Dephasing of resonant energy transfer in a cold Rydberg gas

When the dipole-dipole energy transfer, Rb 25s(1/2)+Rb 33s(1/2)-->Rb 24p(1/2)+Rb 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 Rb 25s(1/2)+Rb 24p(1/2)-->Rb 24p(1/2)+Rb 25s(1/2) and Rb 33s(1/2)+Rb 34p(3/2)-->Rb 34p(3/2)+Rb 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.