Non-adiabatic quantum molecular dynamics: ionization of many-electron systems

Abstract

We propose a novel method to describe realistically ionization processes with absorbing boundary conditions in basis expansion within the formalism of the so-called Non-Adiabatic Quantum Molecular Dynamics. This theory couples self-consistently a classical description of the nuclei with a quantum mechanical treatment of the electrons in atomic many-body systems. In this paper we extend the formalism by introducing absorbing boundary conditions via an imaginary potential.It is shown how this potential can be constructed in time-dependent density functional theory in basis expansion. The approach is first tested on the hydrogen atom and the pre-aligned hydrogen molecular ion H + 2 in intense laser fields where reference calculations are available. It is then applied to study the ionization of non-aligned H + 2 and H 2 . Striking differences in the orientation dependence between both molecules are found. Surprisingly, enhanced ionization is predicted for perpendicularly aligned molecules.

Electron–nuclear correlations in mixed quantum–classical calculations of laser-induced ionization and dissociation

“…Promising approaches for an approximate description of the laser driven dynamics are mixed quantum-classical calculations, where the nuclear system is described classically, whereas the electronic system is treated quantum mechanically, see e.g. [21][22][23][24][25]. Among them is the socalled nonadiabatic quantum molecular dynamics (NA-QMD) [22,25].…”

“…[21][22][23][24][25]. Among them is the socalled nonadiabatic quantum molecular dynamics (NA-QMD) [22,25]. The applicability of this method to a laser driven system was e.g.…”

“…demonstrated in [22] and [26], where results from mixed quantum-classical calculations were compared to those from a full quantum mechanical calculation [9]. Recently NA-QMD has been extended to describe realistically ionization, including so-called charge resonance effects [25]. Yet, to obtain a classical description of the nuclear system, several approximations had to be made.…”

“…For the formulation of the working equations of standard NA-QMD, including the description of ionization processes, we refer to previous publications [22,25]. For the present discussion we stress that the electronic problem is described by timedependent density functional theory.…”

“…In order to extend NA-QMD by going beyond the mean field approach, the quantum solutions are analyzed at fixed times t n = n t. The norm N(t) of the wave function is between zero and one due to the absorbing boundary conditions that have been incorporated in the quantum equations recently [25]. Thus, at any time t the probability that the molecule is in the bound or in the ionized channel can be determined.…”