Near-Theshold Laser Spectroscopy of Rydberg and Autoionization States of Thallium Atom

Abstract

Near-threshold spectra of Rydberg and autoionization states of a thallium atom have been studied with the method of selective stepwise photoionization of atoms with laser radiation. During the investigation, within ionization continuum near the threshold a so-called “transparence window” with the width of 25 ± 2 сm^–1 was found, the center of which was 45 сm^–1 higher than the border of atom ionization (Е_i = 49266.71 cм^–1). The formation of this window is evidently connected with a sharp and deep asymmetry of the discovered earlier Beutler–Fano autoionization resonance having the configuration of 6s6p^{2 4}P_3/2, and placing 659 сm^–1 higher than the ionization border (Е_0.5 = 49925.70 см^–1). The autoionization resonance contour has such profile that Fano parameter (q) in wavelength continuum side turns close to zero. This happens probably due to the strong interaction with the continuum.