Reinvestigation of the (4, 20) band of the O2+ second negative system

High resolution absorption spectra of the (4, 20) band in the second negative system (A2Πu-X2Πg) of O2+ cation were measured and analyzed in the range of 11 900-12 300 cm-1 via optical heterodyne velocity modulation spectroscopy. Precise molecular constants of the levels involved were obtained by a nonlinear least-squares fitting procedure combining with our previous spectra of the (4, 19) and (6, 20) bands.