Atomic ionization potentials in a plane electromagnetic wave

Abstract

We point out that the well-known second-order time-dependent perturbation theory result for the level shift of an atomic system in a plane electromagnetic wave is appropriate for calculating the change in ionization potential due to the presence of the wave. The calculation is valid when the electric field strength of the wave is small compared with atomic field strengths (intensity ≪${10}^{16}$ W/${\mathrm{cm}}^2$). We use this result to show that corresponding calculations carried out using the Kramers-Henneberger transformation yield a result which is valid only when the frequency of the plane wave is high compared with the atomic frequencies.