Interpreting electron-momentum distributions and nonadiabaticity in strong-field ionization

Abstract

We investigate whether nonadiabatic effects, rather than an initial longitudinal momentum spread, can explain the additional final momentum spread measured in strong-field ionization experiments with ultrafast laser pulses. We find that, when used consistently, a well-known nonadiabatic theory which includes an initial velocity offset yields results similar to adiabatic theory. By "consistent use" we mean that nonadiabatic theory is used also for field strength calibration of the experiment. The additional momentum spread can be accounted for by including an initial longitudinal momentum spread, as was done previously in the adiabatic case. Interestingly, when the experimental intensity is calibrated using a common in situ calibration method based on adiabatic assumptions, the nonadiabatic theory improves upon the adiabatic theory. This result highlights the uncertainty associated with using theory-based calibration methods, which are the most common way of calibrating experimental data in attosecond science.

Exit point in the strong field ionization process

“…This separation of the tunnelling ionization process in the quantum-mechanical part, describing the ionization event proper, and the classical part, describing subsequent motion, has been extremely fruitful, as it allows us to consider processes occurring in the strong laser field for systems that are too complex to allow an ab initio quantum mechanical (QM) treatment. It has been demonstrated1718 that for small values of the Keldysh parameter, deep in the tunnelling regime, the results obtained using TIPIS agree very well quantitatively with the results of the Perelomov-Popov-Terentiev (PPT) theory13, which considers all stages of the electron motion fully quantum-mechanically. Despite this success, the concept of the electron exit point remains somewhat elusive.…”

Non-adiabatic imprints on the electron wave packet in strong field ionization with circular polarization

“…The single-active electron approximation was employed, which proved to be very accurate for similar problems (see e.g., [26,31]). Helium was described by the pseudopotential…”

“…Therefore, there has been considerable recent interest in the significance of non-adiabatic effects in strong field ionization, with some experiments finding them insignificant in typical experimental regimes [23,24], while others arriving at the opposite conclusion [25]. A serious source of uncertainty in all these experiments is the calibration of intensity, which is normally achieved using a theoretical model [26,27]. In particular, the in situ field strength has to be reconstructed a posteriori from the same experimental data that one wants to study.…”

How to understand the tunneling in attosecond experiment?

“…Furthermore, in the immediate region γ K ∼ 1, which is narrow [87] (chap 1, p. 2), there is still no clear picture, where both the MPI and tunneling can take place [32,100] and investigations show that non-adiabatic effects affect the tunneling, i.e. differently from the adiabatic approximation, where the electron sees approximately a static electric field during the ionization process [47,[101][102][103][104].…”