Strong field photoionization: Analysis of overlapping ATI and LAPE structures

The interaction of strong and short laser pulses with atoms and molecules has received renewed attention, mainly because the advances in laser technology made possible new experimental investigations of atomic and molecular processes on an ultrashort time-scale and under ultra-intense laser radiation.

When the atom (or molecule) is exposed to an intense laser field, ionization occurs through the process called above-threshold ionization (ATI) where the atom absorbs more than the energetically required number of photons. Furthermore, when an extreme ultraviolet (XUV) pulse and an infrared (IR) laser field overlap in space and time with matter, the so-called laser-assisted photoionization emission (LAPE) processes take place. Depending on the XUV pulse duration, two different scenarios arise: the sideband and the streaking regime. The photoelecton momentum distribution (PMD) can be recorded for different delays between the pulses, so the photoionization dynamics become accessible with attosecond resolution.

An interesting aspect of LAPE ionization processes is that, with the usual choice of IR and XUV laser parameters, the energy domains of XUV and IR-induced ionization are well separated: while ATI structures due to the IR laser extend from the energy threshold up to 2Up (twice the ponderomotive energy), the XUV term is approximately centered at the XUV frequency. Then, by selecting the XUV frequencies, the two domains do not overlap [1]. However, in the general situation of LAPE process, both contributions could appear superimposed.

In this work we present a theoretical study within the strong field approximation to analyze the IR and XUV-IR interference terms, i.e., the interference between ATi and LAPE structures in the photoelectron spectrum.

[1] R Della Picca et al, Phys Rev A 102, 043106 (2020)