Projection microscopy of photoionization processes in gases

We demonstrate a method combining laser ionization of molecules with projection technique and allowing observation of photoionization processes in gases with sub-focal spatial micro-resolution. A bunch of molecular ions created by the nonlinear photoionization of the imaging gas near a tip extends in a divergent electrostatic field producing a magnifying image on the detector. It can be used to observe the profile of the sharply-focused intense laser beam in a wide spectral range. In proof of principle experiment the water molecules are ionized in ~40-μm laser focal spot in the vicinity of the silver needle with a curvature radius 0.5?mm and the resultant ions are counted by a position-sensitive scheme. According to our estimations, ~1.5-μm spatial resolution has been reached. Using a sharp tip, the spatial resolution can be improved to the sub-micrometer scale and such approach can be applied for short wavelength beam diagnostics.