  There is increasing attention to chemical applications of transmission
electron microscopy, which is often plagued by radiation damage. The damage in
organic matter predominantly occurs via ionization (radiolysis). Although
radiolysis is highly important, previous studies on radiolysis have largely
been descriptive and qualitative, lacking in such fundamental information as
the product structure, the influence of the energy of the electrons, and the
reaction kinetics. We need a chemically well-defined system to obtain such
data, and have chosen as a model a variable-temperature and variable-voltage
(VT/VV) study of the dimerization of a van der Waals dimer [60]fullerene (C60)
to C120 in a carbon nanotube (CNT) as studied for individual reaction events at
atomic resolution. We report here the identification of five reaction pathways
that serve as mechanistic models of radiolysis damage. Two of them occur via a
radical cation of the specimen generated by specimen ionization, and three
involve singlet or triplet excited states of the specimen, as initiated by
electron excitation of the CNT followed by energy transfer to the specimen. The
pathways were distinguished by the pre-exponential factor and the Arrhenius
activation energy. The prototypal reaction path is the radical cation reaction
that we saw at <200 K, but, at >350 K, the excited-state reactions dominate.
The results illustrate the importance of VT/VV kinetic analysis in the studies
of radiation damage, and show that chemical ionization and electron excitation
are inseparable but different mechanisms of radiation damage, which has so far
been classified loosely under the single term "ionization."
