Laser-microwave ionisation mass spectroscopy of ion Rydberg states: Ba⁺spectrum

The authors present a novel instrument for the analysis of highly excited Rydberg states of ions. This set-up is applied to study the ns and nd Rydberg levels of Ba + produced in an atomic beam with values of n varying between 19 and 65. The selective excitation is provided by a two-UV-laser-photon process. The excited Ba + ions are ionised by a microwave field and detected by a double time of flight mass spectrometer. A UV optogalvanic spectrum, resulting from an iron hollow-cathode discharge, is used as a reference for calibration. The accuracy of the energy level wavenumber determination is +or-0.1 cm -1 . These measurements lead the authors to a revised value for the Ba II ionisation limit of 80 686.25+or-0.10 cm -1 .