Some simple mechanisms of multiphoton excitation in many-level systems

Results are reported on coherent monochromatic multiphoton excitation in ma ny-level systems, which are representative for some of the basic mechanisms for atomic and molecular multiphoton processes. Numerical solutions are di scussed that use the Floquet and quasiresonant approximations in the framew ork of the URIMIR program package. The excitation schemes include direct th ree-photon excitation, two-photon excitation with diagonal coupling, Gopper t-Mayer-type two-photon processes, multiphoton excitation with off-resonant intermediates, and practically irreversible coherent excitation into dense spectral structures. Several interesting phenomena are observed, such as n onlinear line shifts and broadenings of multiphoton resonances of relevance for multiphoton spectroscopy and almost constant intermediate population i nversions, potentially useful for laser design. The accurate numerical resu lts are compared with approximate solutions from perturbation theory, and w ith simple analytical solutions from Rabi-type formulae.