CO2-laser-induced breakdown in mono- and diatomic gases

Abstract

Several monoatomic and homonuclear diatomic gases absorb energy from a focused CO2-laser photon field. It has been established that the pressure threshold for the energy absorption correlates qualitatively with the known ionization potentials of those gases. The simplified phenomenological theory of the CO2-laser-induced dielectric breakdown of gases is invoked to explain this phenomenon. In the H2−D2 system, the formation of HD is observed under these conditions. The examination of the reaction yields for HD formation demonstrates that the system studied does not reach equilibrium under our experimental conditions. Considerations regarding kinetics of primary processes reveal that ionic species, created originally via an inverse bremsstrahlung mechanism, are converted into atomic transients in fast ionic association processes. The latter species initiate chain reactions with surrounding molecules of substrates leading to the formation of HD. Simple kinetic analysis based on a non-steady-state assumption permitted the derivation of an expression for the yield of HD formation. This equation was fitted to the experimental data assuming that the temperature of the reaction rises with an increase of the amount of D2 in the mixture. Some other aspects regarding the behavior of this system in a focused CO2 laser beam are also discussed.