Hydrogen collision energy resolution

Positrons exhibit resonance phenomena in collisions with some atomic and molecular targets and, as with electrons, an infinite series of resonances is expected to be associated with each degenerate excitation threshold (Mittleman, 1966). For electrons, such thresholds can only arise with hydrogenic targets, but for positrons there are also degenerate thresholds in the excitation of positronium. Several of these resonances have been identified theoretically for a few simple target systems, but they are too narrow to be observed experimentally with the presently available energy resolution of positron beams. 

The energy transfer described by (13.16) can be monitored if M is excited by a short infrared laser pulse and the fluorescence of AB is detected by a fast cooled infrared detector (Sect. 4.5) with sufficient time resolution. Such measurements have been performed in many laboratories [13.6]. For illustration, an experiment carried out by Green and Hancock [13.77] is explained by Fig. 13.16a a pulsed HF laser excites hydrogen fluoride molecules into the vibrational level u = 1. Collisions with other molecules AB (AB = CO, N2) transfer the energy to excited vibrational levels of AB. The infrared fluorescence emitted by AB and HF has to be separated by spectral filters. If two detectors are used, the decrease of the density A(HF ) of vibrationally excited HF molecules and the build-up and decay of A(AB ) can be monitored simultaneously. 

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