Observation of NMR of the Formation of Localized Electronic States in an Ionic Liquid Alloy

${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ NMR results are reported for liquid Cs-Au alloys ranging from pure Cs to 18% excess Au in the ionic liquid compound CsAu. As one approaches CsAu from the Cs-rich side, a rapid drop in the Knight shift is observed, accompanied by a sharp peak in the nuclear relaxation rate at about 5% excess Cs in CsAu. Analysis of the static and dynamic hyperfine fields shows that the conduction electrons abruptly localize, probably as $F$- center analogs, when the concentration of excess Cs falls below about 7%.