We present experimental results on the controlled de-excitation of Rydberg
states in a cold gas of Rb atoms. The effect of the van der Waals interactions
between the Rydberg atoms is clearly seen in the de-excitation spectrum and
dynamics. Our observations are confirmed by numerical simulations. In
particular, for off-resonant (facilitated) excitation we find that the
de-excitation spectrum reflects the spatial arrangement of the atoms in the
quasi one-dimensional geometry of our experiment. We discuss future
applications of this technique and implications for detection and controlled
dissipation schemes.Comment: 6 pages, 5 figure
