Coverage-dependent electronic absorption spectrum of pyrene on Al2O3(11 0)

The electronic absorption spectrum of pyrene adsorbed on Al2O3(1120) was measured at 21 K as a function of coverage. The absorption maximum of the S0→S2 transition shifted from λ = 337 nm at Θ = 0.13 ML to λ = 349 nm at Θ⩾ 3.0 ML. The frequency shifts versus coverage were in agreement with dispersion interactions between the pyrene admolecules. The Bayliss relation was used to obtain the refractive index in the pyrene adlayer from the frequency shift. The refractive index increased from n=1.32 at Θ=0.13 ML to n=1.77 at Θ⩾3 ML; its coverage dependence was also determined using the Lorentz-Lorenz equation after calculating the local density within a sphere on the surface. The sphere radius (8 Å) obtained from fitting these calculations to the observations was consistent with dispersion interactions between pyrene admolecules.