Calculations of resonant multiphoton population transfer in potassium atoms at long wavelengths

We study the interplay between resonant multiphoton population transfer and strong field ionization in potassium atoms, by calculating their response to intense, midinfrared (MIR) radiation. We use wavelengths close to the three or five photon resonance between the ground $4s$ state and the first excited $4p$ state, and find that the amount of population transferred to the excited state as a function of wavelength and intensity shows clear signatures of these resonances. The ionization yield also shows moderate resonant enhancement for intermediate intensities. We can further distinguish between population 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 pronounced sensitivity to the sign of the chirp whereas the total amount of ionization does not. From this we conclude that direct, nonresonant ionization of the ground state competes strongly with the resonant process.