Infrared reflection-absorption spectroscopy of adsorbed molecules

Infrared Reflection-Absorption Spectroscopy (IRAS) is used to perform vibrational spectroscopy of absorbed molecules in submonolayer quantities on well-defined, single crystal metal surfaces. Theoretical and experimental aspects of this method in a single reflection mode are discussed. Apart from its potential to identify adsorbed species through their vibrational spectra, the high sensitivity (< 5/1000 of a CO monolayer) and resolution (1–3 cm-1) make this method particularly suitable to study the interaction between metal substrate and adsorbed molecule as well as intermolecular interactions within an adsorbed layer of molecules. This is illustrated with examples of CO adsorption on metals, where vibrational frequencies of the C-O stretch allow determination of adsorption sites. Intermolecular interactions within the adsorbed layer, which lead to the formation of ordered islands and incommensurate compression structures, can be monitored by characteristic frequency shifts. Further insight into these interactions can be gained by observing the linewidth and intensity as a function of surface coverage and order. Finally examples of other adsorption systems are given to demonstrate the wide range of molecules which have been studied in the past and the potential of IRAS as a method to bridge studies of adsorption of molecules on metal single crystals at UHV pressures to supported catalysts at high, catalytically relevant pressures.