Abstract: (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. Newspectroscopic parameters were derived for 8 vibrational bands which were assigned to the V(¹∑⁺) state, for v'=4 of the E(¹∑⁺) 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.

Key words: REMPI, Hydrogen halides, Laser Spectroscopy, Perturbations, Spectral simulations, Excition mechanisms