ANALYSIS OF MULTIPHOTON IONIZATION OF METAL ATOMS IN THE SATURATION REGIME USING SUBPICOSECOND KRF LASER-PULSES

Singly and multiply charged ion yield curves are reported for resonant and nonresonant two-photonionization processes for a collection of 31 metal atoms. The atoms are created by sputtering from a solid target using an Ar-ion gun. Single and multiple ionization of these atoms is performed using linearly polarized 248.6-nm (KrF) laser pulses with a full width at half maximum duration of 500 fs, employing intensities b etween similar to 10(9) and similar to 10(12) W cm(-2). A four-grid hi gh-resolution reflecting time-of-flight spectrometer is used for ion d etection. This advanced spectrometer has a weil-defined and small sour ce volume. enabling absolute measurements of ionization probabilities and saturation intensities. Because our measurements are not affected by the increase of the interaction volume for increasing intensities, we can discriminate between resonant and nonresonant multiphoton ioniz ation processes without varying the laser pulse duration. For many met als, the intensity dependence of the ion yield can be accurately repro duced by rate calculations based on a resonantly enhanced two-photon-i onization scheme. As a result, we can determine absolute values of the one-photon cross sections in the resonant processes and these are com pared to theoretical values we calculated. For the nonresonant process es, we give generalized multiphoton-ionization cross sections and comp are these to a scaling law of Lambropoulos [J. Opt. Sec. Am. B 4, 821 (1987)]. [S1050-2947(98)00111-5].