R-matrix Floquet description of multiphoton ionization of Li.

We present mixed gauge R-matrix Floquet calculations of multiphoton ionization of Li within a single active electron local model potential. The laser frequency ranges from 15 000 to 15 800 cm, and at least three photons are needed to ionize the electron in the 2s ground state. We found that the measured ionization rate as a function of photon frequency results from ionization processes in different intensity regions of the laser. The experimental angular distribution of the ejected electrons can be explained using the high intensity portion of the laser for both the lowest ionization channel and the first above-threshold ionization channel. The calculated and experimental angular distributions agree well with each other and demonstrate the success of the R-matrix approach to the multiphoton ionization processes. @S1050-2947~96!05810-6#