Near-threshold Photodetachment of Heavy Alkali-metal Atoms

I. I. Fabrikant¹, C. Bahrim², A. A. Khuskivadze¹, U. Thumm²

1) Department of Physics and Astronomy, University of Nebraska, Lincoln, NE, USA
2) Department of Physics, Kansas State University, Manhattan, KS, USA

Low-energy electron scattering by alkali-metal atoms is strongly affected by a ³P^o shape resonance. Its experimental observation is very difficult because of the limited energy resolution in electron scattering experiments. Photodetachment (PD) studies of corresponding negative ions might be advantageous in this regard. However, due to the dipole selection rules, only the J=1 component of the ³P^o_J (J = 0, 1, 2) triplet can be populated in PD experiments with a single photon. Moreover, since the ¹S - ³P transition is forbidden in the LS coupling scheme, the process indicative of the ³P^o resonance becomes very sensitive to the spin-orbit interaction, and the role of the theory in interpretation of experimental data becomes especially important.

In the present work we calculate photodetachment of Rb^-, Cs^-, and Fr^- using the Pauli equation method. The Pauli equation (PE) is a weak relativistic limit of the exact Dirac equation which includes the spin-dependent potential V_LS added to the non-relativistic, spin-independent Coulomb potential V. For a Coulomb potential, V_LS includes a non-physical singularity 1/r³ at r=0, and the PE-approach breaks down. Various regularization functions have been suggested to remove this singularity [1]. Based on the exact analytic solution of the Dirac equation near the nucleus, we formulated boundary conditions for solving the PE for an electron interacting with an atom [1]. By integrating the PE using an effective potential V_eff that is adjusted to reproduce scattering phase shifts provided by exact Dirac R-matrix calculations, we calculated total PD cross sections. Our ³P^o₁ resonance contribution to the PD cross section of Cs^- agrees (in position and width) with recent experiments [2], after fine-tuning V_eff. For Rb^- and Fr^- the resonance contribution is much smaller than for Cs. We therefore also calculate angle-differential cross sections and asymmetry parameters which are much more sensitive to the resonant contribution than the total cross section.

References:

[1] C. Bahrim, I. I. Fabrikant, and U. Thumm, Phys. Rev. Lett. 87, 123003 (2001), and refs. therein.
[2] M. Scheer et al., Phys. Rev. Lett. 80, 684 (1998).

This work was supported by the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

Submitted to the Fano Memorial Symposium, July 2002 in Cambridge, MA

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