Focal overlap gating in velocity map imaging to achieve high signal-to-noise ratio in photo-ion pump-probe experiments

Abstract

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. Velocity Map Imaging (VMI)1,2 is a technique for imaging charged particles which is now widely used in atomic, molecular and chemical physics experiments to study a variety of different processes in gas phase systems. It has become a standard technique to measure ultrafast processes in these systems on attosecond and femtosecond time scales using vacuum-ultraviolet (VUV) and extreme-ultraviolet (XUV) light from high-order harmonic generation (HHG) and Free Electron Laser (FEL) sources 3-7 , and to study X-ray driven processes from synchrotron sources 8 . VMI can provide angle and energy resolved photo-ion and photo-electron spectra in a simple, economical setup without the requirement for extensive post acquisition analysis except an inversion procedure that can be achieved in a few different ways 9-12 . Since the time it was first introduced, VMI has evolved and has been modified and improved, for example, to obtain three dimensional velocity distributions using time-slicing 13-15 , to image ions and electrons in coincidence 16,17 for a complete reconstruction of a photon-molecule interaction event, etc.Here, we present a new technique which involves a simple modification of the input laser beam geometry in a standard VMI which results in significant enhancement of signal-to-noise ratio in time resolved pump-probe measurements. In typical pump-probe experiments using VMI, the pump and probe laser beams are focused on a gas target at the center of the VMI setup where they are temporally and spatially overlapped. The laser beams propagate along a line parallel to the VMI electrodes and the charged particles born along this line are a) Electronic mail: nhshivaram@lbl.gov imaged by the VMI spectrometer. Since these particles are born at the same distance from the repeller electrode, their time-of-flight to the detector for a given mass is the same irrespective of distance along the laser propagation direction. In our method, we make a simple modification of this standard beam geometry by introducing a tilt in the laser beams propagating through the VMI setup (see Fig. 1). This is effectively a 'passive streaking' of the particles which creates a variation in time-of-fl...

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