Multiphoton ionization of the calcium atom by linearly and circularly polarized laser fields

摘要: We theoretically study multiphoton ionization of the Ca atom irradiated by second (photon energy $3.1$ eV) and third 4.65 harmonics Ti:sapphire laser pulses $1.55$ eV). Because dense level structure a are nearly single-photon resonant with $4s4p$ ${}^{1}{P}^{o}$ $4s5p$ states, respectively. Although two-photon takes place through near-resonant intermediate states same symmetry in both cases, it turns out that there significant differences between them. The photoelectron spectra exhibit absence or presence substructures. More interestingly, angular distributions clearly show main contribution to processes harmonic arises from far-off-resonant state rather than state. These findings can be attributed fact dipole moment for $4{s}^{2}$ ${}^{1}{S}^{e}\ensuremath{-}4s5p$ transition is much smaller ${}^{1}{S}^{e}\ensuremath{-}4s4p$ transition.