Pulsed laser excitation of a dense ultracold Cs vapor has been used to study the pairwise interactions between Cs atoms excited to $n{p}_{3/2}$ Rydberg states of principal quantum numbers in the range $n=22–36$. Molecular resonances were observed that correspond to excitation of Rydberg-atom-pair states correlated not only to the $n{p}_{3/2}+n{p}_{3/2}$ dissociation asymptotes, but also to $n{s}_{1/2}+(n+1)s_{1/2}, n{s}_{1/2}+n^{\prime }{f}_j$, and $(n-4)f_j+(n-3)f_j(j=5/2,7/2)$ dissociation asymptotes. These pair resonances are interpreted as arising from dipole-dipole and higher-order long-range-interaction terms between the Rydberg atoms on the basis of (i) their spectral positions, (ii) their response to static and pulsed electric fields, and (iii) millimeter-wave spectra between pair states correlated to different pair-dissociation asymptotes. The Rydberg-atom-pair states were found to spontaneously decay by Penning ionization and the dynamics of the ionization process were investigated during the first 15 $\mu \mathrm{s}$ following initial photoexcitation. To interpret the experimental observations, a potential model was derived that is based on the numerical determination of the eigenvalues and eigenfunctions of the long-range interaction Hamiltonian. With this potential model, which does not include adjustable parameters, all experimental observations could be accounted for, and the results demonstrate that long-range-interaction models provide a global and accurate description of interactions in ultracold Rydberg gases and that they correctly account for, and enable the analysis of, phenomena as diverse as the formation of Rydberg macrodimers, Penning ionization in dense Rydberg gases, and Rydberg-excitation-blockade effects.

• Received 29 June 2015

DOI:https://doi.org/10.1103/PhysRevA.92.032505