Microscopy of photoionisation processes

A method is demonstrated which combines the ionisation of free molecules by a sharply focused femtosecond laser beam and projection microscopy in a divergent electric field. The electric field is produced in vacuum between a metallic tip and a flat positionsensitive charged particle detector. The method enables investigation of photoionisation processes in low-density gases with a subdiffraction spatial resolution and can be used as well in profile measurements for sharply focused, intense laser beams. In a demonstration experiment, a femtosecond laser beam with a peak intensity of ~10¹⁴ W cm^-2 was focused to a 40-μm-diameter waist in vacuum near a millimetre-size tip and ~2-μm spatial resolution was achieved. According to our estimates, the use of a sharper tip will ensure a submicron spatial resolution, which is a crucial condition for the spatial diagnostics of sharply focused short-wavelength VUV radiation and X-rays.