A velocity map imaging detector with an integrated gas injection system

We present the design of a velocity map imaging spectrometer where the target gas is injected from a capillary that is integrated in the repeller plate of the ion optics assembly that drives electrons/ions formed by ionization or dissociation to a two-dimensional detector. The geometry of this design allows the use of gas densities in the interaction region that are two to three orders of magnitude higher than the densities that are used in standard velocity map imaging spectrometers, making the detector suitable for working with weak light sources such as newly developed attosecond pulse sources, or (quasi-)cw sources such as synchrotrons. In a test where monoenergetic photoelectrons were generated by six-photon ionization of Xe (utilizing the second harmonic of a neodymium doped Nd:YAG), the kinetic energy resolution of the spectrometer was found to be Delta E/E=1.8%. This number was found to be in good agreement with Monte Carlo simulations.