  We report and provide justification for the consistently observed four
experimental facts of the mass spectrometric data of carbon cluster emission
from the low-energy Cs irradiated single-walled carbon nanotubes . Firstly, the
diatomic carbon C(2) is the most abundant sputtered species for Cs+ in the
energy range 0.2 keV to 2.0 keV. Secondly, monatomic carbon C(1) is emitted
with the least sputtering yield. Thirdly, at low cesium energies i the emitted
species are C(2), C(3) and C(4). Lastly, as the irradiating Cs+ energy
increases, the normalized yield of atomic carbon monotonically increases while
clusters show gradual decrease and saturation. Sputtering of clusters is proved
here to be due to thermal spikes. Binary collision cascade theory does not
explain cluster sputtering. A statistical thermal model is developed to explain
the experimentally observed data. The probability of a cluster C(x) to be
emitted is shown to be proportional to that for the creation of an x-member
vacancy with formation energy E at temperature T as {exp(E/kT)+1}^(-1). The
energies of formation of single and double vacancies from DFT calculations and
the ratio of normalized experimental yields have been used to estimate spike
temperature. We show that by invoking thermal spikes, cluster emission from,
and the multiple vacancy generation in, the Cs+-irradiated SWCNTs can be
explained. We also suggest modifications to Monte Carlo type calculations of
sputtering.
