New Vibrational Numbering and Potential Energy Curve for the 33ΠgElectronic State of the Li2Molecule

Abstract

An experimental study of the 33Πgelectronic state of7Li2, using the Perturbation–Facilitated Optical–Optical Double Resonance (PFOODR) technique, was recently reported [A. Yiannopoulouet al., J. Chem. Phys.103, 5898, (1995)]. However, due to the very small number of known7Li2A1Σ+u∼b3Πuwindow levels, only 13 ro-vibrational levels (spanning a range of vibrational levels designated υx− 1 to υx+ 3 in that reference) could be observed. Dunham coefficients, based on the assignment υx= 7, were found to fit the observed term values and give a qualitative fit to the intensities of the first six lines of the 33Πg(υ = υx,N= 11) →b3Πuemission spectrum. However, due to the limited number of levels used in the fit, both the absolute vibrational numbering and the 33ΠgRKR potential curve obtained from the Dunham coefficients, must be considered to be uncertain. In the present work, we show that the previously reported 33ΠgRKR curve is unable to reproduce the experimental intensity distribution in the7Li233Πg(υx= 7,N= 11) →a3Σ+uemission continuum. We report new experimental data for the7Li233Πg(υx+ 1,N= 11) →a3Σ+ubound-free continuum and discrete 33Πg(υx± 1,N= 11) →b3Πuspectra obtained using the PFOODR experimental technique. We demonstrate that the correct vibrational numbering and an improved RKR potential curve can be obtained by analyzing the experimental term values in combination with all observed bound-free and discrete spectra. Finally, term values for four6Li233Πgro-vibrational levels were obtained using PFOODR spectroscopy. The measured isotope shifts confirm the absolute vibrational numbering obtained from the present analysis.