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

Abstract

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 $4s4p$ ${}^1{P}^o$ and $4s5p$ ${}^1{P}^o$ 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 $4s4p$ ${}^1{P}^o$ state rather than the near-resonant $4s5p$ ${}^1{P}^o$ state. These findings can be attributed to the fact that the dipole moment for the $4s^2$ ${}^1{S}^e$ - $4s5p$ ${}^1{P}^o$ transition is much smaller than that for the $4s^2$ ${}^1{S}^e$ - $4s4p$ ${}^1{P}^o$ transition.

References in corpus (3)

• Two-, three-, and four-photon ionization of Mg in the circularly and linearly polarized laser fields: Comparative study using the Hartree-Fock and model potentials
• Multiphoton Ionization of Magnesium in a Ti-Sapphire laser field
• Multiphoton ionization through the triplet states of Mg by linearly and circularly polarized laser pulses