  We demonstrate a new technique in velocity map imaging (VMI) that allows
spatial gating of the laser focal overlap region in time resolved pump-probe
experiments. This significantly enhances signal-to-noise ratio by eliminating
background signal arising outside the region of spatial overlap of pump and
probe beams. This enhancement is achieved by tilting the laser beams with
respect to the surface of the VMI electrodes which creates a gradient in flight
time for particles born at different points along the beam. By suitably pulsing
our microchannel plate detector, we can select particles born only where the
laser beams overlap. This spatial gating in velocity map imaging can benefit
nearly all photoion pump-probe VMI experiments especially when
extreme-ultraviolet (XUV) light or X-rays are involved which produce large
background signals on their own.
