A comparison between inelastic electron tunnelling and infrared reflection—absorption spectroscopy for vibrational surface analysis

Abstract

Inelastic electron tunnelling spectroscopy (IETS) and infrared reflection-absorption spectroscopy (IRRAS) have been applied to study several aromatic and non-aromatic carboxylic acids chemisorbed on oxidized aluminium. Various functional groups substituted to the carboxylic acids have been examined to determine the intrinsic differences in sensitivity between IETS and IRRAS spectroscopy. Generally speaking, CH vibrations are stronger in IETS, whereas vibrations involving C and O atoms are stronger in IRRAS. The aromatic CC stretching vibrations are of equal relative intensity in both IETS and IRRAS spectra. IRRAS spectra compare well with bulk transmission IR spectra of related compounds, but IETS spectra differ very substantially from their IR counterparts. IRRAS is rather insensitive to modes at low frequency. Both techniques provide submonolayer sensitivity. However, the very severe restrictions on the metal substrates to be used, the complicated sample preparation, and the need of a liquid-helium cooled dewar make the use of IETS less attractive. Moreover, IRRAS obeys a stringent surface-selection rule, which makes the technique sensitive to the orientation of molecular vibrations at the surface. Unfortunately, the surface-selection rule for IETS appears to be less stringent, so that IETS cannot readily be used to probe the orientation of molecular moieties.