Ion-pair formation from the superexcited states of saturated hydrocarbons has been studied by negative-ion mass spectrometry using synchrotron radiation in the 15–35 eV photon energy range. Negative ion H− has been observed from CH4, C2H6, C3H8, n-C4H10, iso-C4H10, and neo-C5H12. The maximum cross section ranges from 1.6×10−21 to 1.0×10−20 cm2. Strong peaks observed in the photodissociation efficiency curve of H− are assigned as resulting from transitions to the Rydberg states formed by promotion of an electron in a carbon 2s-type molecular orbital. In contrast, the valence-Rydberg transitions from a carbon 2p-type orbital have little (C2H6) or no contribution [CH4 and CmH2m+2 (3≤m≤5)] to the H− formation. This difference can be interpreted as that the latter Rydberg states have short lifetime with respect to autoionization to lower ionic states on account of relatively large overlap between two carbon 2p-type orbitals involved in an electron exchange process.

Two-dimensional photoelectron spectroscopy is performed for studying autoionization of acetylene in the Franck–Condon gap between the X 2Πu and A 2Ag states of C2H2+. The photoelectron spectrum in the photon energy range from 12.8 to 13.6 eV shows exclusive vibrational excitation of the symmetric C–H stretching mode ν1 of C2H2+(X 2Πu), which results from autoionization of the valence state (3σg)−1(3σu)1. Vibrational frequencies with anharmonicities of the ν1 and ν2 (the symmetric C–C stretch) modes are determined by a least-squares fit of the ionization energies of the observed peaks to a second order expansion. At the photon energy of 14.120 eV, autoionization of the Rydberg state (3σg)−1(3pπu)1 leads to a complicated photoelectron spectrum where probably the trans-bending mode ν4 of C2H2+(X 2Πu) as well as ν1 is excited, reflecting a substantial geometrical change during autoionization. Furthermore, a similar excitation of the ν4 mode is observed at ∼13.8 eV. An excellent agreement in positions of the vibrational levels between the spectra at 13.821 and 14.120 eV suggests the presence of the Rydberg state (3σg)−1(3pσu)1 at ∼13.8 eV which has not been identified previously in the photoabsorption or photoionization cross section curves. The constant-ionic-state spectra for the ν1=0–4 levels of C2H2+(X 2Πu) show two spectral features: (a) a weak shoulder (v1=0) or a small maximum (ν1=1–4) at 13.8 eV and (b) two groups of peaks in the range of 14.0–14.4 eV. The ratio of the integrated intensity of the 13.8 eV maximum to that of the two groups differs from level to level. This observation is interpreted in terms of a strong interaction between the Rydberg (3σg)−1(3pσu)1 and valence (3σg)−1(3σu)1 states.

Autoionizing resonance of acetylene is studied by photoelectron spectroscopy using synchrotron radiation. Pronounced vibrational excitation in the C-H stretching mode 1 is observed in the (1 u ) Ϫ1 X 2 ⌸ u band of C 2 H 2 ϩ at a restricted photon energy range from 12.8 to 14.1 eV. It is concluded that the 3 g →3 u autoionizing transition at ϳ13.3 eV gives rise to an anomalously broad maximum in the ͑1 u ͒ Ϫ1 photoionization cross section curve. The strong 1 excitation is explained as that the equilibrium C-H bond length differs from the neutral and ionic ground states to the (3 g ) Ϫ1 (3 u ) 1 resonance state. Constant ionic state spectra for the v 1 ϭ3 and 4 levels of the X 2 ⌸ u state measured over the same energy region show fine structures with regular spacings corresponding to the vibrational levels of the (3 g ) Ϫ1 (3 u ) 1 state.

Predissociation of superexcited states of OCS is studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 15–16.5 eV. A two-dimensional photoelectron spectrum exhibits two kinds of characteristic patterns both of which are ascribed to autoionization of sulfur atoms. This superexcited atom S* is produced by predissociation of a Rydberg state OCS*(RB) converging to OCS+(B̃ 2Σ+). The pattern of the first kind results from predissociation processes in which the effective principal quantum number n of the Rydberg electron is almost conserved. This suggests that the Rydberg electron behaves as a spectator because of its negligibly weak interaction with the ion core (spectator predissociation). On the contrary, n of S* does not accord with that of OCS*(RB) in the pattern of the second kind, indicating that the Rydberg electron participates directly in the electron exchange mechanism controlling conversion from OCS*(RB) to a predissociating state (participant predissociation). With increasing n, OCS*(RB) decays more preferentially by the spectator than by the participant predissociation. The spectator predissociation of OCS*(RB) proceeds through a two-step conversion which involves Rydberg states converging to OCS+(Ã 2Π and X̃ 2Π) and a dissociative multiple-electron-excited satellite state OCS* (SAT) asymptotically correlating with S*+CO(X̃ 1Σ+). In contrast, the participant predissociation may be accounted for by a direct conversion from OCS*(RB) to OCS*(SAT). The quantum yields are estimated to be 0.06 and 0.02 for the spectator and participant predissociation, respectively, at the incident photon energy of 15.95 eV where OCS*(RB) states with n∼12 lie. A simulation is performed to reproduce the partial cross section curve for the spectator predissociation by using a model in which the decay rates for the participant and spectator predissociation are assumed to be proportional to n−3 and n0, respectively. The simulated and experimental cross section curves are in good agreement with each other in the photon energy range of 15.8–16.04 eV.

No abstract