Rabi Oscillations and Few-Level Approximations in Time-Dependent Density Functional Theory

TL;DR

This paper investigates the limitations of time-dependent density functional theory in modeling resonant laser-atom interactions, revealing classical origins of Rabi oscillations and the inadequacy of few-level approximations.

Contribution

It demonstrates that TDDFT with exact exchange captures Rabi oscillations but fails to accurately describe the density, highlighting fundamental limitations in few-level models.

Findings

Rabi oscillations are of classical origin.

TDDFT accurately models dipole oscillations but not density.

Few-level approximations are incompatible with TDDFT for resonant dynamics.

Abstract

The resonant interaction of laser light with atoms is analyzed from the time-dependent density functional theory perspective using a model Helium atom which can be solved exactly. It is found that in exact-exchange approximation the time-dependent dipole shows Rabi-type oscillations of its amplitude. However, the time-dependent density itself is not well described. These seemingly contradictory findings are analyzed. The Rabi-type oscillations are found to be essentially of classical origin. The incompatibility of time-dependent density functional theory with few-level approximations for the description of resonant dynamics is discussed.