Modeling the laser-pulse-induced helium trimer dynamics

Motivated by ongoing pump-probe spectroscopy experiments, this work develops a theoretical framework for describing the rovibrational wave packet dynamics that ensues when a single weakly-bound van der Waals trimer is exposed to a short, sub-picosecond linearly polarized pump laser pulse. The intensity I of the pump laser is chosen such that excitation and ionization of the electronic degrees of freedom are negligible while excitation of the wavepacket in the nuclear degrees of freedom is non-negligible. The numerical treatment, which takes advantage of the fact that the laser pulse is very short compared to typical molecular time scales, is based on a wave packet decomposition that utilizes hyperspherical coordinates. The framework is applied to the extremely floppy bosonic helium trimer. A convergence analysis of the partial wave decomposition is conducted. The kinetic energy release and orientation dynamics are presented. While the dynamics of more strongly-bound van der Waals trimers such as, e.g., the argon trimer display negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture appropriate, those of weakly-bound trimers display non-negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture inappropriate. It is shown that a model that constructs the helium trimer dynamics from the dynamics of the helium dimer captures a number of key characteristics of the alignment signal, including the interference between different angular momentum wave packet components.