General symmetry selection rules for the photoionization of polyatomic molecules

Abstract

General rovibronic symmetry selection rules, which are applicable to any molecular symmetry, have been obtained for the photoionization of polyatomic molecules. The use of the molecular symmetry groups leads to a particularly transparent derivation. The photoelectron is characterized by a partial wave expansion in the orbital angular momentum quantum number l. For a given value of l, one-photon electric dipole transitions can only occur between neutral and ionic states that obey the rovibronic symmetry conditions Γrve (neutral) ⊗ Γrve (ion) ⊃ Γ* for l even and Γrve(neutral) ⊗ Γrve(ion) ⊃ Γ(s) for l odd, where Γ(s) and Γ* represent the totally symmetric and the antisymmetric representations, respectively. Combined with the wellknown angular momentum conservation selection rule Δ J = J + - J = l + , l + ½,…, l - ½, l - [EQUATION](where J + and J represent the total angular momentum quantum number of the ionic and the neutral state between which the photoelectronic transition occurs), these symmetry selection rules give an overall picture of photoionization in molecules. The formalism, which has also been extended to multiphoton ionization processes, is successfully applied to the analysis of rotationally resolved photoelectron spectra of ND4, NH3, C6H6 and H2O.