Charge-transfer band and sers mechanism for the pyridine-Ag system

Abstract

Transmission spectra in the ultraviolet and visible region as well as SERS spectra were measured for chemisorbed pyridine on vacuum evaporated (island) Ag films and on annealed (heated) Ag films. Strong and clear SERS spectra were observed with an enhancement factor of ~10 6 and were definitely assigned to the N-bonded pyridine. On adsorption of pyridine a new peak was detected at ~600 nm as the charge-transfer (CT) band for the pyridine-Ag interaction, which could neither be ascribed to any impurity nor to aggregated Ag particles. The surface morphologies of the Ag films observed by a SEM (highest resolution 1.5 nm) did not show any appreciable changes on adsorption of pyridine, when the Ag substrates were properly prepared. The molar extinction coefficient of the CT band was 10 3–10 4 ℓ mol −1 cm −1. On the annealed Ag films the CT band, which was well isolated from the Ag band, appeared on adsorption of pyridine and disappeared on desorption of pyridine, reversibly. The stronger the CT band, the stronger the SERS spectrum. The excitation profile of the Raman 1010 cm −1 band overlapped well with the CT band. The number of adsorbed pyridine molecules was estimated, from the π — π ∗ band of pyridine at 250 nm, to be (6–15) × 10 15 molecules cm 2 corresponding to a coverage factor of 20–60. The contributions of the electromagnetic (EM) effect and the CT effect to the apparent SERS enhancement factor were estimated to be approximately 10–10 2 and 10 3, respectively. The reason why the CT band had not been detected as an optical absorption band was discussed. The CT band observed is compared with those reported by EELS or reflection measurements.