Theoretical study of photoelectron angular distribution in (1 + 1^′)-photon resonance-enhanced multiphoton ionization of the H₂ molecule

We have studied the effect of laser intensity on the photoelectron angular distribution (PEAD) in (1 + 1^′)-photon resonance-enhanced multiphoton ionization (REMPI) of the H₂ molecule via B¹Σ_u(v=4; j=1, 2) levels. It has been found that both the total and the vibrationally resolved angular distributions [for the formation of H₂⁺ ion in ground X²Σ_g(v⁺=0, 1) levels] vary with laser intensity. Intensity dependence of corresponding asymmetry parameters has also been studied. We have shown that this variation with laser intensity is caused by two effects: (i) the effect of interference of ionization channels via different vibrational levels of B¹Σ_u and C¹Π_u states, and (ii) the effect of coupling between these levels (Raman-like transitions) via continuum. Furthermore, the nature of variation of the PEAD and of the corresponding asymmetry parameters with laser intensity have also been found to depend on the choice of different rotational levels as resonances. This difference in variation is due to the variation in strength of interference and the Raman-like couplings effective in these REMPI processes.