Disorder-order transition and energy transfer in phenanthrene adlayers on alumina (11.hivin.20)

Abstract

The disorder-order transition and energy transfer in phenanthrene adlayers on Al{sub 2}O{sub 3}(11{bar 2}0) at coverages from 10 to 125 monolayers were studied by using laser-induced fluorescence and electronic absorption spectroscopy. After adsorption at 90 K, the absorption spectra of phenanthrene adlayers displayed a broad absorption peak at 304 nm. The corresponding fluorescence spectra were broad and featureless with a maximum at 395 nm and a lifetime of 55 ns. The absorption spectra were consistent with a disordered, glasslike adlayer, and both the featureless fluorescence spectra and the relatively long 55-ns fluorescence decay times were consistent with excimer emission. The absorption spectra changed dramatically and resembled the crystalline absorption spectra when the adlayers were annealed to 200 K. After annealing above 200 K, the fluorescence spectra also revealed vibrational structure originating at 354 nm that was characteristic of the crystalline fluorescence spectra and a fluorescence lifetime of 18 ns. The sudden changes in the absorption and fluorescence spectra at 195-200 K were interpreted as a disorder-to-order transition in the phenanthrene adlayer on Al{sub 2}O{sub 3}(11{bar 2}0). During this transition, the phenanthrene adlayer evolves from a glasslike to a crystalline state. Polarized absorption spectra indicated that the crystalline phenanthrene adlayer wasmore » oriented with the ab plane of the crystal aligned parallel to the Al{sub 2}O{sub 3}(11{bar 2}0) surface. Energy-transfer studies revealed that excimer sites were very effective traps for excited electronic energy. Energy-transfer studies performed with anthracene impurities in the phenanthrene adlayer also demonstrated that electronic energy was immobilized at excimer traps in the disordered film and was very mobile in the ordered crystalline adlayer.« less