Collective effects on ionization of Rydberg sodium atoms by blackbody radiation

Ionization by blackbody radiation of a beam of sodium atoms, prepared in Rydberg states by two-step laser excitation, has been investigated. Ionization by absorption of an additional laser photon was also present, although these ionization channels take place on very different time scales. For atom densities in the range 1–4 x 10¹¹ cm^-3 and power density of the laser used for the second step excitation of the order of MW/cm², we have found evidence of space-charge interactions in the laser-light focus region.