The dipole trap is loaded directly from a subdoppler cooled cloud of atoms overhpped with thecrossed-beams.Weinitiallyload2 x IO'atoms at a temperature of 70 pK and estimated densities -10" cm-' . Forced evaporative cooling of the atomic sample is achieved by lowering the trap beam powers -50-fold over 2 s. The BEC transition occurs at temperature of -300 nK and beam powers of -300 mw. 1. M.D. Barren, LA. Sauer, and M.S. Chapman, "AI-optical formation of an atomic Bose-Einstein condensate:' Phys. Rw. Lett. 87, 010404 (znoi). QTuH4 300 pm QTuH4 Fig. 1. Experimental Rydberg excitation spectrumof"Rb 5P,,2atomsembedded in a laser-generated ion plasma. The lower portion shows how the three types of spectral gaps become hlled in at certain critical wavelengths that depend on the strength of the plasma electric held. ,,m* IPS QTuH4 Fig. 2. Magnitudes of the plasma electric field as a function of time. The plasma is created at time zero.ifests itself in the spectrum through the appearance ofp-lines andof"h-features". Each ofthe latter reflects a hydrogenic manifold of states with the same principal quantum number n. As n increases, the h-features progressively expand and gain more oscillator strength. Eventually, they fill in all spectral regions of the spectrum, including the gaps between the discrete non-hydrogenic I-, pand d-liner. The gaps become lilled at well defined wavelengths, which correspond to well defined principal quantum numbers npd. nmd. and nhr. The n, are solely determined by the spreading behavior of the h-features, and therefore serve as robust indicators of the electric field E present. For example, thepd-gaps, shown in Fig. 1, disappear when the energy spread 3n$ ofthe hydrogenic manifold equals the separation n$ between neighboring hydrogenic manifolds. Using models involving electric-field probability distributions, we have found relations that allow us to obtain themostprobableelectricfield €,,,,in our Gaussian ion clouds from the experimentally observed critical quantum numbers nP nmia and nhr (see Fig. 1).We have varied the time delay between the W excitation pulse and the blue pulse. This has allowed us to study the decay of the electric field as a function of time (Fig. 2). The data clearly show that, as the cloud undergoes Coulomb explosion. the electric field decays on a time ~cale of the order of 1 p. We compare the experimental result with numerical simulations of the explosion of our Gaussian ion clouds.Using crossed and focused laser beams with adjustable polarizations, our time-sensitive method of electric field measurement can be extended to measure both the position-dependence and the directions of the plasma electric field.Temporally and spatidy resolved measurements of the electric field will be an excellent tool to study the electric-field distribution, the plasma microfields, and the dynamics of p h m a expansion.
