Pump-probe studies of fragmentation of a fast HD$^+$ beam

Studies of fast molecular ion beams in strong-field ultrafast lasers are of particular interest for benchmark molecules, like H$_2^+$, H$_3^+$ and HeH$^+$. However, the low target density of a typical ion beam puts severe limits on studies of the time evolution through the implementation of the pump-probe technique. We have recently conducted a first-of-its-kind NIR-pump\,--\,NIR-probe measurement on a few-keV HD$^+$ beam target. Specifically, the first pulse initiates the dissociation of the HD$^+$, while the second ionizes the molecule during its dissociation. We have observed enhancement in the ionization yield of the dissociating wave packet at about 24 and 200 fs, corresponding to internuclear distances estimated classically to be about 15 and 85 a.u., respectively. The unexpected enhancement at very large internuclear separation has not been previously observed in pump-probe studies of neutral hydrogen molecules. \\ \\ \textsuperscript{*}Supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy. BJ was also supported in part by DOE-SCGF (DE-AC05-06OR23100).