Calculations of resonant, multiphoton population transfer in potassium atoms at long wavelengths - art. no. 053411

We study the interplay between resonant multiphoton population transfer and strong held ionization in potassium atoms, by calculating their response-t o intense, midinfrared (MIR) radiation. We use wavelengths close to the thr ee or five photon resonance between the ground 4s state and the first excit ed 4p state,and find that the amount of population transferred to the excit ed state as a function of wavelength and intensity shows clear signatures o f these resonances. The ionization yield also shows moderate resonant enhan cement for intermediate intensities. We can further distinguish between pop ulation transfer to and ionization of the 4p state by using a chirped laser pulse to temporally separate the two processes. We find that the amount of population remaining in the 4p state at the end of the pulse shows a prono unced sensitivity to the sign of the chirp whereas the total amount of ioni zation does not. From this we conclude that direct, nonresonant ionization of the ground state competes strongly with the resonant process.