LETTER TO THE EDITOR

We calculate the 3Se and 1Se scattering lengths in low-energy e + Rb(5s), Cs(6s), and Fr(7s) collisions. Our approach is based on the relativistic version of the modied effective range theory (MERT) and extrapolates eigenphases provided by Dirac -matrix calculations to zero energy. To test the accuracy of our MERT extrapolation method, we compare the ne-structure components for the lowest 3Po resonance of Fr ion with full Dirac -matrix calculations. Low-energy electron scattering by highly polarizable systems can be described by the modied effective range theory (MERT) whose non-relativistic version has been developed by Fabrikant [1, 2]. This theory assumes that the electron wavelength is large compared to the radius of the short-range interaction between the electron and the atom. This is a much weaker restriction than the more common assumption [3] that the electron wavelength is large compared to the radius of the polarization interaction. The non-relativistic version of the MERT method was used to predict the lowest 3Po resonance states for Rb and Cs at a few meV above the detachment threshold [2, 4]. Their positions and widths are in good agreement wit