We have calculated long-range molecular potentials of the
0g+, ${0}_{u}^{\ensuremath{-}}$, and
1u symmetries between highly excited rubidium atoms. Strong
np+np potentials characterized by these symmetries are important in describing interaction-induced phenomena in the excitation spectra of high
np Rydberg states. Long-range molecular resonances are one such phenomenon and they were first reported in S. M. Farooqi et al., Phys. Rev. Lett. 91, 183002 (2003). One class of these resonances occurs at energies corresponding to excited atom pairs $(n\ensuremath{-}1)d+ns$. Such resonances are attributed to
l-mixing due to Rydberg-Rydberg interactions so that otherwise forbidden molecular transitions become allowed. We calculate molecular potentials in Hund's case (c), use them to find the resonance line shape, and compare to experimental results.
We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the 5s ground state to np Rydberg states (n=50-90) reveals resonances at energies corresponding to excited atom pairs (n-1)d+ns. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., approximately 58 000 times the Bohr radius (a(0)) for n=70.
Long-range macrodimers formed by
D-state cesium Rydberg atoms are studied in experiments and calculations. Cesium
[62DJ]2 Rydberg-atom macrodimers, bonded via long-range multipole interaction, are prepared by two-color photoassociation in a cesium atom trap. The first color (pulse A) resonantly excites seed Rydberg atoms, while the second (pulse B, detuned by the molecular binding energy) resonantly excites the Rydberg-atom macrodimers below the
[62DJ]2 asymptotes. The molecules are measured by extraction of autoionization products and Rydberg-atom electric-field ionization, and ion detection. Molecular spectra are compared with calculations of adiabatic molecular potentials. From the dependence of the molecular signal on the detection delay time, the lifetime of the molecules is estimated to be $3--6\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$.