(2-REMPI SPECTRA OF OMEGA=0 STATES OF THE HYDROGEN HALIDES - SPECTROSCOPY, PERTURBATIONS AND EXCITATION MECHANISMS(1))

(2 + I) REMPI spectra of HX (X=Cl, Br and I) have been recorded and an alyzed by simulation calculations to derive rotational constants, band origins and isotope shift values for a number of vibrational bands of Omega = 0 states. Our data for HCl compare nicely with those derived by Green et nl. by conventional analysis methods [D. S. Green et al., J. Mel. Spectrosc. 150, 303, 354, 388 (1991); D. S. Green and S. C. Wa llace, J. Chem. Phys. 96, 5857 (1992)]. New spectroscopic parameters w ere derived for eight vibrational bands which are assigned to the V((1 )Sigma(+)) state, for upsilon'=4 of the E((1)Sigma(+)) state, as well as for five new bands in HBr. New spectroscopic parameters were derive d for four vibrational bands which are assigned to the V state and for upsilon'=1 of the E state in HI. Anomalies observed in energy level s pacings, rotational parameters and isotope shift values are interprete d as being largely due to homogeneous interactions between the V and t he E states. It is argued that the interaction causes a compression of rovibrational levels in the E state manifold but an expansion of leve ls in the V stale manifold, something which might be expected for a Ry dberg to ion-pair interaction. Variations observed in the intensity ra tio of O and S line series to Q line series in vibrational bands of th e E and V slates for HCl and HBr are discussed and mechanisms of two-p hoton excitation processes are proposed. (C) 1998 American Institute o f Physics.