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

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.