Superexcited states of OCS probed by using photoelectron spectroscopy for autoionizing atomic sulfur

Neutral dissociation of superexcited states of OCS has been studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 14.2-16.8 eV. A two-dimensional spectrum exhibits noticeable features which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, from Rydberg states converging to S⁺(²D⁰) to S⁺(⁴S⁰). The precursor molecular states leading to S*+CO are considered to be multiple-electron-excited Rydberg states, OCS*(Dis), converging to OCS⁺ with ²Σ^- and/or ²Δ symmetry. The electron signal counts due to autoionization of S* show enhancement at excitation photon energies for the Rydberg states, OCS*(R_B), converging to OCS⁺(B̃ ²Σ⁺). These results support a predissociation mechanism for the formation of S*: conversion from OCS*(R_B) to OCS*(Dis). The quantum yield for the predissociation is evaluated to be ∼1% at the photon energy corresponding to the 5sσ state of OCS*(R_B).