Desorption kinetics and excimer formation of pyrene on Al2O3(112̄0)

Laser-induced fluorescence (LIF) and electronic absorption spectroscopy (EAS) were used to examine pyrene adsorbed on Al2O3(112̄0) as a function of surface coverage, temperature, and adlayer order. Temperature-programmed LIF and EAS techniques were employed to measure the desorption kinetics of pyrene from Al2O3(112̄0). An activation energy of Ed=18±1 kcal/mol was obtained for pyrene desorption in the submonolayer regime assuming a first-order desorption pre-exponential of ν1=1×1013 s−1. Multilayer desorption studies revealed zero-order desorption kinetics with an activation energy of Ed =20±2 kcal/mol and a pre-exponential of ν0=3×1026±1.5 molecules/cm2 s. The formation of pyrene excimers and the structure of the pyrene adlayer were also investigated using LIF and EAS techniques. The LIF spectra for pyrene adlayers on Al2O3(112̄0) at 21 K exhibited both monomer and excimer emission. The relative amount of excimer emission at 21 K increased vs pyrene coverage from Θ=0.30 monolayers (ML) to Θ=4.5 ML. In addition, the excimer emission increased and the monomer emission decreased vs surface temperature between 21 and 150 K at all measured coverages. The temperature and coverage dependence of the LIF spectra suggested that excimer formation was dependent on pyrene mobility. In support of a mobility mechanism for excimer formation, LIF studies demonstrated that the excimer emission at Θ=0.25 ML was quenched by xenon adsorption on the pyrene adlayer. LIF and EAS measurements also indicated that the pyrene adlayer crystallized when the adlayer was annealed above 230 K. The LIF spectra of the crystalline pyrene adlayers on the Al2O3(112̄0) surface displayed only excimer emission.