Theoretical study of the vibrational-branching ratio and photoelectron angular distribution in (1+1′)-photon resonance-enhanced multiphoton ionization of HD and D₂ molecules

We have studied the isotopic variation of the vibrational-branching ratio (VBR) and the photoelectron angular distribution (PEAD) as a function of laser intensity in (1+1′)-photon resonance-enhanced multiphoton ionization (REMPI) of HD and D₂ molecules via B ¹Σ_u (v=4; j=1,2) levels. We have found that with the increase in laser intensity, the non-Franck-Condon nature of the VBR and the deviation of the total as well as the vibrationally resolved PEAD from that obtained in the lower-intensity regime become much more prominent for D₂ molecules than those for HD molecules. We have considered the effect of the interference of different ionization channels (resonant as well as near-resonant ionization via nearby rovibrational levels) and the effect of Raman-like two-photon coupling between these intermediate vibrational levels via a continuum on this REMPI process. The difference between the vibrational wave functions and the difference between the spacing of rovibrational energy levels in these two isotopes of H₂ molecules lead to different strengths of ionization and the Raman-like two-photon coupling. Hence the isotopic variation in the VBR and the PEAD for these two isotopes shows up as a result of different degrees of interference of parallel ionization channels and different strengths of two-photon coupling via a continuum, both of which become important with an increase in laser intensity. We have also found that the intensity variation of the VBR and the PEAD for both molecules depends on the choice of different rotational levels as resonances. This feature has also been found previously in the REMPI of H₂ molecules [Jainab Khatun, S. Sanyal, and Krishna Rai Dastidar, Phys. Rev. A 49, 4765 (1994); Jainab Khatun and Krishna Rai Dastidar, ibid. 52, 2971 (1995), and references therein; J. W. J. Verschuur and H. B. van Linden van den Heuvell, Chem. Phys. 129, 1 (1989)]. © 1996 The American Physical Society.