UC Berkeley

Ultrafast laser excitation can trigger complex coherent dynamics in molecules. Here, we report attosecond transient-absorption experiments addressing simultaneous probing of electronic and vibrational dynamics in a prototype molecule, deuterium bromide (DBr), following its strong-field ionization. Electronic and vibrational coherences in the ionic XΠ3/22 and XΠ1/22 states are characterized in the Br-3d core-level absorption spectra via quantum beats with 12.6-fs and 19.9-fs periodicities, respectively. Polarization scans reveal that the phase of the electronic quantum beats depends on the probe direction, experimentally showing that the coherent electronic motion corresponds to oscillation of the hole density along the ionization-field direction. The vibrational quantum beats are found to maintain a relatively constant amplitude, whereas the electronic quantum beats exhibit a partial decrease in time. Quantum wave-packet simulations show that decoherence from vibrational motion is insignificant because the XΠ3/22 and XΠ1/22 potentials are nearly parallel. A comparison between the DBr and HBr results suggests that rotational motion is responsible for the decoherence since it leads to initial alignment prepared by the strong-field ionization.