Two-photon resonance enhanced multiphoton ionization photoelectron spectroscopy of the SH (SD) radical below and above the lowest ionization threshold
A two-photon resonance enhanced multiphoton ionization spectroscopic study on the mercapto radical is carried out in the one-photon energy region between 258 and 208 nm. Thirteen previously unobserved Rydberg states converging upon the a 1Δ or b 1Σ+ excited ionic states are reported. Identification and characterization of several states with excitation energies below the lowest ionization limit are performed by analyses of the rotationally resolved two-photon excitation spectra, polarization studies, and photoelectron spectroscopy after excitation of the states. Detailed comparisons between the experimental and simulated excitation spectra, which show for three states clear indications of intensity anomalies, as well as the observation of multiple ionization channels demonstrate that for all states Rydberg-Rydberg and/or Rydberg-valence interactions determine the finer details of their electronic wave functions. Above the lowest ionization limit two-photon resonance enhancement is observed for six Rydberg states. These states are found to ionize by autoionization, rather than by absorption of a third photon. Remarkably, rotationally well-resolved excitation spectra can still be obtained for two of these states. Comparison of the dominant electronic configuration of the six Rydberg states allows us to explain these differences in terms of the possible autoionization channels.