Role of conformation in the electronic properties of chemisorbed pyridine on Cu(110): An STM/STS study

Pyridine was chemisorbed on Cu(110) at 10 K and observed using STM at 5 K as dosed and after annealing to temperatures between 20 and 300 K. At very low coverage, two molecular species with different apparent heights are observed to coexist. The higher species is assigned to a pyridine molecule bonded with its symmetry axis perpendicular to the surface plane, while the lower species is assigned to a pyridine molecule that is tilted down toward the surface plane. At low coverage, the tilted pyridine species predominates on the surface, but as the total surface coverage of pyridine increases, the molecules stand up until the upright geometry becomes favored. Measurements of the STS of the two species show different molecular resonances derived from the lowest unoccupied pyridine pi* orbitals. The tilted pyridine species has a peak in the unoccupied local density of states at 2.6 +/- 0.1 eV, whereas the upright pyridine species has a peak at 2.3 +/- 0.1 eV.