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enhancement). In the formamide aggregates the electronic transitions should also split into A and B symmetry components. These can be mixed vibronically via a B symmetry vibration. We propose that the 1742-cm-l aggregate C=O stretching mode mixes the A component of the n-x* state with the B component of the x-x* state, or vice versa, accounting for the resonance with the n-x* transition. Since the 797-cm-I band shows a similar resonance, it is also assigned to a B symmetry aggregate mode, although the detailed composition is uncertain. The 200-nm peak in the 1742-cm-I profile may likewise be due to n-x*/x-x* vibronic mixing, this time resonant with the r - ~ *state.
it
H-f-H
Figure 12. An aggregate of formamide molecules, which is postulated to occur in concentrated aqueous solution.
Bravais cell would have two formamide molecules, which would form A-type (symmetric) and B-type (antisymmetric) combinations of C=O stretches. These are assigned to the lower and higher frequency Raman bands, respectively. Thus, the 1742-cm-l Raman band in aqueous solution is assigned to the B-type vibration of the proposed linear aggregate. Now we turn to the question of the unique excitation profile peak at 220 nm observed for the 1742- and 797-cm-' Raman bands. Since the 1742-cm-' band is assigned to a B symmetry mode, in both liquid formamide and aqueous solution, its enhancement is expected to arise from vibronic coupling (B term
Conclusions The formamide Raman excitation profiles reveal an upward trend toward the first x-x* transition at 183 nm, but a superimposed structure in the region of the n-x* transition at 220 nm. Most of the bands show a shallow trough at wavelengthsjust above 220 nm, which can be modeled as resulting from destructive interference in the Raman tensor and between the locally resonant weak n-x* transition and the much stronger higher lying x-x* transition. A deep trough seen for the 609-cm-' OCN bending mode is consistent with a larger interference effect arising from a large decrease in the OCN angle in the n-a* excited state, consistent with evidence from related molecules. Deep UV excitation reveals new Raman bands, at 1742 and 797 cm-I for liquid formamide and in aqueous solutions. These are demonstrated to arise from aggregates and are assigned to B symmetry modes of the aggregate. The excitation profile maxima seen uniquely for these bands at 220 nm are consistent with vibronic mixing of the A component of the n-x* state with the B component of the x-x* state, or vice versa. Acknowledgment. This work was supported by NIH Grant GM 25158 (to T.G.S.). P.H. was the recipient of an Otto-Hahn stipend provided by the Max-Planck-Gesellschaft, FRG. Registry No. Formamide, 75-1 2-7.
Nonadiabatic Unimolecular Reactions. 4. Isolated State Decay in the Fragmentation of the Formaldehyde Cation J. C. Lorquet* and T. Takeuch? Dgpartement de Chimie, Universitg de Li?ge, Sart- Tilman. B-4000 Licge 1 Belgium (Received: June 6, 1989; In Final Form: September 25, 1989) ~
The dissociation (by hydrogen loss) of the first excited state (A2BI)of the formaldehyde cation as a function of its vibrational excitation is analyzed. The reaction has been_experimentally observed as a metastable dissociation. Its mechanism involves an electronic predissociation by the ground X2B2state in the tunneling regime. A statistical treatment recently developed for nonadiabatic interactions accounts for the low value of the rate constant as well as for a surprisingly large isotope effect that persists over an energy interval of ca. 0.7 eV. The method requires a partitioning sf the set of degrees of freedom. The CO bond stretch (which assumes very different equilibrium distances in electronic states A and B) introduces a Franck-Condon factor in the expression of the rate constant. The remaining degrees of freedom are treated collectively and give rise to a RRKM-like expression in which the nonadiabatic transition probability is introduced as a transmission coefficient. The hydrogen loss of H2CO+ appears to be the first well-established case of isolated state decay by noncommunicating electronic states, i.e., a process where the rate-limiting step is internal conversion and not dissociation.
I. Introduction Much insight into the dynamics of unimolecular reactions has been gained by the study of the dissociation processes of ionized molecules in a mass spectrometer.'" In particular, the development of photoion-photoelectron coincidence spectroscopy (PIPECO) has been at the origin of a real breakthrough in our 'Permanent address: Department of Chemistry, Nara Women's University, Nara 630, Japan.
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understanding of that field. Another important source of information results from the fact that slow dissociation processes (Le., (1) Lifshitz, C. Adu. Mass Spectrom. 1978, 7 A , 3; J . Phys. Chem. 1983, 87, 2304; Int. Rev. Phys. Chem. 1987, 6, 35; Adu. Mass Spectrom. 1989, 1 1 , 3.3
113.
(2) Bowers, M . T., Ed. Gas Phase Ion Chemistry; Academic Press: New York, 1979, 1984; Vols. 1-3. (3) Derrick, P. J.; Donchi, K. F. In Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1983; Vol. 24.
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I1
X s. This implies an isotope effect of the order of 45 f 20 on the rate constant. At higher values of the ufco vibrational quantum number, the isotope effect cannot be measured, but it certainly remains large over a energy interval of about 0.7 eV. Moreover, in a charge-exchange study, Wankenne et aI.l5 also noticed that the lifetime of H2CO+ was shorter than that of D2CO+, by a factor of 9 or more. Unfortunately, the energy control is not as good as in photoionization experiments. Furthermore, they concluded that most of the metastable ions observed in electron impact experiments Cesult from an autoionization mechanism (presumably to the X2B2 ground state). Thus, a careful distinction has to be made between electron impact and photoionization in the analysis of this problem.
H2 CO’
HCO’
+
H
A
Figure 1. Potential energy curves of electronic states and of t h e formaldehyde ion as a function of the reaction coordinate (CH or CD stretch).
those taking place in the microsecond time scale) are easily detected in a mass spectrometer and are referred to as metastable transitions.’ In conformity with RRKM-QET theory,’”** their energy onset is usually close to the thermodynamic threshold. This is not the case, however, for the reaction D2CO+
-
DCO+
+D
Lorquet and Takeuchi
(1.1)
which presents a metastable transition appearing at an energy of 14.06 eV. This energy is 2 eV higher than the thermodynamic threshold and coincides wiLh the zero-point energy of the first electronically excited state AZBI. From this observation, Guyon et aL9 assiped the metastable transition to a weak predissociation of state A. Furthermore, they noticed that the corresponding process was not detectable for H2CO+ and HDCO+ ions, because it takes place at a rate that is higher than the range where the measurements can be carried out. Ab initio calculations by Vaz Pires et aI.lo and by Barbier et al.” confirmed that the mechanism is controlled by tunneling, as represented in Figure 1 . As a result of a very careful PIPECO study of several aspects of the dissociation of formaldehyde ions,I2*l3Bombach et al. confipned that reaction 1.1 is indeed taking place from the first excited A2Bl state of D2CO+. Furthermore, they could measure its lifetime as a function of the vibrational quantum number of the CO stretching mode (ofco). Their results are given in Figure 2. The lifetimes determined by Bombach et al. have been converted into rate constants of reaction 1. I since the latter is the only available channel1I,l3up to utco = 4. The u’co = 5 level is known to dissociate into CO+ + H2 in the case of H2CO+,and the corresponding level might also contribute very weakly for D2CO+ (by tunneling through a barrierlL.l3),so that we feel that the inverse of the reported experimental lifetime for D2CO+(A2Bl,o&o=5) constitutes an upper limit for the rate constant of reaction 1 .I (as indicated schematically in Figure 2 ) . Bombach et al. also confirmed that the rate constant of the nondeuterated formaldehyde ion? is so large that no metastable signal can be detected. For the (A2Bl;uL0 = 0 ) state of H2CO+, the upper limit of the lifetime was estimatedL4to be about I .3 (4) Baer, T . Adc. Chem. Phys. 1986, 64, 1 1 1. (5) Dannacher. J. Org. Mass Spectrom. 1984, 19, 253. (6) Lorquet, J. C. Org. Mass Spectrom. 1981, 16, 469. Lorquet, J. C.; Barbier, C.; Leyh-Nihant, B. Adu. Mass Spectrom. 1986, IOA, 71. (7) Holmes, J. L.; Terlouw, J. K. Org. Mass Specrrom. 1980, 15, 383. (8) Forst, W . Theory of Unimolecular Reactions; Academic Press: New York, 1973. (9) Guyon, P.M.:Chupka, W. A.; Berkowitz. J. J . Chem. Phys. 1976, 64, 1419. (IO) Vaz Pires, M.; Galloy, C.; Lorquet, J. C. J . Chem. Phys. 1978, 69, 3242. ( I I ) Barbier, C.; Galloy, C.; Lorquet, J. C. J . Chem. Phys. 1984, 81, 2975. ( I 2) Bombach, R.; Dannacher, J.; Stadelmann, J. P.; Vogt, J. Chem. Phys. Letr. 1980, 76, 429; Int. J . Mass Spectrom. Ion Phys. 1981, 40, 275. ( I 3) Bombach, R.; Dannacher, J.; Stadelmann. J. P.; Vogt, J. Chem. Phys, Lett. 1981, 77. 399.
11. Nonadiabatic Reactions Since the tunneling process described in Figure 1 involves a transition between two electronic states, reaction 1 . 1 is called n ~ n a d i a b a t i c . ’ ~ Although ~’~ less thoroughly studied, these reactions are known to present interesting characteristics. Bimolecular reactions have received considerable attention, both experimentally18J9and theoretically.16,20For unimolecular reactions, a statistical theory has recently been d e v e l ~ p e d , which ~ ~ - ~ ~is a natural extension of the usual RRKM t h e ~ r y . I - ~The * ~ role of the transition state is played by the point of lowest energy along the nonadiabatic crossing seam. Just as in RRKM theory, the calculation of the microcanonical rate constant k ( E ) implies a summation over all the possible exit channels, which, according to Eyring’s original suggestion, should, in principle, be weighted by a transmission coefficient. For adiabatic reactions (RRKM theory), this coefficient is nearly always taken equal to unity.s However, just as in the case of t ~ n n e l i n g ?it~sometimes assumes very low va1ues16,20,22*23 in the case of nonadiabatic reactions and therefore plays an essential role. For example, in the case of an electronic predissociation at energies below (or close to) that of the crossing point (Figure l ) , the appropriate expression of the rate constant is21,22 E
kPrd(E) = [ l / h N ( E ) ] C W,*(E,) P ( E - E , - E , )
(2.1)
E,=O
where N ( E ) is the density of states of a system with n degrees of freedom, E, is the energy of the crossing point, E, is that fraction of the internal energy which is stored in the system of (n - 1 ) degrees of freedom other than the reaction coordinate, W,*(E,) is the corresponding degeneracy at energy E,, and PQ(E-E,-E,) is the nonadiabatic transition probability calculated when the energy in the reaction coordinate is equal to ( E - E,). In addition to the usual restrictions encountered in the RRKM theory (existence of a proper transition state, establishment of a state of microcanonical equilibrium, separability of the vibrational wave functions), two further requirements have to be met for the theory developed in ref 3 to apply: (i) the theory is limited (14) Dannacher, J.; Stadelmann, J. P. Private communication. (1 5 ) Wankenne, H.; Caprace, G.;Momigny, J. Int. J. Mass Spectrom. Ion Processes 1984, 57, 149. (16) Tully, J. C. In Dynamics of Molecular Collisions; Miller, W. H., Ed.; Plenum Press: New York, 1976; Part B. Nikitin, E. E. Adu. Chem. Phys. 1975.28, 317. Child, M. S. Atom-Molecule Collision Theory; A Guidefor the Experimentalist; Bernstein, R.B., Ed.; Plenum Press: New York, 1979. (17) Delos, J. B. J . Chem. Phys. 1973, 59, 2365. Desouter-Lecomte, M.;
Dehareng, D.; Leyh-Nihant, B.; Praet, M. Th.; Lorquet, A. J.; Lorquet, J. C. J . Phys. Chem. 1985, 89, 214. (18) Earl, 8. L.; Herm, R. R. J . Chem. Phys. 1974, 60, 4568. (19) Schulz, R. H.; Armentrout, P. B. J . Phys. Chem. 1987, 91, 4433. Tomoda, S . ; Suzuki, S . ; Koyano, I. J . Chem. Phys. 1988,89,7268. Rincon, M.;Kirchner, N . J.; Bowers, M. T. I n t . J . Mass Spectrom. Ion Processes 1988, 86, 369. (20) Zahr, G.E.; Preston, R. K.; Miller, W. H. J . Chem. Phys. 1975.62. 1
v’co implies that-internal energy is delivered to the C O bond of the final state X, at the expense of either the internal energy stored in the set Q+ or, more frequently, at the expense of the energy released in the reaction coordinate. In the latter case, this transfer is described by an upward shift of the potential energy curve along the reaction coordinate, as explained by Caplan and Child (see Figure 4 of ref 26). The summation over u”co in eq 3.3 closes when the dissociation asymptote reaches or exceeds the energy of the initial state. The terms u”co < u’co imply on the other hand thatpart of the vibrational energy deposited in the C O bond of state A by the Franck-Condon transition is transferred to the other degrees of freedom of state X, either in its internal degrees of freedom or in the reaction coordinate (where it appears as translational energy of the XCO+ X fragments). It has been assumed that all of these possibilities are equally probable. Our final equations are, thus, from eq 2.1, 3.3, and 3.5:
+
k(u’C0) =
F I(u~”u”co)12ku~,,o(En)
(3.6)
ua)
k(u’co,n) =
c
(1 / h ) u‘“ 1 ~ u ’ c o l o ’ ’ c o ) lJ24~n[t Q )
v(Q)MQ+) tlm(9) dQ]’ (3.3)
which is a generalization of the golden rule. In this expreision, Y(Q)is the matrix element coupling the electronic states A and X that is responsible for the electronic predissociation, uko and o,’ko are-the quantum numbers of the C O stretch vibration in states A and X, respectively, n denotes a particular collection of ( 3 N (25) Heller, E. J.; Brown, R. C. J . Chem. Phys. 1983, 79, 3336. ( 2 6 ) Caplan, C. E.; Child, M. S. Mol. Phys. 1972, 23, 249.
where E , is defined in eq 3.4. The summation index E , denotes the fraction of internal ensrgy that goes into the set Q+ of ( 3 N - 8) oscillators of state A, and W ( E J is the corresponding degeneracy. The quantity &,,,,,(En) can thus be considered as the rate constant of a fictitious molecule made up of a collection of ( 3 N - 7 ) oscillato_rs (those of the formaldehyde ion with the C O stretch removed). N is the energy-level density of this fictitious molecule. Its dependence on uffc0 results from the fact that, as explained above, the difference v’Ic0 - v’co determines the shift of the potential energy curve along the reaction coordinate and hence the permeability F@ of the potential wall to be tunneled through.
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The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
V. Discussion Clearly, a complete, quantitative ab initio calculation of nonadiabatic rate constants is out of reach. Tunneling processes are notoriously sensitive to the details of the shape of the potential energy surfaces of the two electronic states involved. In the present case, its calculation involves a very uncertain extrapolation lsee I Figure 1 and eq 4.2) of th_e potential energy curve of state X in L5L the energy range of state A (Le., over an energy interval of 3 eV or more). The steepness of this slope is an essential parameter CD20+ii25,, v c o i - DCO’+ D which determines the quality of our calculated results. (Another _~__._____. possibility would be to introduce a slight shift in the equilibrium 0 : 2 3 L v c o positions of coordinate q in states A and X.) Also, the usual Figure 3. Calculated rate constants. The size of the rectangles indicates modelization by a set of separable harmonic oscillators is certainly the dispersion brought about by a change in the assumed vibrational a dangerous oversimplification. The electronic coupling which frequencies. connects the two surfaces is introduced in a general way. Also disregarded is the role played by the rotational degrees of freedom. IV. Calculations Therefore, instead of attempting a multiparameter fit to the The potentis1 energy curves along the reaction coordinate of experimental data, we restrict the discussion to a qualitative states A and X have been modeled by very simple expressions assessment of the importance of the different factors that are responsible for the experimental effects, Le., the low value of the V,(q) = 16.265(4 - qeq)2 13.907 (4.1) rate constant of eq 1.1, and the surprisingly large isotope effect V , ( q ) = D( 1 - exp[-P(q - q,,)]I2+ (1 1.926 - D) (4.2) which persists over an important energy interval. According to eq 3.6 and 3.7, several explanations should bcconsidered: (i) the with Vin electronvolts and q in angstroms. The same equilibrium tunneling process, (ii) the density of states N ( E ) which appears qq has been assumed for both electronic states for simdistance in the statistical equations, (iii) the I(ukolu’ko) l2 Franck-Condon plicity. Several sets of calculations were carried out with different factors, and (iv) the number of terms that contribute to the values of the steepness parameter /3, with two valuesz7of qq (1.09 summation over E , in eq 3.7. and I.lOA) and with twovaluesofD (1.201 and 1.160eV). The The analysis of the magnitude of the different terms that appear I ( u’co~u’’co)~2Franck-Condon factors were calculated with in eq 3.6 and 3.7 reveals that tunneling is the main factor restandard formulas.28 Two values (0.14 and 0.28 A), suggested sponsible for both effects (low rate constant and large iso_tope by ab initio calculations,’0~11~27 were used for the difference betwee? effect). Next comes the high value of the density of states N ( E ) the eguilibrium distances of the CO bond in electronic states A resulting from the presence of a low-energy out-of-plane wagging and X. The nonadiabatic transition probabilities PQ have been mode. Thus, there is an isotope effect on both the numerator and calculated with the weak-coupling double passage f ~ r m u l a . ’ ~ , ~ ’ - ~the ~ denominator of eq 3.7: the-deuterated species tunnels less A valuelo of 40 cm-’ has been adopted for the electronic matrix easily, while at the same time, N D is found to be approximately element V. As a check, the transition probability was also cal4 times larger than NH. Both effects operate in the same direction culated with the semiclassical approximation proposed by Heller and result in a large joint effect. and Brown25for a tunneling process (Le., with state A represented The remaining two factors have a lesser importance. The by a parabola and state X by a linear potential). I(~’cqll.’”~~)l~ Franck-Condon factors also contribute to the overall Th? calculatio_n of the vibrational energy-level densities (W+ lowering of the rate constant. The number of terms that contribute and N) of state A has been done by the Beyer-Swinehardt alto the summation in eq 3.6 is not the same for the two isotopomers. g ~ r i t h mas~well ~ as with Forst’s steepest descent approximation.* This accounts for the fact that the isotope effect does not remain A large number of sets of vibrational frequencies have been tried, constant as a function of ufc0. based, as much as could be, on the available experimental inf o r m a t i ~ n . ~ . ~Of ~ . particular ~’ importance is the presence of a VI. Conclusion low-frequency, out-of-plane wagging mode, detected by Guyon It has been suggested by Bombach et aLI3 that reaction 1.1 ct aL9 in the spectrum of Rydberg states converging toward state constitutes an example of isolated state decay, i.e., a process in A of the ion. The presence of this frequency (which has been which internal conversion to the ground state is much slower than assumed to range between 290 and 370 cm-’ for H2COf and the dissociation step itself, in contradistinction to the situation between 1.50 and 190 cm-’ for D2CO+)increases strongly the commonly encountered. The present calculations confirm entirely density of states N which appears in eq 3.7. To alleviate the this view. The low order of magnitude of the rate constant of the statistical averaging over initial states having the same energy (eq rate-limiting step and the strong isotope effect are natural con3.5) and to simulate different possibilities, the calculations have sequences of the tunneling process and of the relatively high density been repeated with different choices in which accidental degenof states (resulting itself from the presence of a low-frequency eracies have been deliberately created in some cases and avoided out-of-plane bending mode). Thus, the most interesting conclusion in others. that emerges from the present work is that it is possible to adopt Results are given in Figure 3, which should be compared to (more exactly, to adapt) a statistical, RRKM-like formalism to Figure 2. The low value of the rate constant is accounted for, describe an isolated state decay taking place in a very small as well as its overall increase as a function of ubo. The calculated molecular system. ) persists over a large energy isotope effect is strong ( ~ 2 0 and A second, more tentative conclusion is that the conventional interval. I t is consistent with that proposed by Wankenne et al. interpretation of isolated state decay in terms of noncommuni( > 9 ) but is less than that estimated by Bombach et al. (-4.5 cating potential energy surfaces is likely to be valid in the case 20 for utco = 0). of very small molecular ions only, such as that of formaldehyde. In larger systems, the density of electronic states is so high and (27) Feller, D.; Davidson, E. R. J . Chem. Phys. 1983, 80, 1006. Bouma, the number of internal degrees of freedom so large that the slow W. J.; Burger, P. C.; Holmes, J . L.; Radom, L. J . Am. Chem. Soc. 1986, 108, tunneling processes are supplanted by fast internal conversions. 1767. In the high-energy range usually involved3*(3-4 eV or more above (28) Manneback, C. Physica 1951, 17, 1001. the ground state), it would be very unlikely not to find any surface (29) Beyer, T.; Swinehardt, D. F. Commun. A C M 1973, 16, 379; Stein, log k i s - ‘ 1
0
G n [ [ n
A
+
*
S. E.; Rabinovitch, B. S.J . Chem. Phys. 1973, 58, 2438. (30) Baker, A . D.; Baker, C.; Brundle, C. R.; Turner, D. W. Inr. J . Mass Spectrom. Ion Phys. 1968, I , 285. ( 3 I ) Shimanouchi, T. Tables of Molecular Vibrational Frequencies: Narl. Stand. ReJ Data Ser. ( U S . , Nari. Bur. Stand.) 1972, 39.
(32) Eland, J . H. D. Adc. Mass Spectrom. 1980,8A, 17. Stadelman, J. Mass Spectrom. 1980, 8A, 47. Galloy, C . ; Lecomte, C.; Lorquet, J . C. J . Chem. Phys. 1982, 77, 4522.
P.;Vogt, J. Ado.
J. Phys. Chem. 1990, 94, 2283-2290 crossing along any direction in configuration space in the energy range where isolated state decay is found to take place. Acknow'edflent' J'c'L' is indebted to Dr. J* Dannacherv prof. J. Momigny, and Dr. H. Wankenne for fruitful discussions on the experimental aspects of the problem. T.T. thanks the "Relations
2283
Internationales de la Communauti FranGaise de Belgique" for a fellowship. This work has been supported by research grants from the Belgian Government (Action.de Recherche ConcertCe) and from the Fonds de la Recherche Fondamentale Collective,
Registry No. CH,O+, 54288-05-0; CD,O+, 76748-81-7.
Ionic Fragmentation Processes in Organometallic Molecules of Group II-V Elements following ( n - l ) d Core Photoionization Shin-ichi Nagaoka,*,+ Shinzo Suzuki, Umpei Nagashima, Takashi Imamura, and Inosuke Koyano Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: June 12, 1989; In Final Form: September 26, 1989)
Ionic fragmentation following photoionization from the shallowest d core orbital in organometallic molecules with a group 11-V element [ZnMe2, GaMe,, GeMe,, SnMe,,, PbMe,, and BiMe, (Me = CH,)] has been studied in the vapor phase. The threshold electron spectra and photoionizationefficiency curves of these molecules are presented and discussed. Photoionization from the ( n - l)d core orbitals ( n = 4 for ZnMe2, GaMe,, and GeMe,, n = 5 for SnMe,, and n = 6 for PbMe, and BiMe,) is identified in the threshold electron spectra of these molecules. The ( n - I)d core ionized state of the molecules with a group IV element (PbMe, and SnMe,) is split into five sublevels owing to both the spin-orbit coupling and the electrostatic perturbation from the methyl groups. By use of the threshold photoelectron-photoion coincidence (TPEPICO) technique, it is shown that, in the molecules studied in the present work (except BiMe,), the excitation to the [(n - 1)dI9 hole state opens dissociation paths quite different from those following valence photoionization;the metal ions are predominantly produced following the excitation to the [ ( n- 1)dI9hole state. The monomethyl-metal ions are likely to be produced following both core and valence photoionization. The dimethyl-metal and trimethyl-metal ions are shown to originate from the valence-ionized states. The fragmentation processes following the 5d core level ionization of BiMe, seem to be statistical. The quintet splitting of the ( n - 1)d core ionized states of the group IV organometallic molecules and their fragmentation patterns can be explained in terms of the crystal field theory and the hybrid orbitals constructed from the ( n - l)d, ns, and np atomic orbitals.
Introduction In recent years, dynamic processes following core level excitation in molecules have been a topic of much interest.' In contrast to the case of valence electrons delocalized over the molecule, core electrons are localized near the atom to which they belonged originally. As a result, the photoionization from the core orbital is expected to produce dissociation paths quite different from those following the valence photoionization. Volatile organometallic molecules with a group 11-V element are particularly suitable for detailed investigations of the above-mentioned processes in the vapor phase, because the rather small binding energies of the electrons of the shallowest d core orbitals allow the studies to be made in the normal incidence region of the vacuum ultraviolet. Recently, we have studied the d core level photoionization and subsequent fragmentation processes in PbMe,, SnMe,, and GeM, (Me = CHJ23, by use of the threshold photoelectron-photoion coincidence (TPEPICO) Some of the preliminary results obtained are summarized as follows. The ( n - l)d core ionized state of the tetramethyl compound of a group IV element (n = 6 for PbMe, and n = 5 for SnMe,) is split into five sublevels owing to both the spin-orbit coupling and the electrostatic perturbation from the methyl groups. Ions of the central metal atom are predominantly produced following the ( n - 1)d photoionization. The monomethyl-metal ion seems to originate from both the d core ionized state and the valenceionized state. Dimethyl-metal and trimethyl-metal ions are produced following valence photoionization. There remained, however, a few questions to be investigated further. First, the reason for the predominant production of the metal ion has not been clarified so far. This problem is particularly serious in order to elucidate the dissociation paths following d core 'Present address: Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790, Japan.
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photoionization. Second, the detailed explanation for the quintet splitting of the d core ionized states of PbMe, and SnMe, has not been given. The interesting splitting evidently poses a challenge to us. Third, it is desirable to examine whether or not the predominant production of the central metal ion is generally found throughout other organometallic molecules. This problem would be of interest from the viewpoint of photochemical vapor deposition using synchrotron radiation in the processes of semiconductor production. In the present work, we extend our studies on the fragmentation processes following the ( n - l)d core photoionization to include similar compounds of group 11-V elements ( n = 4 for ZnMe2, GaMe,, and GeMe4, n = 5 for SnMe,, and n = 6 for PbMe, and BiMe,). We propose a probable explanation for the quintet splitting of the ( n - l)d core ionized states of the tetramethyl compounds of group IV elements, as well as that for the fragmentation patterns following ( n - 1)d core level ionization. The explanation is based on the crystal field theory and the hybrid orbitals constructed from the (n - l)d, ns, and np atomic orbitals.
Experimental Section The experiments on GaMe,, GeMe,, SnMe,, PbMe,, and BiMe3 were performed by use of the TEPSICO-I1 apparatussv9installed ( I ) Nenner, 1.; Beswick, J. A. In Handbook on Synchrotron Radiotion; Marr, G . V., Ed.; North-Holland: Amsterdam, 1987; Vol. 2, Chapter 6, and references cited therein. (2) Nagaoka, S.; Suzuki, S.; Koyano, I. Phys. Rev. Lett. 1987, 58, 1524. (3) Nagaoka, S.; Suzuki, S.; Koyano, I. Nucl. Instrum. Methods 1988, A266, 699. (4) Tanaka, K.; Koyano, I . J . Chem. Phys. 1978,69, 3422. ( 5 ) Koyano, I.; Tanaka, K. J. Chem. Phys. 1980, 72, 4858. (6) Nenner, I.; Guyon, P.-M.;Baer, T.; Govers, T. R. J . Chem. Phys. 1980, 72, 6587. (7) Tanaka, K.; Kato, T.; Guyon, P.-M.; Koyano, I. J . Chem. Phys. 1983, 79, 4302, and references cited therein. (8) Koyano, I.; Tanaka, K.; Kato, T.; Suzuki, S.; Ishiguro, E. Nucl. Instrum. Methods 1986, A246, 507.
0 1990 American Chemical Society
