Nonresonant multiphoton ionization of cesium in strong fields: Angular distributions and above-threshold ionization

Multiphoton ionization of cesium atoms by intense (5×{}10¹¹ W cm^-2), short (54 ps) laser pulses at the neodymium-doped yttrium aluminum garnet laser frequency is studied. Experimentally, the energy spectrum and angular distribution of the photoelectrons are recorded using a time-of-flight spectrometer. Strong-field effects are clearly demonstrated by evidence of simultaneous four- and five-photon ionization (above-threshold ionization) and by intensity-dependent angular distributions. Theoretically, differential generalized cross sections for four- and five-photon ionization are computed and are shown to be strongly affected by light shifts and high-order coupling of nonresonant bound states. Comparison between theory and experiment includes effective order of nonlinearity, intensity dependence of four- and five-photon electron signal around the saturation intensity, relative values of angular-distribution coefficients, and the ratio of five- to four-photon signals. Intensity effects are generally well predicted both for angular distributions and for saturation. Some discrepancy remains between experimental and theoretical angular-distribution coefficients. The ratio of the total number of electrons emitted in the five- and four-photon processes is measured to be (2.9±{}0.5)×{}10^-2, at 6×{}10¹¹ W cm^-2, encompassing the theoretical value of 3.3×{}10^-2.