Nonadiabatic Electron Transfer at Metal Surfaces

The present article treats the role of the density of electronic states ρF at the Fermi level of a metal in affecting the rate of nonadiabatic electron transfer. The rate constant kET is calculated for the electron transfer across an alkanethiol monolayer on platinum and on gold. The ρF of platinum is about 7.5 times that of gold, the difference being mainly due to the d band of Pt. In spite of this difference, the electron transfer rate constant kET calculated in the present paper increases only by a factor of about 1.8, instead of the factor of about 7.5 expected using ρF alone. Implications of these results for present and past experiments are described. Bands which are weakly coupled (e.g., the d-band of Pt in the present case) contribute much less to the rate constant than is suggested by their density of states ρF. Thereby, kET is approximately independent of ρF in two cases: (1) adiabatic electron transfer and (2) nonadiabatic electron transfer when the extra ρF is due to the d electrons. Experiments which can test the latter are discussed.