Radiative cooling ofAl4−clusters

Abstract

The radiative cooling of isolated, negatively charged four-atom aluminum clusters has been measured using an electrostatic ion beam trap. Stored $\mathrm{Al}_{4}{}^{\ensuremath{-}}$ ions were irradiated by a short laser pulse at different times after their production in a hot ion source, and delayed electron emission was observed up to hundreds of microseconds after the laser pulse. The decay curves could be well reproduced using an Arrhenius decay law and allowed us to deduce the cluster temperatures at the time of the laser pulse. Using this sensitive molecular thermometer, the cluster temperature could be determined as a function of storage time. The radiation intensity is found to decrease from $40\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{s}$ at $T=1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{s}$ at $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with a temperature dependence as given by ${T}^{b}$ with $b=3.5\ifmmode\pm\else\textpm\fi{}0.2$---i.e., similar to what would be expected from a blackbody. This cooling behavior requires the presence of either electronic transitions or very collective infrared-active vibrations at transition energies around $\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$.