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

TL;DR

This theoretical study investigates multiphoton ionization of calcium atoms using harmonics of Ti:sapphire lasers, revealing how near-resonance and transition dipole moments influence ionization pathways and electron spectra.

Contribution

It provides new insights into the role of near-resonant states and transition dipole moments in calcium atom multiphoton ionization under different harmonic excitations.

Findings

Photoelectron spectra show substructure differences between harmonics.

Ionization mainly involves far off-resonant states for the third harmonic.

Transition dipole moments significantly affect ionization pathways.

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 ${4s4p}$ $^1P^o$ 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.