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/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 ${ 4s^2}$ $^1S^e$ - ${ 4s5p}$ $^1P^o$ transition is much smaller than that for the ${ 4s^2}$ $^1S^e$ - ${ 4s4p}$ $^1P^o$ transition.
Categories: physics.atom-ph