Thermally Activated Decay Processes of Isolated Superhot C60 in Molecular Beams

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 (T_o=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 E_o = 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 A_o = A₁ = 2.5 × 10¹³ sec^-1. the emissivity coefficient for black body like radiation was found to be ε = 4.5 × 10^-5.