Dipole-dipole interactions between excited Rb atoms at long range ($\sim 300a_0–2150a_0$) have been observed with molecular wave packets and a coherent nonlinear optical process. Fourier analysis of the parametric four-wave mixing (PFWM) signal wave intensity produced in femtosecond pump-probe experiments demonstrates the appearance of sidebands associated with the Rb $7s\mathrm{\text{−}}5d_{5/2}$ quantum beating frequency of $\sim 18.3\mathrm{THz}$. Calculations show that the observed sideband splittings and Fourier domain profiles result from multiple atom, dipole-dipole interactions, and ensembles comprising five or fewer Rb ($7s$, $6p$) atoms account for virtually all of the data.

• Received 13 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.99.143201