Abstract
Over the past several years, another type of spectrometer has been developed that proves to be superior to conventional spectrometers. In this “momentum imaging” spectrometer, electrons and target-recoil ions produced in ionizing collisions are accelerated to opposing position-sensitive detectors by an external electric field. The momentum imaging spectrometer essentially projects (or images) the initial three-dimensional (3D) electron and recoil-ion momentum vectors onto the 2D plane of each corresponding detector. Because the spread in electron arrival time is quite small in comparison with the spread in recoil-ion arrival time, one can utilize the electron signal as a timing marker to extract the full 3D momentum vector of the recoiling ion. This technique has proven to be quite successful in cold-target recoil-ion momentum spectroscopy. Momentum imaging methods have also been recently utilized in the search for evidence of saddle-point electron emission. Experimental studies of and incident on He were carried out by Abdallah et al. [Phys. Rev. A 56, 2000 (1997)]. Rather surprisingly, their results exhibited projectile-charge dependent shifts in the opposite direction than that implied by the saddle-point mechanism. However, as we shall demonstrate, proper saddle-point shifts may be observed if one takes into account the time of flight of the electron.
- Received 9 February 1999
DOI:https://doi.org/10.1103/PhysRevA.60.1135
©1999 American Physical Society