MODIFICATION OF MULTIPHOTON IONIZATION SPECTRA VIA 3RD-HARMONIC GENERATION IN FOCUSED LASER-BEAMS

The modification of multiphoton ionization spectra through third harmonic generation in focused laser beam geometries is reported. A strong enhancement of the ionization peaks corresponding to the decay of the nf autoionizing series of Kr and the observation of a double resonance in Xe, which is missing in low atomic density spectra, is shown to originate from excitation channels involving absorption of third harmonic photons. This excitation is demonstrated to occur in a significantly extended volume outside the focal region, thus strongly increasing the ionization yield.