Zero-Energy Resonance Effects in Break-up Collisions
We show that the analysis of cusp and threshold structures in breakup collisions of nuclei, atoms and molecules represents an alternative approach for the study of elastic two-particles collisions at extremely low-energies. We illustrate these general ide
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by Springer-Verlag 2001
Zero-Energy Resonance Effects In Break-up Collisions J. Fioll, C. Courbin 2 , V. D. Rodriguez 3 ,* and R. O. Barrachina1 ,* 1
2
3
Centro At6mico Bariloche and Instituto Balseiro t , 8400 S. C. de Bariloche, Rio Negro, Argentina. Laboratoire de Dynamique des Ions, Atomes et Molecules, Universite P et M Curie, 4 Place Jussieu, TI2-B75, F-75 252 Paris Cedex 05, France. Departamento de Fisica, FCEyN, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina.
Abstract. We show that the analysis of cusp and threshold structures in breakup collisions of nuclei, atoms and molecules represents an alternative approach for the study of elastic two-particles collisions at extremely low-energies. We illustrate these general ideas by showing that the "electron capture to the continuum" cusp in the ionization of atoms by neutral projectiles can be understood in terms of a resonance mechanism. This approach allows for the extraction of the s-wave scattering length for the electron-projectile interaction. 1
Introduction
The rapid development of Laser cooling and trapping techniques has opened new avenues in the physics of ultracold atomic and molecular collisions. At a very low energy range, only a few partial waves (typically only one) contribute to the corresponding cross sections. Therefore, they can be characterized by a small number of constants. The knowledge of these microscopic constants is important since, for instance, they dominate the macroscopic behaviour of BoseEinstein condensation. In particular, the sign of the s-wave scattering length ao is known to control the stability of the entire quantum gas. Even though these new experimental techniques represent the direct way to the study of these ultracold elastic collisions, there is an alternative approach that could yield some additional clues or, at least, provide a different point of view. Without the need of tuning the initial energy of a scattering process to any precise ultra-low value, two particles can move at an arbitrarily small relative velocity * Also member of the Consejo Nacional de Investigaciones Cientificas y Tecnicas, Argentina.
tComision Nacional de Energia Atomica and Universidad Nacional de Cuyo, Argentina.
47 in the final state of a break-up collision. A large number of scattering processes in nuclear, atomic and molecular physics can lead to a final state with three particles. This is the case - for instance - of a proton-deuteron collision at a center-of-mass energy of more than 2.22 MeV, the ionization of atoms by electron or ion impact, or the fragmentation of molecules. We suggest that coincidence measurement of two particles in the final state of these processes may provide an alternative method for extracting information on low-energy two body collisions. By employing this technique, Bader et al. [1] measured the s-wave scattering lengths ao for the collisions of electrons by neutral Helium atoms in the metastable 21 8 and 23 8 states. In particular, for the 21 8 state, ao ranges in the order of 300 atomic un
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