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(2+1) resonance enhanced multiphoton ionization spectra have been recorded, simulated, and used to derive energies of rovibrational levels in the F([sup 1]Δ), E([sup 1]Σ[sup +]), and V([sup 1]Σ[sup +]) states for HCl (H[sup 35]Cl and H[sup 37]Cl) and HBr (H[sup 79]Br and H[sup 81]Br). Spectroscopic parameters derived for the F states compare nicely with those derived by others using conventional analysis methods. Clear evidence for near resonance interactions between the F and the V states is seen for the first time, both in HCl and HBr. Shape of curves for rotational level energy spacings versus rotational quantum numbers are found to depend characteristically on the nature of off-resonance interactions observed between the E and the V states. Model calculations for state interactions, based on perturbation theory, are performed for HCl. These prove to be useful to interpret observed perturbations, both qualitatively and quantitatively. Interaction strengths are evaluated for F to V and E to V state interactions. Variations observed in the intensity ratios of O and S line series to Q line series in vibrational bands of the V state for HCl are discussed and mechanisms of two-photon excitation processes are proposed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]