Spatially resolved measurement of ionization yields in the focus of an intense laser pulse

We introduce a novel technique for measuring spatially resolved photoionization yields of gas-phase ions created in an intense-laser focus. Overcoming the limitations of traditional experiments where the ionization yield is integrated over the entire focal volume, the technique provides precise information on the ionization dynamics over a wide range of intensities between the appearance intensity of the lowest charge state up to relativistic intensities. The new method provides insights into the ionization process beyond the saturation intensity and, at the same time, a precise way for noninvasive, in situ focus diagnostics. We demonstrate these advances for the case of strong-field ionization of argon. The data are analyzed using the Ammosov–Delone–Krainov (ADK) formula (Ammosov et al 1986 Zh. Éksp. Teor. Fiz. 91 2008).