“…Since then numerous experiments have been performed probing several valence, ion-pair, 11,17,[25][26][27][28][29][30][31][32][33][34][35] and Rydberg 31,36,37 states via numerous techniques such as visible absorption spectroscopy, emission spectroscopy, laser-induced fluorescence spectroscopy, vacuum ultraviolet spectroscopy, and micro-and millimeter-wave spectroscopy.…”