Observation and Analysis of Small Inclination of Thymine Molecules on Graphite

The orientation of vacuum-sublimated thymine molecules on graphite was studied by ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). MAES gave pronounced band intensity enhancement for π molecular orbitals, which confirms the nearly lying-down orientation of the molecules proposed by Uchihashi et al. (Appl. Surf. Sci. 2000, 157, 244). In UPS, a lowering of the vacuum level was observed upon monolayer formation and attributed to inclination of the molecular dipoles from the surface. The tilt angle of the molecular plane was estimated to be 2.1−4.4° by applying the Helmholtz equation to the vacuum level shift.