2-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 s tudy on the mercapto radical is carried out in the one-photon energy r egion between 258 and 208 nm. Thirteen previously unobserved Rydberg s tates converging upon the a 1 Delta or b (1) Sigma(+) excited ionic st ates are reported. Identification and characterization of several stat es with excitation energies below the lowest ionization limit are perf ormed by analyses of the rotationally resolved two-photon excitation s pectra, polarization studies, and photoelectron spectroscopy after exc itation of the states. Detailed comparisons between the experimental a nd simulated excitation spectra, which show for three states clear ind ications of intensity anomalies, as well as the observation of multipl e ionization channels demonstrate that for all states Rydberg-Rydberg and/or Rydberg-valence interactions determine the finer details of the ir electronic wave functions. Above the lowest ionization limit two-ph oton resonance enhancement is observed for six Rydberg states. These s tates 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 t he dominant electronic configuration of the six Rydberg states allows us to explain these differences in terms of the possible autoionizatio n channels. (C) 1996 American Institute of Physics.