Partitioning method applied to the computation of coupled electronic-nuclear dynamics in LiH induced by strong pulses including photoionization

The excitation of molecules by strong field pulses leads to complex dynamics involving both the motion of electrons and nuclei on different timescales. Such pulses can induce a beating of the electronic density on the attoseconds to few femtoseconds (fs) timescale. The non-stationary electronic density corresponds to a coherent superposition of several electronic states of the molecule. Within a few tens of fs, nuclei then explore the potential energy surfaces of the electronic states involved in the superposition, and their motion is coupled by the Non-adiabatic interactions. The complex dynamics induced upon excitation by strong pulses is thus essentially non-Born Oppenheimer and is hardly described by methods based on a semi-classical formalism. We will report on the computation of coupled electronic-nuclear dynamics in the LiH and HCN molecules on several excited electronic states and including multiphoton ionization. We compute the dynamics by numerically integrating the nuclear Time-dependent Schrödinger Equation on a spatial grid and on several potential energy surfaces. We include the photoionization process using the partitioning method. The ionized states wave function is represented as an antisymmetrized product of the electronic state of the cation and the state of the photoelectron. The wave function of the photoelectron is represented by a set of orthogonalized plane waves. The coupling between the wave function of the neutral and the wave function of the cation is computed at each grid point in nuclear coordinates as the transition dipole between the photoelectron wave function and the Dyson orbital that is defined between the electronic state of the neutral and the electronic state of the cation. This method allows us to compute the dynamics of LiH and HCN upon excitation by strong IR and UV femtosecond pulses. Depending on the pulse parameters, the dissociation of the LiH molecule yields a lithium atom in different electronic states. The dynamics of HCN in its excited states is more complex because of numerous conical intersections, and can lead to photo-isomerization or to photo-dissociation.