Threshold processes of sodium ion emission from NaAu surface alloy

We have studied threshold processes of Na$^+$ ion emission from a semiconductor Na$_x$Au$_y$ film formed on the surface of a gold substrate. In contrast to the classical notions of threshold processes involved in the surface ionization of alkali metal ions from heated metal surfaces, the diffusion exchange of atomic species between the surface and volume of the Na$_x$Au$_y$ film ensures stable emission of Na$^+$ ions from the substrate in the region of threshold temperatures. A diffusion mechanism of self-regulation of the surface coverage of alkali metal in the Na$_x$Au$_y$ film is proposed.