Absolute strong-field ionization probabilities of ultracold rubidium atoms

Understanding strong-field ionization requires a quantitative comparison between experimental data and theoretical models which is notoriously difficult to achieve. Optically trapped ultracold atoms allow to extract absolute nonlinear ionization probabilities by imaging the atomic density after exposure to the field of an ultrashort laser pulse. We report on such precise measurements for rubidium in the intensity range of 1 × 1011 – 4 × 1013 W cm−2. The experimental data are in perfect agreement with ab-initio theory, based on solving the time-dependent Schrodinger equation without any free parameters. We investigate the strong-field response of 87Rb atoms at two different wavelengths representing non-resonant and resonant processes in the demanding regime where the Keldysh parameter is close to unity.