Surface laser plasma accompanying the heating and evaporation of microdefects

An investigation is made of the mechanism whereby a surface laser plasma is formed when short ( ~ 1 μs) laser pulses having a rise time of the order of tens of nanoseconds interact with a target. A model is suggested for breakdown on the surface of a metal in a molecular gas atmosphere due to the heating and evaporation of thermally insulated surface microdefects which, under certain conditions, are the most probable centers of plasma formation and lead to a significant lowering of the threshold intensity compared with breakdown in the absence of a surface. It is shown that two plasma formation regimes are possible, with E = const and with q = const (where E is the surface energy density of the laser pulse and q is its intensity). A comparison is made with an experiment in which such regimes were observed.