A recoil ion momentum spectrometer for probing ionization, e-capture, and capture-ionization induced molecular fragmentation dynamics.

The development of a recoil ion momentum spectrometer (RIMS) along with a post-collision projectile charge state analyzer (CSA) and its performance for carrying out studies of molecular fragmentation following direct ionization, electron-capture, and capture-ionization have been demonstrated here. This is a two-stage Wiley-McLaren type spectrometer with slight modification introduced by adding a lens to achieve higher momentum resolution as well as larger angular acceptance. Along with the time and position sensitive detector, it can measure all the three momentum components of singly charged recoil ions of energy up to 10 eV emitted in all directions. The CSA assembly is designed for separating out any neutral or singly or doubly charged post-collision projectiles typically of keV energy. The RIMS with initial trigger ("start") from CSA or an ionized electron can uniquely determine the dynamics of molecular fragmentation following different electron-capture or direct ionization events, respectively. To check the performance of the setup, we carried out an experimental study of the fragmentation of N molecules under the impact of 250 keV protons. Apart from the single-electron-capture channel, we could clearly identify three more capture-ionization channels, which lead to fragmentation. The essential features of the momentum distributions and the kinetic energy release distributions of all three fragmentation channels are discussed in detail. These results are compared with the findings from the ionization induced fragmentation experiments and with the available results from theoretical calculations as well as high resolution experiments. The branching ratios of these fragmentation channels are determined.