(2+1) REMPI spectra of =0 states of the hydrogen halides: Spectroscopy, perturbations and excitation mechanisms

(2+1) REMPI spectra of HX (X=Cl, Br and I) have been recorded and analyzed by simulation calculations to derive rotational constants, band origins and isotope shift values for a number of vibrational bands of =0 states. Our data for HCl compare nicely with those derived by Green et al. by conventional analysis methods [D. S. Green et al., J. Mol. Spectrosc. 150, 303, 354, 388 (1991); D. S. Green and S. C. Wallace, J. Chem. Phys. 96, 5857 (1992)]. New spectroscopic parameters were derived for eight vibrational bands which are assigned to the V(¹Σ⁺) state, for v^′=4 of the E(¹Σ⁺) state, as well as for five new bands in HBr. New spectroscopic parameters were derived for four vibrational bands which are assigned to the V state and for v^′=1 of the E state in HI. Anomalies observed in energy level spacings, rotational parameters and isotope shift values are interpreted as being largely due to homogeneous interactions between the V and the E states. It is argued that the interaction causes a compression of rovibrational levels in the E state manifold but an expansion of levels in the V state manifold, something which might be expected for a Rydberg to ion-pair interaction. Variations observed in the intensity ratio of O and S line series to Q line series in vibrational bands of the E and V states for HCl and HBr are discussed and mechanisms of two-photon excitation processes are proposed.