title: Electron impact ionization of sodium Rydberg atoms below 2 eV.
abstract: We have observed electron impact ionization of highly excited sodium Rydberg atoms in ns and nd states, n=35-51, below E=2 eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from sigma(35d) approximately 7 x 10(-10) to sigma(50d) approximately 4 x 10(-9) cm(2). The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and sigma(n) follows a power law in n. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.
authors.affiliation: Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506-0055, USA.
authors.forename: K
authors.initials: K
authors.lastname: Nagesha
authors.forename: K B
authors.initials: KB
authors.lastname: MacAdam