Ultrafast dynamics of LiH molecule induced by ultrashort optical pulses. Influence of photoionization on the electronic coherence

Upon excitation by strong IR and UV few femtosecond pulses, LiH molecules can undergo both electronic excitation and photoionization. The simultaneous excitation into several electronic excited states leads to a coherent superposition of electronic states in the neutral and in the cation1 (Fig. 1). The evolution of the coherence after the end of the pulse is determined both by the energy difference between the states involved in the superposition and by the phase relation between each component of the superposition2. The phase and the amplitude of each component is itself built by the complex transient dynamics taking place during the initial excitation that involves interaction with the photoionization continuum. It is also influenced by the non- adiabatic coupling induced by nuclear motion3. To suitably represent the evolution of the coherence induced upon excitation by such pulses, methodologies that include both electronic excitation, nuclear motion and photoionization are needed to compute the dynamics.
We will report on the computation of coupled electronic-nuclear dynamics in the LiH molecule 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 method4. The wave function of the ionized states is represented as an antisymmetrized product of the electronic states of the cation and of the states of the photoelectron. A set of orthogonalized plane waves is used for the photoelectron. 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 as the overlap between the electronic state of the neutral and the electronic state of the cation. This method allows us computing the dynamics of LiH 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.
[1] B. Mignolet, R. D. Levine, and F. Remacle, Phys. Rev. A 89, 021403(R)
[2] S. van den Wildenberg, B. Mignolet, R. D. Levine, F. Remacle, Phys. Chem. Chem. Phys., 2017,19, 19837-19846
[3] Astrid Nikodem, R. D. Levine, and F. Remacle, Phys. Rev. A 95, 053404
[4] R. D. Levine, Quantum Mechanics of Molecular Rate Processes (Oxford University Press, 1969).