Time-resolved photoelectron spectra as probe of excited state dynamics: A full quantum study of the $Na2F$ cluster

The excited state dynamics of the $Na2F$ cluster initiated by a femtosecond laser pulse is studied by quantum wave packet propagation within a pump–probe setup. The probe pulse is supposed to ionize the system to yield a photoelectron spectrum that depends on the time delay between the pump- and probe pulse. It is shown that the time dependence of the photoelectron spectrum is an extremely sensitive tool to study intramolecular motion of the cluster like the energy flow between different internal modes. The potential energy surfaces involved in the photoinduced process are calculated from a pseudopotential model with repulsive and Coulomb potentials for the ions and a quantum description of the excess electron via electron-$Na+$ and electron-$F−$ pseudopotentials, polarization of the ionic cores as well as electron-ion correlation being added perturbatively. The nuclear motion upon laser excitation is described by full-dimensional quantum wave packet propagation using realistic laser pulse parameters. We find that the pump–probe signal is essentially dominated by the bending motion of the cluster with a periodicity of 180 fs. In addition, we also report detailed structures in the photoelectron spectrum which can be attributed to the excitation of the stretching mode. The possibility of experimental observation of the predicted dynamical behavior is discussed.