The thermal energy dependence (10–20 eV) of electron impact induced fragmentation of C60 in molecular beams: Experiment and model calculations

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Abstract
We have studied the dependence of electron impact induced ionization and fragmentation of C60 molecules in effusive molecular beams upon the initial thermal excitation in the temperature range of 1190–1875 K, corresponding to an average vibrational energy of 10–20 eV. This is the largest energy range of parent molecule thermal excitation ever reported for electron‐impact mass‐spectrometric studies. The normalized curves of electron energy (Ee) dependent ion currents of C+60 and C+58 were measured and analyzed for the temperatures (T0) of 1190, 1435, 1570, 1695, and 1875 K. Similar measurements were done for C+2n (n=26–28) fragments for T0=1190 and 1875 K. We have developed an expression for the dependence of C+58 fragment ion current i58(Ee,T0), formed via the decay process C+60→C+58+C2, on electron energy and initial temperature. Using this expression and the strong temperature dependence observed, we have proposed a simple experimental method for estimating the energy deposition function—the probability density of vibrational excitation ε by an ionizing electron of energy Ee. The effective (apparent) value of maximum deposited energy was found to be εm(Ee)=Ee−E*, where E*=30±5 eV. Possible interpretations for this surprisingly low value are discussed. Comparing the experimental i58(Ee,T0) curves with the calculated ones over the range of Ee=30–80 eV we find that for T0≤1600 K, good agreement is obtained assuming that the C60 initial internal excitation is determined by the source temperature alone. For the higher temperature range 1600 K≤T0≤1900 K, we had to use a modified calculation taking into account radiative cooling and ensemble evaporative cooling processes along the molecular beam flight path. As a result, we have obtained an accurate simulation of the complete family of i58(Ee,T0) curves over all the temperature range measured, using a single set of independently measured physical quantities, and without any adjustable parameter. Uniqueness and sensitivity were thoroughly checked and demonstrated. The good agreement between experiment and calculation basically confirms our description of the underlying process and provides an additional support for the values of the independent physical parameters used. We have used maximum energy deposition parameter of E*=31 eV, an activation energy of E0=4.3–4.5 eV for the neutral fragmentation channel C60→C58+C2 and E1=4.0 eV for the ion fragmentation channel C+60→C+58+C2, and pre‐exponential factors of A0=A1=2.5×1013 s−1. These values are very close to former ones obtained by us from analysis of time‐of‐flight distributions and integrated flux decay measurements of hot C60 molecular beams. Correspondence with other results reported in the literature is discussed and a two‐step dissociation mechanism is proposed.