Dipole-Dipole Interactions Between Excited Atoms
Probed by Molecular Wavepackets and Parametric Four Wave Mixing

J. Gary Eden

University of Illinois, Department of Electrical and Computer Engineering, Laboratory for Optical Physics and Engineering,
1406 West Green Street, Urbana, Illinois 61801, USA

J. G. Eden, J. Gao, F. Shen, and Y. Xiao Dipole-dipole interactions between excited Rb atoms have been observed, and absolute interaction energies measured, with molecular wavepackets and parametric four wave mixing (PFWM). Fourier analysis of the PFWM signal wave intensity produced in femtosecond pump-probe experiments reveals sidebands associated with the Rb 7s-5d5/2 quantum beating frequency (~18.3 THz). Measurements of the temporal history of the wavepacket over a range in Rb number density of almost four orders of magnitude yield sideband splittings following the expected R-3 dependence and in quantitative agreement with values predicted by calculations of the Rb2 molecular potentials correlated with Rb (7s, 5d3/2, 5/2) + Rb (6p1/2, 3/2) in the separated atom limit, and the internuclear separation (R) in the ~150-1150 Å interval. The results demonstrate that atomic dipole-dipole interactions can be observed at long range (R > 1000 Å) with a new form of FM spectroscopy.

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