Many-body dipole-dipole interactions between excited Rb atoms probed by wave packets and parametric four-wave mixing.

Dipole-dipole interactions between excited Rb atoms at long range ( ∼ 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-5d 5/2 quantum beating frequency of ∼18.3 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.