When the dipole-dipole energy transfer, $\mathrm{Rb}\hspace{0.5em}{25s}_{1/2}+\mathrm{Rb}\hspace{0.5em}{33s}_{1/2}\to \mathrm{Rb}\hspace{0.5em}{24p}_{1/2}+\mathrm{Rb}\hspace{0.5em}{34p}_{3/2},$ resonant at fields of 3.0 and 3.4 V/cm, is observed in a cold Rydberg gas, the resonances are broader than expected for this dipole-dipole interaction alone. Using a variant of the Ramsey interference method, in which the system is detuned from the resonance, we observe a density-dependent dephasing which we attribute to the inhomogeneities in the always resonant dipole-dipole excitation exchange interactions $\mathrm{Rb}\hspace{0.5em}{25s}_{1/2}+\mathrm{Rb}\hspace{0.5em}{24p}_{1/2}\to \mathrm{Rb}\hspace{0.5em}{24p}_{1/2}+\mathrm{Rb}\hspace{0.5em}{25s}_{1/2}$ and $\mathrm{Rb}\hspace{0.5em}{33s}_{1/2}+\mathrm{Rb}\hspace{0.5em}{34p}_{3/2}\to \mathrm{Rb}\hspace{0.5em}{34p}_{3/2}+\mathrm{Rb}\hspace{0.5em}{33s}_{1/2}.$ Since the dephasing rate is comparable to the observed widths, it appears that all of these interactions contribute to the observed linewidths.

• Received 10 February 2002

DOI:https://doi.org/10.1103/PhysRevA.65.063404