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Original Articles

Thermally Activated Decay Processes of Isolated Superhot C60 in Molecular Beams

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Pages 67-102 | Received 27 Aug 1997, Published online: 23 Apr 2008
 

Abstract

We have studied thermally activated decay processes of an ensemble of isolated superhot C60 molecules in molecular beams by several different methods. Highly vibrationally excited C60 molecules in effusive or supersonic beams (with average vibrational energy of 10-20 eV) were generated in an all ceramic, two-stage high temperature nozzle source. the decay kinetics due to various decay processes of the initially canonical ensemble was followed by a mass spectrometric methods for a large range of initial temperatures (To=1100 − 1950 K). the processes studied are: (1) fragmentation (C2 emission) of the neutral C60 (2) C2 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 C60 flux. (b) Analysis of C60 time-of-flight distributions. (c) Dependence of electron impact induced ionization/ fragmentation of C60 upon its initial thermal excitation, and (d) Thermal energy dependence of delayed electron emission. It is shown that thermal kinetics models using a single set of independently measured parameters uniquely reproduce all the experimental observations. the models take into account the different cooling processes and their time evolution. We analyze in detail the evolution of the initially canonical vibrational energy distribution during the flight time to the detector as it is gradually being distorted due to evaporative and radiative cooling mechanisms. It is concluded that the correct parameters to be used for describing the thermally activated decay kinetics of superhot C60 are activation energy of Eo = 4.3 − 4.8 eV for the neutral fragmentation channel C60 → C58 + C2 and E1=4.0 − 4.3 for the ion fragmentation channel C+ 60→ C+ 58 + C2, and corresponding pre-exponential factors of Ao = A1 = 2.5 × 1013 sec−1. the emissivity coefficient for black body like radiation was found to be ∊ = 4.5 × 10−5.

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