Efficiency of Anti-Stokes Fluorescence in Dyes

Abstract

ACCORDING to Kautsky and his collaborators¹, the majority of the molecules of dyes investigated by them, among which were also the molecules of fluoresceine, show an ability to phosphoresce when ‘energetically isolated’, for example, when adsorbed by convenient adsorbents. We can assume therefore that in such molecules there must be at least one metastable energy level M (Fig. 1), situated lower than the level F reached immediately after absorption. From the state F the molecules can pass either to a normal state N, emitting the band F—N (fluorescence), or to the metastable state M. The probability of the transition M—N is very small. Therefore when the temperature is sufficiently high, a great majority of molecules will be raised thermally from the level M to F and will be able to emit the band F—N (phosphorescence at room temperature). At low temperatures, direct transitions M—N take place. These transitions are accompanied by the emission of a phosphorescence band which is displaced towards the red relatively to band F—N; the duration of phosphorescence increases greatly (phosphorescence at low temperatures).