Neutral Rydberg matter clusters from K: extreme cooling of translational degrees of freedom observed by neutral time-of-flight

Neutral electronically excited clusters K-N(+) with N = 2-7 are formed and studied by neutral time-of-flight (TOF) caused by laser induced Coulomb explosions in a cloud of Rydberg Matter (RM). A thermal velocity distribution would give TOF peaks with widths of the order of 100 mus. Instead, the minimum half-width is 5 mus, indicating supersonic velocities with an effective temperature of <20 K. This cooling is proposed to be caused by energy transfer from the core ions to the electrons in the conduction band in RM and by evaporation of thermal electrons over the work function barrier of less than or similar to0.5 eV. The small size of the clusters also implies that normally no electrons exist above the Fermi level, corresponding to very low local electron temperatures. Evidence for the geometrical extension of the cloud of RM in the chamber is given. (C) 2002 Elsevier Science B.V. All rights reserved.