Dynamical hole in ultrafast photoassociation: Analysis of the compression effect

Abstract

Photoassociation of a pair of cold atoms by excitation with a short chirped laser pulse creates a dynamical hole in the initial continuum wave function. This hole is manifested by a void in the pair wave function and a momentum kick. Photoassociation into loosely bound levels of the external well in ${\mathrm{Cs}}_{2}\phantom{\rule{0.3em}{0ex}}{0}_{g}^{\ensuremath{-}}(6S+6{P}_{3∕2})$ is considered as a case study. After the pulse, the free evolution of the ground triplet state $a^{3}\ensuremath{\Sigma}_{u}^{+}$ wave packet is analyzed. Due to a negative momentum kick, motion to small distances is manifested and a compression effect is pointed out, markedly increasing the density of atom pairs at short distance. A consequence of the hole is the redistribution of the vibrational population in the $a^{3}\ensuremath{\Sigma}_{u}^{+}$ state, with population of the last bound level and creation of pairs of hot atoms. The physical interpretation makes use of the time dependence of the mass current and population on each channel to understand the role of the parameters of the photoassociation pulse. By varying such parameters, optimization of the compression effect in the ground-state wave packet is demonstrated. Due to an increase of the short-range probability density by more than two orders of magnitude, we predict significant gain in the photoassociation rates into deeply bound levels of the excited state by a second pulse, red-detuned relative to the first one, and conveniently delayed.