Time-dependent calculation of ionization in potassium at midinfrared wavelengths

We study the dynamics of the potassium atom in the midinfrared (12-, 13-, a nd 14-photon ionization), high-intensity, short laser pulse regime. We asce rtain numerical convergence by comparing the results obtained by the direct expansion of the time-dependent Schrodinger equation onto B splines to tho se obtained by the eigenbasis expansion method. We present ionization curve s in the 12-, 13-, and 14-photon ionization range for potassium. The ioniza tion curve of a scaled system, namely hydrogen starting from the 2s, is com pared to the 12-photon results. In the 13-photon regime, a dynamic resonanc e is found and analyzed in some detail. The results for all wavelengths and intensities, including hydrogen, display a clear plateau in the peak heigh ts of the low-energy part of the above threshold ionization spectrum, which scales with the ponderomotive energy Up and extends to (2.8+/-0.5) U-p. [S 1050-2947(99)00212-7].