Electron-induced secondary electron emission yield from condensed rare gases: Ne, Ar, Kr, and Xe

The secondary electron emission (SEE) yield $\ensuremath{\delta}$ has been measured for condensed layers of Ne, Ar, Kr, and Xe excited by primary electrons of energy ${E}_{0}$ ranging from 0.05 up to $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. The deposited thickness $D$ ranges from 0 up to a few thousands of monolayers. For very thick specimens, the evolution of the $\ensuremath{\delta}({E}_{0})$ curves is very similar for all the four gases and it may be described by a single analytical expression. At $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, the maximum yield is not attained for the thickest layers despite the fact that the measured yields exceed several tens. These observed high yields are analyzed in terms of escape depth for secondary electrons (\ensuremath{\sim} the micrometer range) and of escape probability $(>75%)$. Such results are consistent with those previously obtained by proton bombardment and by x-ray photon irradiation. More surprising is the observed increase of $\ensuremath{\delta}$ with $D$ when all the primary electrons remain confined in the film, $D>R$ (range of incident electrons), and several hypotheses are suggested to explain this effect: increase of the crystallite grain size, charging mechanism, SE reflection effects at the film∕solid interface. For very thin films where the constant thickness $D$ is $D⪡R$, the observed decrease of $\ensuremath{\delta}$ as a function of ${E}_{0}$ is qualitatively explained but, when $D$ is changed, additional calculations are needed.