Design of Velocity Map Imaging Spectrometer Equipped with a Mass Gate Discriminating Particular Photofragments

Abstract

A photoionization spectrometer for velocity map imaging has been developed for measuring the scattering distribution of fragment ions from polyatomic molecules. The spectrometer contains a mass gate and an ion reflector which are able to discriminate ions with a particular mass-to-charge ratio m/z . The basic functions and feasibility of these devices were tested experimentally and theoretically. First, the photoions from Kr and C 60 were extracted into a time-of-flight (TOF) mass spectrometer by a transient or continuous electrostatic field. When the pulse application on the mass gate was tuned to the arrival timing of ions with a specific m/z , the peak of the selected ions alone was present on a TOF spectrum. Second, compatibility between velocity map imaging and ion discrimination was investigated by the computer simulations of the ion trajectories of photofragments from C 60 . A pulsed voltage was applied to the mass gate synchronously with the arrival timing of C 5 8 + ions. The initial three-dimensional velocity distribution of C 5 8 + was projected onto the image plane with an energy resolution better than 10 meV. The C 5 8 + image was free from the contamination of other ions such as C 6 0 + and C 5 6 + .