Wavelength Dependence of Nanosecond IR Laser-Induced Breakdown in Water: Evidence for Multiphoton Initiation via an Intermediate State
Lubeck Medical Univ Lubeck Germany
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Investigation of the wavelength dependence 725 1025 nm of the threshold for nanosecond optical breakdown in water revealed steps consistent with breakdown initiation by multiphoton ionization, with an initiation energy of about 6.6 eV. This value is considerably smaller than the autoionization threshold of about 9.5 eV, which can be regarded as band gap relevant for avalanche ionization. Breakdown initiation is likely to occur via excitation of a valence band electron into a solvated state, followed by rapid excitation into the conduction band. Theoretical analysis based on these assumptions suggests that the seed electron density required for initiating avalanche ionization drops from 2.51015 cm-3 at 725 nm to 1.11012 cm-3 at 1025 nm. These results demand changes of future breakdown modeling for water including the use of a larger band gap than previously employed, the introduction of an intermediate energy level for initiation, and consideration of the wavelength dependence of seed electron density.