Characterization of Fermi Resonances in Adsorbate Vibrational Spectra Using Cluster Calculations: Methoxy Adsorption on Al(111) and Cu(111)

摘要

The optimized geometries and computed vibrational frequencies for CH_3O adsorbed at fcc and hcp threefold sites were determined for the Al(111) and Cu(111) surfaces using cluster calculations. The frequencies of the first overtone for the three CH bending modes are found to be very close to the three fundamental CH stretching frequencies, suggesting that a Fermi resonance between the two types of modes might take place. The presence of the multiple peaks arising from Fermi resonances often complicates the assignment of adsorption structures on surfaces. To provide a more precise evaluation of whether a Fermi resonance will actually occur, we illustrate a simple computational method for obtaining numerical estimates to the energy third derivatives most responsible for coupling between the fundamental and the overtone modes.