Ponente
Descripción
The injection of energetic electrons in dense gases and liquids is a technique widely used to accomplish several goals. In particular, how hot electrons thermalize in a dense gas or in a liquid is a subject that has attracted much attention. Hot electrons can lose their energy along multiple energy degradation paths. Dense rare gases efficiently convert the electron kinetic energy by exciting atomic levels. Collisions of excited noble gas atoms with ground state ones may lead to the formation of excimers in high-lying molecular levels. The excimer deexcitation to the dissociative molecular ground state releases a relevant fraction of the excitation energy to a relatively narrow band in the vacuum ultraviolet (VUV) range. In a previous experiment in an electron beam excited, dense Xe gas we have been able to detect an infrared (IR) band originating from a bound-free electronic transition between higher lying molecular states. The location of the relatively broad IR excimer band has been found to depend on the host gas density. Namely, the band maximum linearly shifts to longer wavelength as the density is increased. We explained the experimental observation by accounting fort two effects, one classical and one quantum mechanical. The excimer is considered as consisting of an ionic core plus an electron in a large, Rydberg-like orbit. Several host gas atoms atoms are encompassed within the orbit so that they act as a dielectric screen that reduces the Coulombic interaction between the optically active electron and the ionic core thereby changing its energy. Furthermore, the quantum wavelength of the delocalized Rydberg electron is so large as to make it interact with many atoms at once. As a result of the combined effect of the atomic polarizability and the electron atom-scattering length in pure Xe gas the energy of the electronic transition in the excimer is reduced as density is increased. We confirmed this model by also measuring the Xe excimer in a 10%Xe-90%Ar mixture. The model holds true if the polarizability and scattering length of Xe are replaced by those of Ar because in the mixture the Xe2 excimer is mainly surrounded by Ar atoms. In order to further confirm our model we are now carrying out spectroscopic measurements of the cathodoluminescence of the Xe2 excimer in several mixtures with different noble gases. In particular, we are studying mixtures with He, whose atomic polarizability is roughly one order of magnitude smaller than that of Xe and its electron-atom scattering length has opposite sign with respect to Xe. Thus, we should be able, by exploiting the law of ideal mixtures, to tailor the density dependent shift of the excimer spectrum maximum by simply adjusting the Xe-He concentration. Now, we report the first results obtained with several Xe-noble gas mixtures.
