Resonance enhanced multiphoton ionization of the hydrogen halides: Rotational structure and anomalies in Rydberg and ion-pair states of HCl and HBr

(2 + 1) resonance enhanced multiphoton ionization spectra have been recorde d, simulated, and used to derive energies of rovibrational levels in the F( (1)Delta), E((1)Sigma(+)), and V((1)Sigma(+)) states for HCl ((HCl)-Cl-35 a nd (HCl)-Cl-37) and HBr ((HBr)-Br-79 and (HBr)-Br-81). Spectroscopic parame ters derived for the F states compare nicely with those derived by others u sing conventional analysis methods. Clear evidence for near resonance inter actions between the F and the V states is seen for the first time, both in HCl and HBr. Shape of curves for rotational level energy spacings versus ro tational quantum numbers are found to depend characteristically on the natu re of off-resonance interactions observed between the E and the V states. M odel calculations for state interactions, based on perturbation theory, are performed for HCl. These prove to be useful to interpret observed perturba tions, both qualitatively and quantitatively. Interaction strengths are eva luated for F to V and E to V state interactions. Variations observed in the intensity ratios of O and S line series to Q line series in vibrational ba nds of the V state for HCl are discussed and mechanisms of two-photon excit ation processes are proposed. (C) 2000 American Institute of Physics. [S002 1-9606(00)01824-9].