Anderson Localization and Mobility Edges in Ruby

When the $R_1$ line in ruby is swept with a tunable laser from the center to the wings, the ratio of trap emission to $R_1$ emission remains constant up to a point in the line where it suddenly drops to a lower constant value out to the wings. We suggest that this break corresponds to mobility edges separating delocalized states in the central region from "localized" states beyond this break.