In situ measurement of three-dimensional ion densities in focused femtosecond pulses

We image spatial distributions of Xe$^{q+}$ ions in the focus of a laser beam of ultrashort, intense pulses in all three dimensions, with a resolution of $\sim$3 $\mu$m and $\sim$12 $\mu$m in the two transverse directions. This allows for studying ionization processes without spatially averaging ion yields. Our \emph{in situ} ion imaging is also useful to analyze focal intensity profiles and to investigate the transverse modal purity of tightly focused beams of complex light. As an example, the intensity profile of a Hermite-Gaussian beam mode HG$_{1,0}$ recorded with ions is found to be in good agreement with optical images.