Interference and non-Franck-Condon effects in ionization of H₂molecules by photon impact

Abstract

Abstract. We analyze theoretically interference effects in the spectra of electrons emitted in the H2 photoionization by high energy linearly polarized photons. Molecular bound and continuum states are accurately described by means of B-spline basis allowing the inclusion of the nuclear degrees of freedom. One interesting feature is observed: the usual FranckCondon behavior is not followed when the H2 internuclear axis is parallel to the polarization direction. Moreover, this is related to the fact that for this molecular orientation and under certain conditions, the electron cannot be emitted in the direction of the radiation field. On the contrary, for H2 molecules perpendicular to the polarization direction, the angular distribution of electrons is analogous to the one observed in the two slits Young's experiment. IntroductionNowadays, the study of interference effects in the electronic emission from diatomic molecules is a very active subject of research (see for instance [1]). In the sixties, Cohen and Fano explained anomalies in the photoelectron spectra of diatomic molecules [2]. They argued that the coherent emission from both molecular centers produced interferences when the de Broglie wavelength of the ejected electrons is of the order of the internuclear distance of the molecule in analogy with the Young's two slit experiment. They elaborated a simple model based on the following assumptions: i) ionization is basically a one-electron process, ii) the lowest molecular orbital ψ g is well described in the Linear combination of atomic orbitals (LCAO) approximation (ψ g ∼ 1s A +1s B , where 1s A and 1s B are atomic orbitals centred in nucleus A and B, respectively) and iii) the ionized electron is well described by a single-centre plane wave. In this way, Cohen and Fano showed that the total photoionization cross section is given by the analytical form