Crystallization kinetics of benzophenone and naphthalene multilayers on Al2O3 (0001)

The crystallization of naphthalene and benzophenone on Al2O3 (0001) was stu died by monitoring the emission from optically pumped excited electronic st ates. Adlayers of these molecules were initially prepared in their amorphou s state by vacuum deposition onto the Al2O3 surface at 90 K;. Prepared in t his way, naphthalene exhibits excimeric emission, as characterized by the b road and featureless spectrum that peaks near 400 nm. The emission of the a morphous naphthalene is attributed to two excimeric forms: one that contrib utes about 70% of the intensity due to the multilayer, and the second that is observed when the adlayer is crystallized. The peak of this second excim er is red shifted by about 30 nm and is tentatively assigned to the monolay er. Both of these excimers have a non-radiative decay channel whose rate co nstant increases with temperature. Using the Arrhenius model, the energy of activation for the annealing process was determined to be 36 +/- 2 kJ mol( -1) When prepared at 90 K, amorphous benzophenone rapidly relaxes to a more sta ble form, as indicated by the decay in the emission to about 50% of the ini tial intensity. The spectrum of the amorphous state shows little change fro m that of the crystalline form, except that the latter is red shifted by ab out 8 nm. The intensity is also thermally quenched, decreasing by about 3% per degree rise in temperature. (C) 2000 Elsevier Science B.V. All rights r eserved.