Spectator- and participant-like behavior of a Rydberg electron on predissociation of superexcited states of OCS

Abstract

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.

Neutral photodissociation of superexcited states of molecular iodine

“…Using this technique, these authors also observed autoionization of atomic sulfur Rydberg states from the dissociation of carbonyl sulfide. 16 Regan et al 17 observed H͑n =2͒ +I͑ 2 P 3/2 ͒ products following multiphoton excitation of HI, while Romanescu and co-workers 18,19 observed a similar process in HCl. Both Cl and H Rydbergs have been observed more recently by Chichinin et al 20 Finally, using a version of MATI ͑Ref.…”

The formation of fluorescent and metastable fragments by photoexcitation of some diatomic molecules in the vacuum ultraviolet region

“…In particular, the determination of fragment's internal state by fluorescence dispersion is often useful for investigating the details of the dissociation process [2]. Photoelectron spectroscopy can also probe the neutral dissociation process, when the dissociation fragment has an autoionizing decay pathway [3,4]. In contrast, knowledge of neutral dissociation forming stable fragments is very limited as these particles are difficult to observe, yet this process is apparently the most general in the total neutral dissociation cross section.…”

O‐loss photodissociation of the OCS+ ion in the low‐lying electronic states studied using multiconfiguration second‐order perturbation theory

“…Numerous experimental studies have been devoted to the OCS + ion that had an old formula of COS + in the early literature. The experimental studies 3–25 all indicated that X 2 Π, A 2 Π, B 2 Σ + , and C 2 Σ + were the four lowest‐lying electronic states of the OCS + ion. In 2005, Chen et al 10 calculated the low‐lying electronic states of the OCS + ion using multiconfiguration second‐order perturbation theory.…”

“…Their calculation results supported the assignments of the X, A, B, and C states of OCS + to the 1 2 Π, 2 2 Π, 1 2 Σ + , and 2 2 Σ + states, which were characterized as primary ionization states with the most important configurations (MICs) of (3π) −1 , (2π) −1 , (9σ) −1 , and (8σ) −1 , respectively. The dissociation of the OCS + ion in the low‐lying states to the S + + CO, S + CO + , CS + + O, and CS + O + product groups, were experimentally studied by many groups 3–9, 11–20, 22, 23, 26–31. In 2006, Chen et al 32 reported their theoretical study on S‐loss dissociation from the X 2 Π, A 2 Π, B 2 Σ + , 1 4 Σ − , 1 2 Σ − , and 1 2 Δ states of the OCS + ion, and discussed the S‐loss dissociation mechanisms of the A 2 Π and B 2 Σ + states.…”