We report on precise measurements of absolute nonlinear ionization probabilities obtained by exposing optically trapped ultracold rubidium atoms to the field of an ultrashort laser pulse in the intensity range of $1 \times 10^{ 11}$ to $4 \times 10^{ 13}$ W/cm$^2$. The experimental data are in perfect agreement with ab-initio theory, based on solving the time-dependent Schr\"odinger equation without any free parameters. Ultracold targets allow to retrieve absolute probabilities since ionized atoms become apparent as a local vacancy imprinted into the target density, which is recorded simultaneously. We study the strong-field response of $^{ 87}$Rb atoms at two different wavelengths representing non-resonant and resonant processes in the demanding regime where the Keldysh parameter is close to unity.
Categories: physics.atom-ph cond-mat.quant-gas