Momentum imaging spectrometer for molecular fragmentation dynamics induced by pulsed electron beam.

A momentum imaging spectrometer has been built for studying the electron impact molecular fragmentation dynamics. The setup consists of a pulsed electron gun and a time of flight system as well as a two-dimensional time and position sensitive multi-hit detector. The charged fragments with kinetic energy up to 10 eV can be detected in 4π solid angles and their three-dimensional momentum vectors can be reconstructed. The apparatus is tested by electron impact ionization of Ar and dissociative ionization of CO2. By analyzing the ion-ion coincidence spectra, the complete and incomplete Coulomb fragmentation channels for CO2(2+) and CO2(3+) are identified. The kinetic energy release (KER) and angular correlation for the two-body breakup channel CO2(2+*) → O(+) + CO(+) are reported. The peak value of total KER is found to be 6.8 eV which is consistent with the previous photoion-photoion coincidence studies, and the correlation angle of O(+) and CO(+) is also explicitly determined to be 172.5°.