Gas breakdown by a short laser pulse

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Abstract
The breakdown of gases by short (typically sub-nanosecond) laser pulses when the two dominant mechanisms governing the plasma growth are multiphoton and cascade ionization is treated theoretically. It is shown that two important pressure régimes result: a low-pressure régime where the breakdown is governed solely by the multiphoton ionization process; and a high-pressure régime where the expression for threshold breakdown intensity involves both ionization processes as well as the effect of impurities in the gas. The predictions of the theory are compared with published experimental data for ruby and Nd3+ laser pulses. Good agreement is obtained in the case of ruby radiation, and a possible explanation of deviation from the experimental data obtained using a Nd3+ laser is offered.