Gauge-invariant absorption of light from a coherent superposition of states

TL;DR

This paper develops a gauge-invariant theoretical framework for analyzing light absorption and emission in atoms superposed in coherent states, revealing how angular momentum, phase, and transition types influence absorption behavior.

Contribution

It introduces a gauge-invariant formulation of transient absorption theory based on Yang's gauge theory, and applies it to simulate and interpret atomic responses to attosecond pulses.

Findings

Absorption depends strongly on angular momentum and phase of superposed states.

Non-resonant transitions cause asymmetry in energy and phase.

Resonant transitions to the continuum contribute symmetrically to absorption.

Abstract

Absorption and emission of light is studied theoretically for excited atoms in coherent superposition of states subjected to isolated attosecond pulses in the extreme ultraviolet range. A gauge invariant formulation of transient absorption theory is motivated using the energy operator from Yang's gauge theory. The interaction, which simultaneously couples both bound and continuum states, is simulated by solving the time dependent Schr\"odinger equation for hydrogen and neon atoms. A strong dependence on the angular momentum and the relative phase of the states in the superposition is observed. Perturbation theory is used to disentangle the fundamental absorption processes and a rule is established to interpret the complex absorption behaviour. It is found that non-resonant transitions are the source of asymmetry in energy and phase, while resonant transitions to the continuum contribute symmetrically to absorption of light from coherent superpositions of states.