Direct experimental evidence of multiphoton ionization of impurities as the initiation process of laser-induced gas breakdown

Abstract

Using a focused Q-switched ruby laser, multiphoton ionization of the background gas of an ordinary vacuum system was observed by the time-of-flight method and an electron multiplier. The number of free electrons created at the focal region was found to be at least more than 20. Since a few free electrons would be sufficient for the initiation of the laser-induced spark in gases (Young and Hercher), it was thus concluded that multiphoton ionization of gas impurities is the initiation mechanism of the optical breakdown of gases, as is generally believed.