Emission of molecular nitrogen upon electron bombardment of pyrolytic aerogel SiO$_2$ and aluminum

It is found that the emission intensity of the $2^+$ band system of the N$_2$ molecule (C$^{3}\Pi_{u} \to \text{B}^{3}\Pi_{g}$ transition) under electron bombardment of pyrolytic aerogel SiO$_2$ in air at atmospheric pressure is $40$–$70$% higher than that in the case of bombardment of aluminum under the same conditions. The dependences of the emission intensity at $337$ nm (0-0 vibrational transition in the $2^+$ band system of the N$_2$ molecule) on the target temperature in the range of $30$–$60^\circ$ C are different for the aluminum and SiO$_2$ targets, which indicates different excitation mechanisms of the N$_2$(C$^3\Pi_u$) state. It is suggested that when the SiO$_2$ target is bombarded, the N$_2$(C$^3\Pi_u$) state is excited not only by direct collisions with the electrons of the primary beam and secondary electrons, but also by the energy transfer from SiO$_2$ excitons and conduction electrons to N$_2$ molecules upon their contact with the SiO$_2$ surface.