A Theoretical Study of Time Delays in Above threshold ω-2ω Ionization

S. D. LÓPEZ; D. G. ARBÓ

París

Simposio; The 5th International Symposium on Intense Short Wavelenght Processes in Atoms and Molecules (ISWAMP); 2019

Comité Organizador ISWAMP

An experiment in atomic ionization by two-color lasers by Zipp et al.1 has revealed that a pump-probe scheme can be used to characterize time delays in the emission of electrons in the above-threshold ionization regime for visible frequency of the pump and its first harmonic as a probe. To shed more light about the ionization processes of the mentioned experiment, in this work we perform a theoretical analysis of the time delays in Ar ionization by a twocolorlaser [Ti:Sapphire laser (800 nm) together with the first harmonic (400 nm)] for a typical ω−2ω configuration 2. We perform simulations with the time dependent Schödinger equation and compare these results with the strong-field and Coulomb-Volkov approximations. We find that the strong assumption of additive time delays adopted in streaking or RABBITT techniques needs to be revisited when the atomic system is subject to ω−2ω fields. We show that time delays depend on the definition used to extract them from electron momentum distributions by considering the asymmetry in a particular direction of the electron emission or by integrating in hemispheres, and also considering a single energy at the multiphoton peaks or integrating over energy around the multiphoton peaks. Besides, we also find a large discrepancy between the results predicted by the strong-field approximation and the numerical solutions of the time dependent Schrödinger equation even at the highest simulated energies. With the help of the Coulomb-Volkov approximation, we see that the contribution of the long-range potential effects is almost negligible at intermediate to high emission energies with respect to the strong field approximation. We have also compared with ab initio simulations using short-range Yukawa potentials.