Toward a theory of atomic gas breakdown by laser radiation near a metal surface

The threshold intensity of laser radiation causing the breakdown of atomic gases near a metal surface is investigated as a function of the kind of metal and gas, pressure and radius of focusing spot. The initial stage of breakdown is viewed as a clearly defined transition from the state of a gas with a very low degree of vapor ionization to a state with considerable ionization. This transition is determined by the relationship between the generation of electrons in ionization events and diffusion losses. The average electron energy and diffusion loss radius are found through two-dimensional balance equations written for a cylindrical geometry at a given surface temperature with allowance for the process of ionization and recombination, ambipolar diffusion, heat conductivity, elastic and nonelastic electron energy loss, and absorption of laser radiation energy. The laser radiation intensity is found for which ionization and recombination processes predominate over diffusion losses near the surface. The intensities thus derived are compared with experimental data for different gases and metals.