Imaging the evolution of an ultracold strontium Rydberg gas

Abstract

Clouds of ultracold strontium $5s48s\phantom{\rule{4pt}{0ex}}{}^{1}{S}_{0}$ or $5s47d\phantom{\rule{4pt}{0ex}}{}^{1}{D}_{2}$ Rydberg atoms are created by two-photon excitation of laser-cooled $5{s}^{2}\phantom{\rule{4pt}{0ex}}{}^{1}{S}_{0}$ atoms. The spontaneous evolution of the cloud of low orbital angular momentum (low-$\ensuremath{\ell}$) Rydberg states towards an ultracold neutral plasma is observed by imaging resonant light scattered from core ions, a technique that provides both spatial and temporal resolution. Evolution is observed to be faster for the $S$ states, which display isotropic attractive interactions, than for the $D$ states, which exhibit anisotropic, principally repulsive interactions. Immersion of the atoms in a dilute ultracold neutral plasma speeds up the evolution and allows the number of Rydberg atoms initially created to be determined.