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

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