Determination of electron-atom interaction from photo- and collisional-detachment experiments

P. A. MACRI; R. O. BARRACHINA

San Petersburgo

Conferencia; 21 International Conference on Spectral Line Shapes; 2012

The energy dependence of cross sections near the threshold of opening reaction channels has been of great interest for years. A complete description of the two-body threshold behavior is widely believed to be governed by a law proposed by Wigner more than 60 years ago. In practice, however, it is found that the Wigner law often describes observations over a very small range of a few meV above threshold. This shortcoming is of great theoretical and practical interest since particles interacting in the vicinity of a threshold provide the essential link between the continuum and spectral properties of atomic and molecular systems. In photodetachment of negative ions, for example, the knowledge of the threshold behavior is critical in the determination of electron affinities. At a first sight, it might appear that only the data closest to threshold are useful. However, these data have usually the worst signal-to-noise ratio. The proper procedure is to extrapolate measurements down to the threshold, thereby making use of all the experimental information. As one can expect, the extended threshold law depends sensitively on the long-range interaction of the two-body fragments. In this sense, a threshold law in a broader energy range can be regarded as a useful tool to extract different interaction properties as scattering length, potential range, number of supported bound states, etc. In this work we analyze how the Wigner law has to be modified at threshold by the presence of long range polarization potentials as appears in electron-atom/molecule collisions. We also consider the range of validity of this generalized threshold law. In particular, we analyze the threshold structures in detached electron spectra arising from fast collisions of Li- with gas targets [1] and from photo-detachment of Li-[2]. Using our generalization of the Wigner threshold law, we were able to fit the experimental data to a high degree of accuracy and obtain compatible electron-Li interaction parameters for both experiments. Moreover, we employ these parameters to obtain a reliable estimation of electron-Li excitation cross sections as compared with benchmark numerical computations [3].