Two-photon dissociative photoionization and photodissociation of single rotational states

Abstract

We report on an experimental study of dissociative photoionization and neutral dissociation of H 2 by two-photon excitation from single B 1 + u (v = 5) rotational states (photon energy ≈ 4 eV). The B-state rovibrational levels are excited in the same light pulse by threephoton absorption from X 1 + g (v = 0). Angle resolved photoelectron and ion kinetic energy distributions allow us to discriminate between the different decay channels. Neutral decay is detected by one-photon ionization of excited atomic hydrogen product states. The kinetic energy distribution of photoelectrons from dissociative photoionization shows an oscillatory structure which depends on the initial B-state rotational quantum number J . An indication of a corresponding structure in the ion kinetic energy distributions is only found for J = 0. Similar to results reported by Verschuur et al we find neutral decay preferentially into high principal quantum number atomic states. The results indicate that the H 2 Q 1 doubly excited states play a crucial role in the decay processes.

Dissociative autoionization in (1+2)-photon above-threshold excitation of H2molecules

“…To get an estimate, we have calculated electronic dipole transition moment for continuum-continuum transition (Gordon 1929) shows an out of phase oscillation. The presence of three peaks in PES, above 0.7 ev of photoelectron energy, has also been experimentally observed (Rottke et al 1997). From the figure for PES (Fig.…”

“…In this transition, the doubly excited states of ungerade symmetry are excited and hence the dissociative autoionization process via these AI states contributes to the PES. Our aim here is to show that the dissociative autoionization process can give rise to oscillatory photoelectron spectrum, as observed in the recent experiment (Rottke et al 1997).…”

“…The experimental PES is given in arbitrary units and here we compare our theoretical PES (scaled by the factor I 3 F ) with experiment. It is to be mentioned here that, for photoelectron energies less than 0.7 ev, the contribution from the ionization of neutral atoms is dominating (Rottke et al 1997) and hence the effect of dissociative autoionization will be prominent only for energies ≥ 0.7 ev. Therefore, we will discuss the features obtained in PES for energies above 0.7 ev ( experimental results also show PES above 0.7 ev, see fig.2 of Rottke et al, 1997).…”

“…Therefore, in this REMPI ( resonance enhanced multiphoton ionization ) technique, the initial condition for dissociative ionization can be changed and the molecule can be excited to different continuum energies, much below the energy, reached in the single photon transition. Recently, experimental study (Rottke et al 1997) on (3 + 2)-photon dissociative ionization of H 2 molecules has been done by selectively exciting the…”

Observation of Direct Dissociative Ionization in Molecular Hydrogen

“…The introduction of intense strong laser systems made it possible to populate different intermediate high lying states via multiphoton excitation pathways, from which dissociation and ionization can be driven with less energetic photons. Experiments have been reported via the B 1 S 1 u state (using one or three photons) [6,7] and via the E, F 1 S 1 g state (using two photons) [8][9][10]. In many of these studies, doubly excited neutral states [11,12] positioned in the ionization continuum have key roles, masking the observation of direct dissociative ionization.…”