HD+in a beam in intense pulsed laser fields: Dissociation and ionization with high-energy resolution of the fragments

Abstract

The isotopically mixed hydrogen molecular ion ${\text{HD}}^{+}$ was investigated in intense pulsed laser fields with high, i.e., vibrational resolution of the fragments. An ion beam of ${\text{HD}}^{+}$ with a translational energy of 11.1 keV was exposed to femtosecond laser pulses (100 fs) of intensities in the range from ${10}^{13}--{10}^{15}\text{ }\text{W}/{\text{cm}}^{2}$. Fragments from the two dissociation channels ${\text{HD}}^{+}\ensuremath{\rightarrow}\text{H}+{\text{D}}^{+}$, and ${\text{HD}}^{+}\ensuremath{\rightarrow}\text{D}+{\text{H}}^{+}$, as well as the Coulomb explosion channel, ${\text{HD}}^{+}\ensuremath{\rightarrow}{\text{H}}^{+}+{\text{D}}^{+}+{e}^{\ensuremath{-}}$, were projected onto a two-dimensional multichannel plate detector to measure their velocity distributions. The fragments from the three channels were spatially clearly distinguished because of the different velocities of H $({\text{H}}^{+})$ and D $({\text{D}}^{+})$ fragments. Fragments from most of the populated vibrational states of ${\text{HD}}^{+}$ can be discerned as well-resolved peaks in the speed and angular distributions. The typical light induced potential (LIP) effects such as bond softening and level shifting already observed in the studies of ${\text{H}}_{2}^{+}$ and ${\text{D}}_{2}^{+}$ are here also observed. In addition to these one-photon peaks, also peaks due to three- (net two-) and finally also to direct two-photon absorption, typical of the asymmetric ${\text{HD}}^{+}$, were found in the vibrationally resolved fragment spectra of the dissociation channels. Besides the fragments from one-photon bond softening, also fragments formed by two- and three-photon processes were found to have narrow angular distributions, in contrast to the approximately ${\text{cos}}^{2}$ distribution of fragments from vibrational levels above the LIP crossing being fragmented by classical dissociation. The relative probability of the two dissociation channels was investigated, addressing the question of the higher electron affinity to the proton or the deuteron during the dissociation, but could not be decided because of too low accuracy here. The Coulomb explosion channel was also clearly discerned by its fragment velocities as well as the narrow angular distributions of ${\text{H}}^{+}$ and ${\text{D}}^{+}$ fragments, which is quite analogous to those found in the cases of ${\text{H}}_{2}^{+}$ and ${\text{D}}_{2}^{+}$.