Research Papers in Physics and Astronomy


Date of this Version

October 1996


Published in Two-Center Effects in Ion–Atom Collisions: A Symposium in Honor of M. Eugene Rudd; Lincoln, NE May 1994, edited by Timothy J. Gay and Anthony F. Starace. AIP Conference Proceedings 362; American Institute of Physics, Woodbury, New York, 1996. Pages 19–28. Copyright © 1996 American Institute of Physics. Used by permission.


The saddle-point model of ionizing ion-atom collisions is based on the idea that when a charged projectile ionizes target electrons. some of the ejected electrons find themselves on the transient, moving saddle-point of Coulomb potential with a velocity that matches that of the saddle point. Feeling no force at this position, they "ride" the saddle out of the collision volume and are thus ionized. In the case of the prototypical H+ + H system, or in any proton–neutral target collision, the saddle point moves at half the projectile velocity, so these electrons are often referred to as "v/2" electrons. This model has been the topic of numerous papers, and has proven to be useful for the insights it provides into the problem of collisional ionization. It has also been applied successfully in the analysis of scaling laws for prediction of total ionization cross sections and specific phenomena such as the Barkas effect. But the model, its implications and interpretation, and some of the experimental evidence cited to support it have been controversial, and no consensus or comprehensive picture yet exists about the nature of saddle-point effects.

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