Two-photon spectroscopy, Rydberg∼valence interactions, and superexcited state dissociation of HCl probed by resonance enhanced multiphoton ionization

The 1Σ+(0+) and 3Σ−(0+) Rydberg states as well as the V 1Σ+(0+) valence state of hydrogen chloride isotopomers are studied using (2+1) resonance enhanced multiphoton ionization (REMPI) and time-of-flight mass spectrometry (TOFMS). Pulsed supersonic expansion provides sufficient rotational cooling to facilitate rotational analysis. Polarization selection in conjunction with fragment and isotope discrimination provide the first unambiguous identification of all vibrational bands from v′=3 to 32 of the V 1Σ+(0+) state. REMPI excitation spectra from the v″=0 level of the X 1Σ+(0+) state through vibrational levels of the broad V 1Σ+(0+) state suggest non-Franck–Condon transitions. Perturbations in intensity, line position, and rotational constants are consistent with the near-adiabatic nature of the short-range E 1Σ+(0+) Rydberg state and long-range V 1Σ+(0+)state. Evidence for Rydberg∼valence interaction and repulsive superexcited states is provided by the detection of strong H+ and Cl+ ion yields. The onset of a distinct Cl+ channel is detected for the first time and confirms the presence of at least two dissociation pathways at the three-photon energy and at large internuclear distance.