BLACKBODY EXCITATION OF AN ATOM CONTROLLED BY A TUNABLE CAVITY

We have used a gold cavity to control the blackbody spectrum and hence to alter thermal excitation of a Rydberg atom. We suppress the cesium transition 13S-13P or 14S-14P when the cavity width is less than half the transition wavelength, 37 or 50 mu m. Our measurements resolve th e fine structure to a few percent. The experimental curves can be unde rstood by summing multiple reflections of the atomic dipole in the wal ls of the cavity. Changing the temperature shows explicitly that the e xcitation rates are indeed proportional to the number of photons per m ode of the blackbody field. [S0031-9007(98)07187-7].