The Role of Projectile Coherence in the Few-Body Dynamics of Simple Atomic Systems

The reaction dynamics in simple atomic systems have been studied extensively in order to address the fundamentally important, but yet unsolved few-body problem. For the description of electron impact-induced reactions theory has made remarkable progress over the last decade. However, describing the reaction dynamics for the same processes induced by ion impact proved to be much more challenging. Surprising discrepancies between experiment and theory were found even for cases which were considered to be "easy" for theory. Only a few years ago an important step toward a resolution of this puzzle was made when strong experimental indications were reported that measured cross sections can sensitively depend on the projectile coherence properties, while theory usually assumes that the projectile beam is always coherent. Owing to the much larger de Broglie wavelength electron projectiles tend to be much more coherent relative to the target dimension, which results in much better agreement between experiment and theory. In this chapter, recent studies on the effect of the projectile coherence properties on the reaction dynamics are reviewed.