A new direct method for probing the thermal dissociation kinetics of hot clusters is presented. It is based on analysis of time-of-flight distributions of effusive molecular beams. The method was applied to superhot ${\mathrm{C}}_{60}$ up to 1890 K, and it was found that the decay rate constants decreased during the flight time. Several cooling mechanisms were studied in order to account for this phenomenon, and thermal photon emission, modeled as blackbodylike radiation, was found to be most effective. The Arrhenius parameters obtained were $E_0=4.0\mathrm{}\pm 0.3$ eV and $A=2.8\mathrm{}\times {10}^{13}$ ${\sec }^{-1\mathrm{}}$.

• Received 7 July 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.510