  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$_{1/2}+n'$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 10 $\mu$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.
