We studied the properties of a cold (ϳ100 mK) and dense (ϳ10 8 10 cm 23) atomic Rydberg Cs gas, and found that the observed widths and shapes of resonances in population transfers cannot be explained in the framework of a usual gas model. We propose a "frozen Rydberg gas" model, where the interplay between two-body and many-body phenomena affects in an unexpected way the width and the shape of spectral lines. [S0031-9007(97)04903-X]

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We develop a quantum-mechanical approach to the photoassociation phenomenon in a sample of cold atoms initially at thermal equilibrium with temperature T . We calculate the rate of formation of molecules in the weak field regime using a perturbative approach. Analytical expressions are obtained for the overlap between the initial and final molecular wavefunctions, and are shown to agree with numerical calculations. The photoassociation rate is found to decrease with increasing atomic mass m, from Li to Cs. Also considered is the heating of the atomic sample through spontaneous photodissociation. † This index n is simply used to number the rovibrational molecular states and has nothing to do with the principal quantum number which characterizes the atomic ns and np states.