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peak at 304 nm, and 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 the broad featureless fluorescence spectra were attributed to excimer emission. The relatively long 55-11s fluorescence decay times were also consistent with excimer emission.The absorption spectra changed abruptly and resembled the crystalline absorption spectra when the adlayer was annealed to 200 K. The main feature of this change was a dramatic reduction of the absorbance of the So -S2 electronic transition. After annealing above 195 K, the fluorescence spectra also revealed vibrational structure characteristic of the crystalline fluorescence spectra originating at 354 nm.The sudden changes in the absorption and fluorescence spectra at 200 K were interpreted as a disorder-to-order transition in the phenanthrene adlayer on A1203( 1120). During this transition, the phenanthrene film evolves from a glasslike to a crystalline state. Polarized absorption spectra indicated that this crystalline phenanthrene adlayer was oriented with the ab plane of the phenanthrene crystal aligned parallel to the AI,O,( 1 120) surface.Energy-transfer studies revealed that excimer defect sites in the adlayer were very effective traps for excited electronic energy. Energy-transfer studies performed with anthracene impurities in the phenanthrene adlayer demonstrated that electronic energy was immobilized at excimer traps in the disordered film. In contrast, the electronic energy was observed to be mobile in the ordered crystalline adlayer. Acknowledgment.

The homonuclear decoupled proton NMR spectrum of benzene associatively chemisorbed on a 5.0% wt/wt Pt/A1203 catalyst is reported. The structure of the resonance is attributable to heteronuclear dipolar or pseudodipolar couplings to platinum-195 nuclei in the surface of the particle. Comparison of simulated spectra for benzene molecules in various sites coupled by the heteronuclear dipolar interaction to an array of platinum atoms indicates that only benzene in the "on top" geometry is consistent with the observed spectrum. If the coupling is attributed solely to direct dipolar couplings, the predicted Pt-benzene distance is 1.56 f 0.02 A. Considering both dipolar and pseudodipolar couplings leads to the conclusion that this value is a minimum for the Pt-benzene distance.