THERMALLY ACTIVATED DECAY PROCESSES OF ISOLATED SUPERHOT C-60 IN MOLECULAR-BEAMS

We have studied thermally activated decay processes of an ensemble of isolated superhot C-60 molecules in molecular beams by several differe nt methods. Highly vibrationally excited C-60 molecules in effusive or supersonic beams (with average vibrational energy of 10-20 eV) were g enerated in an all ceramic, two-stage high temperature nozzle source. The decay kinetics due to various decay processes of the initially can onical ensemble was followed by a mass spectrometric methods for a lar ge range of initial temperatures (T-o=1100-1950 K). The processes stud ied are: (1) fragmentation (C-2 emission) of the neutral C-60 (2) C-2 emission from the C-60(+) ions (3) black-body like radiative cooling, and (4) delayed electron emission. The experiments described here are: (a) Depletion of the integrated C-60 flux. (b) Analysis of C-60 time- of-flight distributions. (c) Dependence of electron impact induced ion ization/fragmentation of C-60 upon its initial thermal excitation, and (d) Thermal energy dependence of delayed electron emission. It is sho wn that thermal kinetics models using a single set of independently me asured parameters uniquely reproduce all the experimental observations . The models take into account the different cooling processes and the ir time evolution. We analyze in detail the evolution of the initially canonical vibrational energy distribution during the flight time to t he detector as it is gradually being distorted due to evaporative and radiative cooling mechanisms. It is concluded that the correct paramet ers to be used for describing the thermally activated decay kinetics o f superhot C-60 are activation energy of E-o=4.3-4.8 eV for the neutra l fragmentation channel C-60 --> C-58 + C-2 and E-1=4.0-4.3 for the io n fragmentation channel C-60(+) C-58(+) + C-2, and corresponding pre-e xponential factors of A(o)=A(1)=2.5x10(13) sec(-1). The emissivity coe fficient for black body like radiation was found to be epsilon=4.5x10( -5).