Transient spectroscopy from time-dependent electronic-structure theory without multipole expansions

TL;DR

This paper introduces a gauge-invariant, multipole-expansion-free method for transient absorption spectroscopy that efficiently models laser-driven electron dynamics beyond the dipole approximation, demonstrated through advanced electronic structure simulations.

Contribution

It develops a computationally efficient, gauge-invariant framework for transient spectroscopy without multipole expansions, applicable to complex electron dynamics.

Findings

Accurate simulation of core-level absorption spectra.

Efficient modeling of electron dynamics beyond the dipole approximation.

Successful application to circular dichroism spectra.

Abstract

Based on the work done by an electromagnetic field on an atomic or molecular electronic system, a general gauge invariant formulation of transient absorption spectroscopy is presented within the semi-classical approximation. Avoiding multipole expansions, a computationally viable expression for the spectral response function is derived from the minimal-coupling Hamiltonian of an electronic system interacting with one or more laser pulses described by a source-free, enveloped electromagnetic vector potential. With a fixed-basis expansion of the electronic wave function, the computational cost of simulations of laser-driven electron dynamics beyond the dipole approximation is the same as simulations adopting the dipole approximation. We illustrate the theory by time-dependent configuration interaction and coupled-cluster simulations of core-level absorption and circular dichroism spectra.