Alignment and dissociation of electronically excited molecular hydrogen with intense laser fields

Abstract

ABSTRACT The dissociation of aligned, electronically excited H2 (E,F ), followed by the ionisation of the produced H atom, is analysed via the velocity mapped imaging technique. The dissociation and ionisation processes are accomplished, respectively, by a two- and a one-photon absorption from a single 532-nm laser pulse, while the alignment is induced by a separate 1064-nm laser pulse. The velocity of the produced H+ photofragments shows a weak perpendicular alignment at low alignment laser field values, evolving to strongly parallel for larger fields. We modelled this alignment behaviour with a simple two-state model involving the Stark mixing of the initially-prepared J = 0 with the J = 2 rotational state. This model is able to reproduce all of the observed angular distribution and permits us to extract from the fit the polarisability anisotropy of H2 (E,F) electronic state. We determine this value to be (3.7 ± 1.2) × 103 a.u. As this value is extremely large in comparison to what one would expect from the pure H2 (E,F) electronic state, we hypothesise that this value comes from the 1064-nm laser beam mixing nearby electronic states with the initially laser prepared (E,F) state generating a mixed state (EF**) with an extremely large polarisability anisotropy.