Abstract
Laser-induced breakdown has been observed since the advent of the Ruby laser and a considerable body of literature aimed at understanding the physical mechanisms has since developed.1 To date, experiments have centered largely on measurements of breakdown thresholds. For longer, nanosecond laser pulses, thresholds are found to vary inversely with pulse length and pressure indicating a collisional mechanism such as electron-impact ionization, but for shorter pulses or low pressures, thresholds are considerably higher and only weakly pressure dependent indicating direct multiphoton ionization. The various mechanisms may act simultaneously and their relative contributions not only depend on the initial conditions but also change during the growth of the plasma. Theoretical models of laser-induced breakdown have included all these effects, but the point of comparison with the experiment is generally the dependence of the breakdown threshold on the initial conditions of gas, pressure, laser wavelength, and intensity.2
© 1991 Optical Society of America
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