers a unique opportunity to probe the electronic structure of the isolated molecule. This has proved to be the case in previous gas-phase PES studies of other metal oxides.'" As part of our continued interest in this area we report a PES investigation of vanadium monoxide, a molecule which is known to be a major constituent in the atmospheres of cool red stars4 Vanadium oxides of different stoichiometry are also of importance in the solid state, being used industrially as catalysts in the selective oxidation of hydrocarbon^.^ The ground electronic state of VO has been established previously as 42-by means of an ESR study of the molecule trapped in an inert gas matrix.6 This is consistent with some more recent gas-phase spectroscopic studies on V07s8as well as with the results of some limited basis set ab initio calculationsg which indicate that the 42-state arises from the molecular configuration .A2u', where the outermost 6 and u molecular orbitals are essentially vanadium 3d and 4s in character, respectively. A number of high-temperature vaporation and thermodynamic studies on vanadium oxides with a range of stoichiometries have been It has been found that V203(s) is the only congruently evaporating vanadium oxide with V and VO being the major vapor species whereas V 0 2 is a much smaller vaporphase con~tituent.'~In contrast, stoichiometric VO(s) vaporizes incongruently to give VO as the major vapor component with smaller partial pressures of V and V02." Only one direct measurement of the first adiabatic ionization energy of VO has been made previously and this was by electron impact mass spectrometry.'2 A value of 8.4 0.5 eVi2 was obtained for VO whereas in the same study the first ionization
*
(1) Dyke, J. M.; Gravenor, B. W. J.; Josland, G. D.; Lewis, R. A.; Morris, A. Mol. Phys. 1984, 53, 465. (2) Dyke, J. M.; Gravenor, B. W. J.; Lewis, R. A.; Morris, A. J . Chem. SOC.Faraday Trans. 2 1983, 79, 1083. (3) Dyke, J. M.; Morris, A.; Ridha, A. M. A.; Snijders, J. G. Chem. Phys. 1982, 67, 245. (4) Spinrad, H.; Wing, R. F. Annu. Rev. Astron. Astrophys. 1969, 7, 249. (5) (a) Cullis, C. F.; Hucknall, D. J. "Specialist Periodical Report on Catalysis"; The Royal Society of Chemistry: London, 1982; Vol. 5, p 273. (b) Rey, L.; Gambaro, L. A.; Thomas, H. J. J . Catal. 1984, 87, 520. (6) Kasai, P. H. J . Chem. Phys. 1968, 49, 4979. (7) (a) Cheung, A. S.C.; Taylor, A. W.; Merer, A. J. J. Mol. Spectrosc. 1982, 92, 391. (b) Cheung, A. S. C.; Hansen, R. C.; Merer, A. J. J. Mol. Spectrosc. 1982, 91, 165. (8) Cheung, A. S. C.; Hansen, R. C.; Lyyra, A. M.; Merer, A. J. J . Mol. Spectrosc. 1981, 86, 526. (9) Carlson, K. D.; Moser, C. J . Chem. Phys. 1966, 44, 3259. (10) Bennett, S. L.; Lin, S. S.;Gilles, P. W. J. Phys. Chem. 1974,78,266. (11) Berkowitz, J.; Chupka, W. A. J. Chem. Phys. 1957, 27, 87. (12) (a) Balducci, G.; Gigli, G.; Guido, M. J . Chem. Phys. 1983, 79, 5623. (b) Balducci, G.; Gigli, G.; Guido, M. J. Chem. Phys. 1983, 79, 5616. (13) Farber, M.; Uy, 0. M.; Srivastava, R. D. J. Chem. Phys. 1972, 56, 5312.
0022-3654/85/2089-4613$01.50/0
*
energy of V 0 2 was measured as 10.5 f 0.5 eV. However, a lower value for the first I P of VO can be derived from a recent determination of the dissociation energy, Do, of VO' in its electronic ground state.I4 The experimental method used in this latter study involved measuring the reaction cross section of the V+ plus C O reaction as a function of ion translational energy. The value obtained for D,,"(VO+) was 5.68 f 0.22 eV.14 Our ab initio molecular orbital calculations indicate that the ground state of VO' is X3Z- and it is reasonable to assume that this dissociates to V+(5D) and O(3P), the ground states of the vanadium ion and the oxygen atom, respectively. Assuming also that VO(X4Z-) dissociates to O(3P) and V(4F), then the first ionization energy of atomic vanadium of 6.74 eVIS and Doo of VO(X4Z-), 6.49 f 0.09 eV,12,16,17 can be combined with the above value of Doo of VO' to yield the first adiabatic IP of VO as 7.55 f 0.3 1 eV, clearly lower than the electron impact mass spectrometric value of 8.4 f 0.5 eV.12 The aim of this present work was, therefore, to resolve the problem over the first ionization energy of VO and to characterize the states of VO+ observed in the photoelectron spectrum. The only other previous spectroscopic work on VO' has involved the study of two transitions of VO', 1Z+-'2+and IA--'A, observed in emission from a microwave discharge in flowing V0Cl3 vapor mixed with helium.]* Unfortunately, these states will not be seen in the photoelectron spectrum of VO because application of the photoelectron spin selection rule shows that an initial quartet state in the neutral molecule will only give rise to triplet and quintet states in the ion.
Experimental Section Vanadium oxide was produced in the vapor phase by vaporizing stoichiometric vanadium monoxide (Cerac. Inc. 99.5%) into flowing helium from an inductively heated tungsten furnace of similar design to that used earlier.1-3s'9 Spectra have been recorded with both a single detector20 and multidetector photoelectron spectrometer2I and sufficiently high vapor pressures to record spectra of VO were obtained at furnace temperatures of 2150 f 30 K for the single detector instrument and 1980 30 K for the multidetector instrument. These temperatures were measured by focussing a calibrated optical pyrometer onto the hottest part of the furnace. Spectral calibration was achieved using
*
(14) Aristov, N.; Armentrout, P. B. J . Am. Chem. SOC.1984, 106, 4065. (15) Sugar, J.; Corliss, C. J. Phys. Chem. ReJ Data 1978, 7, 1191. (16) Cdppens, P.; Smoes, S.; Drowart, J. Trans. Fararaday S o t . 1967,63, 2140. (17) Jones, R. W.; Gole, J. L. J . Chem. Phys. 1976, 65, 3800. (18) Merer, A. J.; Cheung, A. S. C.; Taylor, A. W. J . Mol. Spectrosc. 1984, 108, 343. (19) Bulgin, D.; Dyke, J. M.; Gocdfellow, F.; Jonathan, N.; Lee, E.; Morris, A. J. Electron. Spectrosc. Relat. Phenom. 1977, 12, 67. (20) Dyke, J. M.; Jonathan, N.; Morris, A. In?. Reu. Phys. Chem. 1982, 2, 3. (21) Morris, A.; Jonathan, N.; Dyke, J. M.; Francis, P. D.; Keddar, N.; Mills, J. D.; Reu. Sci. Instrum. 1984, 55, 172.
0 1985 American Chemical Society
4614
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
TABLE I: Computed Vertical Ionization Potentials (eV) of VO(X~Z-Y ionizationb 9a 16 37r 8a
ionic state uroduced
35'A
5n
sz-
ASCF I P
ASCF + C I IP
exptl (this work)
6.54 8.01 7.25 6.94
6.06 7.51 10.04 9.25
7.25 f 0.01 8.42 ic 0.01
e
Dyke et al. TABLE II: Experimental and Calculated Spectroscopic Constants for VO(X4Z-), VO+(X3Z-). and VO+('A)" parameterd re, 8, P,, cm-'
c
"See text for details of these calculations. bOne-electron ionizations were considered from the VO(X4Z-) (...8a237r41629a') configuration. Only the lowest energy ionic state arising from a particular configuration was considered in each case.23.24 C A broad band was observed centered at 11.41 f 0.02 eV associated with VO; see text.
the He Ia spectrum of vanadium,22the He Ia and He I@spectra of methyl iodide, and the He(I1) ionization of helium. As well as the evaporations performed on stoichiometric VO(s) some experiments were also performed with V203(s) and V205(s)as well as VIVO and V/V203 mixtures.
Computational Details The electronic ground state of VO is known from theoretical and spectroscopic ~ t u d i e s ~to- be ~ a 4Z- state arising from the configuration
VO(X~Z-) VO+(X~Z-) V O + ( ~ A ) exptl a b initio H FS exptl ab initio
1.589b 1.534 1.567 lollb 1288 1068
HFS vert. IP, eV exptl a b initio, SCF, ASCF'
H FS
1.54 f 0.01 1.501 1.536 1060 f 40 1380 1150 7.25 6.12
1232 1099 8.42 7.61
6.28
7.24
1.517 1.546
OThis work unless otherwise stated. bFrom ref 7, 8, 25, and 26. CThese ASCF values differ from those shown in Table I because a slightly larger basis set has been used to derive the parameters listed in Table I1 than that used in Table I. d r , = equilibrium bond length; Pe = vibrational wavenumber; vert. I P = vertical ionization potential.
, I
, i
1.0 iai
1.0
Exptl
I
0.4 4
...8a23a416*9a' where the 16 molecular orbital is essentially a V 3d atomic orbital and the 9u molecular orbital is mainly V 4s in character. The 3 7 and 8u orbitals are largely 0 2p in character with small contributions from V 3d orbitals. One-electron ionization from the 9a level will give the 32-,IF, and lZ+ states whereas oneelectron ionization from the 16 level will produce 3A and ]A ionic states. The (3a)-' and (8u)-I ionizations gave 5.311,' s 3 x ,13311(2), and 1,3Z+,ls3Z-, 1*31',5,3Z-states respectively. However, in view of the fact that the ground state of VO is 4Z-, the photoelectron spin selection rule imposes the condition that only quintets and triplets will be accessible by one-electron ionization. Hence, for one-electron ionization from the VO 9u, 16, 3a, and 8u molecular orbitals, it is expected that at most the following ionic states will be observed in the photoelectron spectrum; 3Z- from (9u)-', 3A from (la)-], 5,311,3x, 311(2),and 3+ from (37)-', and 32?, 32-, 31', and 5-3Z-from (8a)-l. Furthermore, for the states produced by the (37r-I and (Sa)-' ionizations the 511 and 5Z- states are expected to be the lowest in energy for each m a n i f ~ l d . ~ ~ . ~ ~ In this work, vertical ionization energies of V O ( X 4 Z ) to the VO+ states 32-,3A, 511, and 5z1- arising from the (9u)-l, (3a)-l, and (8u)-' ionizations have been calculated by using the ASCF method. This involved performing S C F calculations for VO(X4Z-) and the ionic state under consideration, the ASCF vertical IP being obtained by subtracting the two computed total energies. All calculations were carried out at the experimental equilibrium bond length of VO(X4Z-) of 1.5894 The basis set used was a contracted GTO basis similar to that used previously for calculations on Cr0.2,27 For vanadium a total contracted GTO basis set of [5s, 2p, 3d] was used, whereas for oxygen a contracted GTO basis of [3s, 2p] was adopted. The results of these calculations are listed in Table I. In order to make some allowance for the correlation energy change on ionization, limited configuration interaction calculations have been performed for each ionic state as well as the neutral molecule. For VO(X4Z-) a configuration interaction calculation was performed by constates generated from the reference determinant sidering all 42c-
i
I
I
f
i
020
0.01 1 U,'
0.09,
85
8.0
75
7.0
65
/
, 6.0
Is3+
A.7*8325,26
(22) Dyke, J. M.; Gravenor, B. W. J.; Hasting, M. P.; Josland, G. D.; Morris, A. J . Elecrron Spectrosc. Relat. Phenom. 1985, 35, 6 5 . (23) Raftery, J.; Scott, P. R.; Richards, W. G. J. Phys. B 1972, 5, 1293. (24) Scott, P. R.; Raftery, J.; Richards, W. G. J . Phys. B 1973, 6, 881. (25) Lagerqvist, A.; Selin, L. E. Ark. Fys. 1957, Z2, 553. (26) Huber, K. P.; Herzberg, G. "Molecular Spectra and Molecular Structure; Constants of Diatomic Molecules"; Van Nostrand: New York, 1979. (27) (a) Roos, B.; Siegbahn, P. Theor. Chim. Acta 1970, 17, 209. (b) Roos, B.; Veillard, A,; Vinot, G. Theor. Chim. Acta 1971. 20, 1
1.E.leVI
Figure 1. Comparison of computed and experimental PES band envelopes for the first two bands of VO. The most intense component of each band has been set to 1.OO; no allowance has been made for the relative intensity of each band.
through single and double excitations with the constraint that only the orbitals 8u to 13u, 3 a to Sa,and 16 to 26 were open to orbital substitution. These were the orbitals whose energies were in the region -1.15 to 1.31 au. Exactly the same approach was adopted for the four VO" states listed in Table I with the same orbitals open to substitution. The corrected ASCF vertical ionization potentials obtained by this approach are also listed in Table I. In order to further assist in the assignment of the VO photoelectron spectrum, vibrational band envelopes of the bands of VO have been calculated. The procedure involved computing wavefunctions and total energies for the states VO(X4X-), VO+(X3B-), and VO+(3A)at five points within fO.l A of the potential energy minimum and fitting the points obtained to a fourthmorder polynomial in (r-re). Potential energy curves were obtained by means of both ab initio Hartree-Fock (HF) and Hartree-FockStater (HFS) calculation^.^^^^^ In the ab initio Hartree-Fock calculations a double-{ STO basis was usedz7 with added polarization functions (0 3d and V 4d) whereas in the HartreeFock-Slater claculations a triple-f STO basis with added polarization functions (two 3d functions and one 4f function on oxygen and two 4d functions and one 4f function on vanadium) was utilized. The spectroscopic constants, We and re, derived from both the ab initio H F and HFS methods are listed in Table 11. Using the method described p r e v i o ~ s l ythese , ~ ~ parameters were then used to compute the vibrational component intensities for ionization from the lowest vibrational level in VO(X4Z-) to t h e (28) Baerends, E. J.; Snijders, J. G.; de Lange, C. A,; Jonkers, G. In "Local Density Approximations in Quantum Chemistry and Solid State Physics"; Dahl, J. P., Avery, J. Eds.; Plenum Press: New York, 1984. (29) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1. (30) Dyke, J. M.; Kirby, C.; Morris, J . Chem. Soc., Faraday Trans. 2 1983, 79, 438.
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4615
PES Study of Vanadium Monoxide
100
Avo I 0,2
30
G,1
0,0
120
Bvo
U
w
Lo
\ Lo
I-
z
U
3 0
w
VI
U
\ Ln I-
z
3 0 U
h
Help)
7
(
O
‘
0
I
I
I.5
7.0
L
L
6.5
I. E.(e V ) Figure 3. Single detector photoelectron spectrum obtained from a stoichiometric VO sample recorded in the 6.5-8.0-eV ionization energy region at a furnace temperature of 2150 K. Ordinate: counts s-’; ab-
Figure 2. Single detector photoelectron spectrum obtained from a V:VO (1:l) mixture recorded in the 6.0-9.0-eV ionization energy region at a furnace temperature of 2150 K. Ordinate: counts s-’; abscissa: ioni-
scissa: ionization energy, eV.
zation energy, eV. states VOt(X3Z-) and VO+(A3A). The results of these Franck-Condon calculations are shown schematically in Figure 1.
Results and Discussion The He1 photoelectron spectrum recorded in the 6.0-9.0 eV ionization energy region for the vapor above a stoichiometric V:VO mixture at a furnace temperature of 2150 f 30 K is shown in Figure 2. Bands associated with atomic vanadium, marked A,, B,, C,, and D, in Figure 2, can be clearly seen in this spectrum and are useful calibration features. As has been described in detail elsewhere,22bands B, and C, arise from a (4s)-I ionization of the ground state of atomic vanadium, the a 4 F state, and bands A, and D, arise from a (3d)-I ionization of an excited state, the a 6Dstate, which is approximately 0.27 eV above the ground state.Is Also shown in Figure 2 are two bands labeled A,, and B,, whose relative intensity remained constant on varying the inert carrier gas pressure and furnace temperature. Similar spectra to that shown in Figure 2 have also been recorded from solid VO except that the relative intensity of bands attributed to atomic vanadium were a lot weaker relative to the bands A,, and B,,. An expanded scan of the 6.5-8.0-eV IP region obtained from solid VO is shown in Figure 3. The vertical ionization energies of the A,, and B,, bands were measured as 7.25 & 0.01 and 8.42 & 0.01 eV, respectively. These bands were assigned to ionization of VO on the basis of the calculations shown in Table I and because VO is known to be the main vapor species above solid VO at the temperatures used for evaporation. For both A,, and B,, the adiabatic and vertical components coincide. No vibrational structure was observed in the second band of VO although three components were observed in the first band (see Figure 3). Also, on the low ionization energy side of the first band a broad feature was observed at 7.12 0.02
*
8.5
8.0
7.5
7.0
I E lev1
Figure 4. Multidetector photoelectron spectrum obtained from stoichiometric VO recorded in the 7.0-8.5-eVionization energy region at a
furnace temperature of 1980 K. It represents 1900 accumulated scans in a total time of 110 s. Ordinate: counts; abscissa: ionization energy, eV. eV whose intensity relative to the other A,, components increased on increasing the furnace temperature. Because of the position of this peak, it cannot arise solely from the atomic vanadium band, B,, which is expected at 7.06 eVZ2and as a result it was thought that it arises from two contributions, the atomic vanadium band, B,, as well as a vibrational “hot band” u r = 0 u” = 1 associated with the first ionization of VO. From the known photoelectron spectrum of the relative intensity of the bands A, and B, can be determined at the experimental furnace temperature and this ratio has been used to perform an approximate deconvolution of the band at 7.12 eV into two components (see Figure 3). A much simpler spectrum is, however, obtained by using a multidetector photoelectron spectrometer (see Figure 4). Figure
-
4616
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
4 was recorded for stoichiometric VO(s) and represents 1900 scans accumulated in a total time of 110 s. The only major difference between the conditions used to record the single detector (Figure 3) and multidetector (Figure 4) spectra was that the multidetector spectra were recorded at a furnace temperature of 1980 f 30 K, approximately 170 K lower than the temperature used to obtain Figure 3. This implies that the improved sensitivity of the multidetector instrument allows spectra to be recorded at lower temperature than that used on the single detector instrument and this reduces dissociation of the VO molecules to atomic vanadium and oxygen which apparently takes place to a much greater extent at higher temperature. This result also means that degradation or reaction of high-temperature components used in the furnace assembly will be reduced in the multidetector instrument. The experimental relative intensity of the first two VO bands is 1:(6.1 f 0.4), when allowance for the analyzer transmission function is made. Our ab initio calculations indicate that the first ionization process can be described approximately as a metal (4s)-’ ionization whereas the second VO band can be attributed essentially to a metal (3d)-’ ionization. Hence, the relative intensity of the first two bands seen in Figure 4 is consistent with the vanadium 3d orbitals having a much greater photoionization cross section than the vanadium 4s orbitals at the He I photon energy as has been established from the atomic vanadium photoelectron spectrum.zz A similar result has also been obtained recently for Ti and Ti0.l As shown in Figure 4, the second band of VO exhibits no vibrational structure whereas the first band shows three clear vibrational components as well as a weak feature to the low energy side of the adiabatic component attributed to the u’ = 0 u” = 1 “hot band”. These experimental envelopes shown in Figure 4 are consistent with the computed envelopes shown in Figure 1 and add further support to the assignments shown in Table I. For the first band of VO, measurement of the vibrational separations gave We = 1060 f 40 cm-’ for the VO+(X3L’-) state. Also use of the vibrational component intensities, obtained from spectra which showed a very small “hot band” contribution, via a series of Franck-Condon calculations’ allowed the equilibrium bond length in VO+(X3Z-) to be calculated as 1.54 f 0.01 A. For the multidetector spectra which showed the highest “hot band” intensity, the u’ = 0 u” = 1 component was at most equal in D” = 0 component. The computed intensity to the u’ = 2 Franck-Condon factors for these ionizations, obtained by using the parameters re and #e derived for VO’(X3Z-) in this study and the corresponding values for VO(X4L’:-)measured in other studies,7,8~zS,z6 have been used to estimate the maximum vibrational excitation temperature in the multidetector experiment as 860 f 20 K, considerably less than the furnace temperature. A similar result has been observed in the Ti0 case’ and is due to collisional deactivation of the vibrationally excited metal oxide before ionization occurs, a process which is much more efficient than collisional deactivation of electronically excited vanadium.22 Experiments on the oxides VO(s) and V,O,(s) and the mixtures V/VO and V/Vz03 all showed bands attributable to V and VO in the ionization energy region 6.0-9.0 eV. Also, for all these samples a broad band was observed in the 10.0-13.0-eV ionization energy region which was approximately proportional to the first two bands of VO (see Figure 5). The peak maximum of the broad band is estimated at 11.41 f 0.02 eV although subsidiary maxima were also observed at 10.58 f 0.02 and 11.06 f 0.02 eV. Detailed assignment of this region is not possible but it is thought that this broad, composite band can be attributed to components arising from the (8u)-I and (37r-’ ionizations of VO. Experiments were also performed on vanadium and vanadium monoxide to investigate the suggestion made earlier2*that a band at 10.47 f 0.02 eV, observed on evaporation of atomic vanadium, arises from V 0 2 . No bands were observed in the 10.0-1 1.O-eV IP region which could be positively associated with VOz and hence we are forced to conclude that the band at 10.47 f 0.02 eV in ref 22 does not arise from ionization of this molecule. A number of experiments were also performed on VZOS. For these samples high pressures were experienced in the ionization chamber in the temperature region
-
--
15
14
~
11
-_-_-
~~
12
11
10
1
3
I E leVi
Figure 5. Single detector photoelectron spectrum obtained from stoichiometric VO recorded in the 8.0-14.0-eV ionization energy region. Ordinate: counts s-I; abscissa: ionization energy, eV.
1000-1400 K and, as a result, spectra were not recorded in this temperature range because the pressure was too high for the electron detector to be operated safely. It is well-known that in this temperature range, V205decomposes to Vz03(s),V,Olo(g), and V408(g).13At temperatures greater than 1400 K, the residual sample behaved as expected, in an exactly analogous way to v203(s). Comparison of the experimental and ab initio calculated vertical ionization potentials of VO shows that at the ASCF level the 511 and 52-states obtained from the (3a)-’ and ( 8 ~ ) one-electron ~’ ionizations lie lower than the ’A state obtained from the (16)-’ ionization (see Table I). However, when some allowance is made for the correlation energy change on ionization in each case, the ’lI and 52-states of VO’ are predicted above the 3A state. A similar result has been obtained recently for TiO’.’ Further support for the assignment of the first two bands of VO presented in this work comes from the computed Franck-Condon envelopes shown in Figure 1 and the calculated spectroscopic constants listed in Table 11. As shown in this table, ab initio calculations for VO(X4Z-) and VO+(X3Z-) near the Hartree-Fock limit overestimate We and underestimate re, a result which is well established for calculations of this type on small diatomic molecule^.^^ In contrast, the corresponding parameters computed by the HFS method are much closer to the experimental values. This result arises, as has been noted e l s e ~ h e r e ,because ~ ~ , ~ ~of partial allowance for electron correlation in the HFS method. It is also interesting that the second band of VO experimentally shows no vibrational structure and this is consistent with the VO+ A3A state having spectroscopic parameters re and We very close to those of the electronic ground state of the neutral molecule, VO X42-, as expected from the HFS computed parameters listed in Table 11. As can be seen in Table I, for the first two bands of VO, the ASCF computed vertical ionization potentials are in better agreement with the experimental positions than the ASCF plus CI values. This apparent anomaly arises because the ASCF values are obtained from S C F ab initio calculations not at the Hartree-Fock limit and also because only limited configuration interaction has been performed in each state. As indicated in the Introduction, measurement of the first adiabatic ionization energy of VO as 7.25 f 0.01 eV allows the dissociation energy Do of VO’ to be determined. The value obtained, 5.98 f 0.01 eV, compares reasonably well with a value determined in a V+/CO beam-gas study of 5.68 f 0.22 eV.I4 The first IP of VO measured in this present PES study also compares favorably with 7.55 f 0.31 eV derived from ref 14 but unfavorably (31) Richards, W. G . ; Raftery, J.; Hinkley, R. K. Spec. Period. Rep.: Theor. Chem. 1974, 1, 1. (32) Dyke, J. M.; Morris A,; Ridha, A. M. A,; Sniiders, J. G. Chem. Phys. 1982, 67, -245. (33) Post, D.;Baerends, E. J . J . Chem. Phys. 1983, 78, 5663
J . Phys. Chem. 1985, 89, 4617-4621 with a value of 8.4 f 0.5 eV obtained from an electron impact mass spectrometric investigation.', From this present study and recent studies on Ti, TiO,' and V,,, the reason for the discrepancy is clear. In ref 12, the current of VO' (Ivo+) was studied as a function of incident electron energy and, in order to estimate the first ionization potential of VO, Zvo+ is extrapolated back to zero to give the appearance potential of VO+ from VO. However, in the incident electron kinetic energy range 10.0-20.0 eV it is likely that the cross section for the second one-electron ionization of VO is much greater than the cross section for the first one-electron ionization, as indicated in the present work. It is not surprising therefore that the linear extrapolation method used in ref 12 erroneously yields a value which is very close to the second IP of VO even though the measurements were calibrated by use of atomic vanadium. In the only previous spectroscopic study on V0',Is two new electronic transitions of VO' have been observed in emission (IZ'-IZ+ and IA-IA). In both cases the lower electronic states are thought to be perturbed by the A3A state and it seems fairly
4617
clear that both the A3A and lower IA state arise from the electronic configuration . . . 8 ~ ~ 3 ~ ~ 1 For 6 ~ the 9 a lower ~ . ' A state, values of AG,,,of 960 cm-l and re of 1.601 A were obtained from rotational analysis of the observed spectrum. The corresponding values obtained in this work for the VO' X3Z- state are 1050 f 40 cm-l and 1.54 f 0.01 A. Clearly further work is needed to accurately establish the position of the excited singlet states of VO+ observed in ref 18 relative to the VO' 3Z- ground state and it is hoped that this present study will stimulate both theoretical and experimental interest in this problem. Acknowledgment. We thank the C.E.G.B. and S.E.R.C. (U.K.) for financial assistance, the S.E.R.C. for the award of a studentship to B.W.J.G. and a C.A.S.E. studentship (with C.E.G.B.) to M.P.H. Dr. A. Paul and Mr. A. M. Ellis are also acknowledged for assistance in the final stages of this work. Professor E. J. Baerends and Dr. J. Snijders are also thanked for valuable discussions and making copies of their HFS programs available. Registry No. VO, 12035-98-2.
Dynamics of CS(A'n) Formation in the Dissociation of CS2 Konrad T. Wu Department of Chemistry, State University of New York, College at Old Westbury, Old Westbury, New York 11568 (Received: May 6, 1985) CS(AIII) product energy distributions are analyzed from the fluorescence observed in an Ar(3P2,0)+ CS2 afterglow and compared with results from photodissociationat various wavelengths. The formation mechanism of CS(A) in the dissociation of CS2 originates from predissociation of the Rydberg state converging into the ground state of CS2+below the ionization limit, via a very steep repulsive potential curve. Energy disposal into translation of the fragments in this bound-continuum region is consistent with the result predicted from the simple impulsive model. Vibrational energy distributions of CS(A) fragment are found to be influenced principally by the Franck-Condon effects; such distributions fit well with the results from golden rule calculations.
Introduction Photofragment dynamics of polyatomic molecules has received considerable interest in recent years.'" New technological advances in the development of powerful light sources and sophisticated detection systems have improved our understanding of the dissociation processes under investigation. Detailed dynamical information can now be obtained from such measurements as final-state distributions (translational and internal), product angular distribution, dissociation lifetime, polarized fluorescence, etc. As a result of rapidly increasing disclosure of the microscopic details of photofragmentation, many theoretical models have been developed for interpreting the experimentally observed dynamical effects. It is believed that accurate theoretical description of the photofragment dynamics may soon become possible for simple polyatomic m o l e c ~ l e s . ~ - ~ Analysis of the vibrational and rotational structures in the fluorescence emission of electronically excited fragments provides a simple way of probing the dynamics of excited-state photofragmentation; the amount of such information available is com(1) (2) (3) (4)
Simons, J. P. J . Phys. Chem. 1984,88, 1287. Leone, S. R. Adu. Chem. Phys. 1982, 50, 255. Bersohn, R. J . Phys. Chem. 1984, 88, 5145. Shapiro, M.; Bersohn, R. Annu. Reu. Phys. Chem. 1982, 33, 409. (5) Greene, C. H.; Zare, R. N. Annu. Rev. Phys. Chem. 1982, 33, 119. (6) Freed, K. F.; Band, Y. B. "Excited States"; Lim, E. C., Ed.; Academic Press: New York, 1977; Vol. 3, p 109. (7) Morse, M. D.; Freed, K. F. J . Chem. Phys. 1981, 74, 4395. (8) Band, Y.B.; Freed, K. F.; Kouri, D. J. J . Chem. Phys. 1981, 74,4380. (9) Morse, M. D.; Freed, K. F.; Band, Y. B. J. Chem. Phys. 1979, 70, 3604, 3620.
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parably much less than for photodissociation processes leading to the ground-state fragments. Even for simple triatomic molecules, limited information is a ~ a i l a b l e . ' - ~ J ~ *It' is thus highly desirable to expand the data base for these molecules, which will be useful for further development of dynamical models. Analogous to the photodissociative excitation process of triatomic molecules, other fragmentation methods such as electron-12 and particle-impactI3-l5dissociative excitation provide complementary dynamical information. We have recently demonstrated13 that the vibrational energy distribution of SO(A) produced from the argon-metastable impact dissociation of SO, is comparable to that resulting from the photodissociation process. The formation mechanism of SO(A) via predissociation of SO, is identical with the photofragment dynami~s.'~It is interesting to extend this kind of study in order to obtain a better understanding of the dynamics of triatomic molecular fragmentation. Since a great deal of dynamical information is available concerning the photodissociative excitation of CS2 for the formation of CS(A'II),1'~12,'6~'7 it would be reasonable to investigate the excited-state fragmentation of
'
(10) Lahmani, F.; Lardeux, C.; Solgadi, D. J . Chem. Phys. 1982, 77, 275. (11) Ashfold, M. N. R.; Quinton, A. M.; Simons, J. P. J . Chem. SOC., Furuduy Trans. 2 1980, 76, 905 and references therein. (12) See, e.g.: Ajello, J. M.; Srivastava, S. K. J . Chem. Phys. 1981, 75, 4454. (13) Wu, K. T. Chem. Phys. 1984, 87, 109. (14) Ozaki, Y.; Kondow, T.; Kuchitsu, K. Chem. Phys. 1983, 77, 223. (15) Snyder, H. L.; Smith, B. T.; Parra, T. P.; Martin, R. M. Chem. Phys. 1982, 65, 397. (16) Lee, L. C.; Judge, D. L. J . Chem. Phys. 1975, 63, 2782. (17) Okabe, H. J . Chem. Phys. 1972, 56, 4381.
0 1985 American Chemical Society
