Multiphoton-double-ionization probability linearly depends on laser intensity: Experimental studies of barium

TL;DR

This study demonstrates that barium's multiphoton double ionization probability scales linearly with laser intensity, revealing a simplified single-photon-like behavior despite complex underlying dynamics.

Contribution

The paper provides experimental evidence that double ionization in barium under infrared laser excitation behaves as a linear process, challenging the expectation of nonlinear multiphoton dependence.

Findings

Double ionization probability is proportional to laser intensity.

Single-electron ionization involves a four-photon resonant transition.

Double ionization occurs via a two-electron mechanism.

Abstract

Despite inherently complex multiphoton dynamics, our observations show that Ba double ionization with an infrared laser (8800-8920 cm^{-1}) resembles a single-photon process; namely, its probability is proportional to the laser intensity. In this regime, single-electron ionization is due to a four-photon resonant transition through the highly perturbed state 6p^2 1D_2, whereas double ionization is realized by the two-electron mechanism. Furthermore, we argue that these conclusions are valid for other alkaline-earth-metal atoms and other parameters of laser radiation.