Multiphoton ionization and optical breakdown of xenon in annular laser beams

By measuring selective laser-induced breakdown in xenon near the three-photon 6s resonance it has been shown that the ionization spectra experience remarkable transformations and the breakdown threshold in some spectral regions is reduced when the excitation is performed by laser beams with a blocked central part (annular beams). These effects have been explained by considering an internally-generated third harmonic. Due to the self-phase-matching mechanism the third harmonic in a focused annular beam can be generated in a much broader spectral range than in ordinary Gaussian beams. It leads to the appearance of additional ionization pathways and transforms the ionization spectra.