Non-Equilibrium Dynamics in Two-Color, Few-Photon Dissociative Excitation and Ionization of D$_2$

TL;DR

This study investigates the complex non-equilibrium dynamics of D$_2$ molecules under femtosecond XUV and NIR excitation, revealing detailed dissociation pathways and molecular alignments through combined experimental and theoretical approaches.

Contribution

It identifies and isolates specific dissociation pathways and molecular orientations in D$_2$ using advanced spectroscopy and ab initio calculations, advancing understanding of molecular dynamics under strong light fields.

Findings

Distinct dissociation pathways with specific molecular alignments identified.

Molecular orientation influences the dissociation sequence.

Real-time probing of nuclear wave packet evolution on the B state.

Abstract

D$_2$ molecules, excited by linearly cross-polarized femtosecond extreme ultraviolet (XUV) and near-infrared (NIR) light pulses, reveal highly structured D$^+$ ion fragment momenta and angular distributions that originate from two different 4-step dissociative ionization pathways after four photon absorption (1 XUV + 3 NIR). We show that, even for very low dissociation kinetic energy release $\le$~240~meV, specific electronic excitation pathways can be identified and isolated in the final ion momentum distributions. With the aid of {\it ab initio} electronic structure and time-dependent Schr\"odinger equation calculations, angular momentum, energy, and parity conservation are used to identify the excited neutral molecular states and molecular orientations relative to the polarization vectors in these different photoexcitation and dissociation sequences of the neutral D$_2$ molecule and its D$_2^+$ cation. In one sequential photodissociation pathway, molecules aligned along either of the two light polarization vectors are excluded, while another pathway selects molecules aligned parallel to the light propagation direction. The evolution of the nuclear wave packet on the intermediate \Bstate electronic state of the neutral D$_2$ molecule is also probed in real time.