Fluorescence quenching of the phenanthrene excimer on Al2O3(0001): Coverage and distance dependence

The fluorescence from disordered phenanthrene adlayers on Al2O3(0001) was examined in ultrahigh vacuum at 20 K using laser-induced fluorescence techniques. The fluorescence spectra was consistent with emission from a phenanthrene excimer. The fluorescence lifetime of the phenanthrene excimer was studied versus phenanthrene coverage and versus distance from the Al2O3(0001) surface using xenon spacers. As a function of coverage on Al2O3(0001), the fluorescence lifetime decreased from τ=34±0.5 ns at phenanthrene coverages of Θ≥20 ML to τ=7±0.5 ns at Θ=1 ML. As a function of xenon spacer distance from the Al2O3(0001) surface, the fluorescence lifetime also decreased from τ=34±0.5 ns at distances of d>100 Å to τ=7±0.5 ns at d=7 Å. Fluorescence measurements versus phenanthrene coverage on xenon, butane, acetonitrile and methanol multilayers revealed that the phenanthrene excimer fluorescence lifetime was constant at τ≊35 ns on the molecular multilayer surfaces. These results indicated that the reduction of the fluorescence lifetime was particular to the Al2O3(0001) surface. The fluorescence lifetimes versus phenanthrene coverage and xenon spacer distance on Al2O3(0001) were equivalent when the phenanthrene coverage was converted to total adlayer thickness. This correspondence suggested that the excited electronic energy in disordered phenanthrene adlayers transfers rapidly to phenanthrene excimers at the phenanthrene–vacuum interface. Subsequently, competition occurs between fluorescence quenching by the Al2O3(0001) surface and phenanthrene excimer fluorescence. In addition, fluorescence lifetime measurements vs phenanthrene coverage on CaF2 thin films displayed similar fluorescence quenching. Fluorescence lifetimes versus phenanthrene coverage on O2 molecular multilayers also revealed fluorescence quenching that was attributed to a charge-transfer mechanism. The observed fluorescence quenching on Al2O3(0001) indicates that the surface states of this ionic crystal may be accessible for electronic energy transfer even though Al2O3 is a known insulator. A Förster electronic energy transfer mechanism was used to analyze the observed fluorescence lifetimes vs phenanthrene coverage or xenon spacer distance on Al2O3(0001).