Femtosecond Collisional Dissipation of Vibrating D2+ in Helium Nanodroplets

We explored the collision-induced vibrational decoherence of singly ionized D2 molecules inside a helium nanodroplet. By using the pump-probe reaction microscopy with few-cycle laser pulses, we captured in real time the collision-induced ultrafast dissipation of vibrational nuclear wave packet dynamics of D2+ ion embedded in the droplet. Because of the strong coupling of excited molecular cations with the surrounding solvent, the vibrational coherence of D2+ in the droplet interior only lasts for a few vibrational periods and completely collapses within 140 fs. The observed ultrafast coherence loss is distinct from that of isolated D2+ in the gas phase, where the vibrational coherence persists for a long time with periodic quantum revivals. Our findings underscore the crucial role of ultrafast collisional dissipation in shaping the molecular decoherence and solvation dynamics during solution chemical reactions, particularly when the solute molecules are predominantly in ionic states.