Plasma formation by high-repetition rate pulsed periodic laser irradiation of metals in air at atmospheric and reduced pressures

High-speed spectroscopy with spatial and temporal resolution is used to determine the basic plasma parameters and to examine the shift from an air plasma to an erosion plasma during multipulse laser irradiation of various metals at repetition rates ranging from 5 to 50 kHz with irradiances q > 2∙10⁸ W/cm². The formation of a distinctive surface plasma structure is observed which (a) facilitates the expulsion of the shielding layer of "air" plasma produced during the first of a series of laser pulses by vapor from the target and (b) inhibits the propagation of a metal vapor plasma front into the air during the subsequent pulses.