REMPI spectra of the hydrogen halides

(2+1) resonance multiphoton ionization (REMPI) spectra of HX(X=Cl, Br and I) were recorded and analyzed by simulation calculation to derive rotational constants, band origins and isotope shift values for a number of vibrational bands of Ω states. Our data for HCl compared nicely with those derived by Green et al. using conventional analysis methods. New spectroscopic parameters were derived for 8 vibrational bands which were assigned to the V(¹Σ⁺) state, for v′=4 of the E(¹Σ⁺) state as well as for 5 new bands in HBr. New spectroscopic parameters were derived for 4 vibrational bands which are assigned to the V state and for v′=1 of the E state in HI. Anomalies in energy level spacings, rotational parameters and isotope shift values were observed as being largely due to homogeneous interactions between the V and the E states. 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 were discussed and mechanisms of two photon excitation processes were proposed.