X-ray and UV photoelectron-spectroscopic studies of pyridine adsorbed on evaporated nickel and palladium in the temperature range 140–385 k

The states of pyridine adsorbed on evaporated nickel and palladium films have been investigated as a function of temperature in the range 140–385 K by means of X-ray and UV photoelectron spectroscopy. At ～ 140 K, pyridine “N-bonded” on the metal surfaces gives C 1s and N 1s peaks whose binding energies are very close to those for condensed pyridine and “N-bonded” pyridine on pre-oxidized nickel. The high-lying valence orbitals, 2b₁ (π) and 1a₂ (π) + 7a₁ (n), of pyridine show shifts similar to those for the “N-bonded” molecule on pre-oxidized nickel. At ～ 290 K, “π-bonded” pyridine shows large shifts in the C 1s and N 1s peaks and in the high-lying valence orbitals, as observed for “π-bonded” benzene on nickel. The assignments of the adsorbed states are supported by work-function change data. A large proportion of pyridine converts from the “N-bonded” to the “π-bonded” form between 220 and 290 K. Formation of “α-pyridyl” is suggested at ～ 375 K on nickel.