Probing two-center interference in H2+using chirped pulses

Abstract

The influence of intermediate resonant dissociative channels in the few-photon ionization dynamics of H${}_{2}$${}^{+}$ is demonstrated in a pump-probe scenario. Two-photon ionization of H${}_{2}$${}^{+}$ by two sequentially applied pump and probe vuv femtosecond ${10}^{11}$ W/cm${}^{2}$ laser pulses is reported. The kinetic energy distribution of the ejected protons is calculated by solving the time-dependent Schr\"odinger equation within the Born-Oppenheimer approximation, including the electronic three-dimensional and vibrational one-dimensional motion. Population is effectively transferred from the $1s{\ensuremath{\sigma}}_{g}$ to the $3p{\ensuremath{\sigma}}_{u}$ potential surface, via resonant one-photon absorption, by applying a chirped pump pulse. The molecule is ultimately ionized by the probe pulse. It is found that a double-peaked structure appears in the resulting kinetic energy release spectra of the nuclear fragments. A corresponding modulated structure also appears in the dissociative channels, merely demonstrating that the double-peaked structure in the spectra originates from the molecular $1s{\ensuremath{\sigma}}_{g}$-$3p{\ensuremath{\sigma}}_{u}$ electronic dynamics, and an inherent two-center interference in the underlying electric-dipole coupling. It turns out that the dipole coupling between the $1s{\ensuremath{\sigma}}_{g}$ and $3p{\ensuremath{\sigma}}_{u}$ electronic states vanishes at an internuclear distance of $2.25$ a.u., i.e., close to the equilibrium internuclear separation. The resulting node in the $R$-dependent dipole coupling imposes a minimum in the corresponding kinetic energy release spectra of the dissociating nuclei. By creating a negative or positive chirp in the applied pump pulse, it is found that the modulations can be made more or less salient.