SERS and infrared reflection‐absorption spectroscopy of NO on cold‐deposited Cu

Abstract

Surface enhanced Raman spectroscopy (SERS) of NO on cold-deposited Cu yields only bands of the dissociation products of NO, but not the stretching bands of NO. In contrast, infrared reflection-absorption spectroscopy (IRRAS) on similarly prepared samples displays strong NO stretching bands and only a very weak band of N 2 O from multilayer NO. The differences can be explained neither by thermal or photodesorption nor by photochemical effects in the laser focus. Whereas the SERS results may be explained by the action of special sites, it is still unclear why the IRRAS signal from the dissociation products of NO at the 'catalytically active sites' is below the noise level.

Electronic Mechanisms of SERS

“…In these cases, IRRAS detects neither [66] the dinitrogen species as decay products of N 2 O nor the activated anionic CO − 2 species [65]. The observations that IRRAS does not detect the dinitrogen species from the NO dissociative reactions (see Fig.…”

“…The observations that IRRAS does not detect the dinitrogen species from the NO dissociative reactions (see Fig. 14), but delivers a clear and pronounced spectrum, when a cold-deposited Cu film is exposed directly to molecular nitrogen [66] gives an important hint for the further explanations. All observations can be explained by the assumption that the protocatalytic reactions take place at special adsorption sites that cannot be monitored by IRRAS.…”

“…However, it seems that the larger part of NO is not dissociated, because a good IRRAS spectrum of dimerized NO was obtained [63], see Fig. 14.…”

“…In this sense, at transition metal surfaces, there should exist a genuine first-layer effect of SERS, albeit weaker than at the coinage metals, but not restricted to SERS-active sites. Figure 13 shows the SERS spectra of 5 different Cu films, deposited at 30 K, annealed at the indicated temperatures, recooled to 30 K and exposed to 5 L NO (nitric oxide) [63]. The 1284 cm −1 and 2228 cm −1 bands are assigned to the N-O and N-N stretch frequency of N 2 O (nitrous oxide) (this molecule has the configuration NNO); the 585 cm −1 structure on [64].…”

“…14. The small signal of N 2 O in the IRRAS spectra occurs only after the formation of the second layer of NO [63]. Thermal desorption of NO and photoreaction or photodesorption in the laser focus can be eliminated as reasons of the missing NO signal in SERS [63].…”