Theoretical study of two photon ionization of atomic argon in pump-probe spectrometry.

In an experimental poster presented in parallel to this theoretical study, an exploration of two-photon ionization using pump-probe spectroscopy in atomic argon exhibits oscillations with the time delay of the probe. These experimental observations show two relevant features. First, there is a significant phase difference between the signal corresponding to the ionization leaving the core on each spin-orbit split state. Second, the angular distribution of the electrons has a higher degree of asymmetry than what a two-photon process would suggest. We propose a theoretical model based on multichannel quantum defect theory that describes this process. We used MQDT to incorporate the full complexity of the bound states involved in the process in the calculation. Since the process is outside the limits of perturbation theory, we perform time propagation of the quantum state on a Hilbert space composed of the most relevant states of the atom based on the bandwidth of the two pulses. By exploiting the linearity of the Schrödinger equation, we managed to obtain an efficient way of generating the observed spectrograms achieving a high degree of match between the predicted signal and the experimental observation. We replicate the relative yield of ionization between each ionic state and the phase difference between the two peaks. To describe the high degree of asymmetry we included the atomic states required to sustain four-photon processes. These processes seem to be strongly driven due to the strong coupling of nearly resonant transitions, but the theory underestimates the effects, yielding a lower degree of asymmetry. Further study is required to determine the driving force behind this feature.