ABSTRACT

	(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.