Probing Nonadiabatic Effects in Strong-Field Tunnel Ionization

We investigate experimentally the validity of proposed theories extending the tunneling approximation towards the multiphoton regime in strong-field ionization of helium. We employ elliptically polarized laser pulses and demonstrate how the influence of the ion potential on the released electron encoded in the measured observable provides the desired sensitivity to detect nonadiabatic effects in tunnel ionization. Our results show that for a large intensity range the proposed nonadiabatic theories contradict the experimental trends of the data, while adiabatic assumptions are confirmed.