(2+1) REMPI spectra of HX; X = Cl, Br and I have been recorded and
analysed by simulation calculations to derive rotational constants, band
origins and isotope shift values for number of vibrational bands of W = 0
states. Our data for HCl compare nicely with those derived by Green et. al by
conventional analysis methods 1-4. New spectroscopic parameters were derived
for 8 vibrational bands which are assigned to the V(1S+) state, for vī= 4 of
the E(1S+) 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 vī= 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 mechanims of two photon excitation processes are proposed.