Two-body dissociation of formic acid following double ionization by ultrafast laser pulses

We studied the fragmentation of planar formic acid (HCOOH) molecules following their double ionization by intense ultrashort laser pulses. Deuterium tagging (i.e., HCOOD) combined with coincidence momentum imaging measurements of all fragment ions enabled determination of the role of the hydroxyl and carboxyl hydrogen atoms in the breakup. Specifically, we observe a strong preference for the hydroxyl (OD) group to remain intact in a HCOOD2+ -> OD+ + HCO+ fragmentation, which is an order of magnitude more likely than OH+ + DCO+. An even larger preference for breaking the H-C bond over the O-H bond is observed in the H+ + DCO2+ and D+ + HCO2+ deprotonation channels. Bond rearrangement, leading to H-2(+) or H2O+ formation, exhibits no isotopic preference. The kinetic-energy-release distributions of the OH+ + DCO+ and O+ + H2CO+ breakup channels suggest that more than one process contributes to these final products, although further theoretical work is needed to identify the specific paths.