Keldysh parameter, photoionization adiabaticity, and the tunneling time

Ultrafast photoionization dynamics within a driver field cycle is shown to be the key to understanding the multifaceted role of the Keldysh parameter. as the borderline between multiphoton and tunneling ionization, ionization adiabaticity degree, and a measure for the electron tunneling time. We demonstrate that, when applied to subcycle ionization dynamics, the gamma << 1 condition automatically translates into a criterion of both adiabaticity and photoionization via tunneling. We also show that the ratio of the Keldysh gamma to the frequency of the driver field indeed defines an important time scale of photoionization. However, instead of connecting to the electron motion beneath the potential barrier, it relates to the time needed for an electron to acquire a pondermotive energy equal to the ionization potential. This time scale is shown to manifest itself in the experimentally measurable buildup time of the photoelectron yield.