The dependence of the metal—molecule vibrational frequency on the mass of the adsorbate and its relevance to the role of adatoms in surface-enhanced raman scattering

It has been suggested by Lombardi, Shields Knight and Birke that one can distinguish between an adsorbed molecule bonded “directly” to a metal surface as opposed to one bonded to a metal adatom which is itself bound to a metal surface by plotting the adsorbate—adsorbent vibrational frequency for a series of cognate molecules, alternatively against m_ads^?1/2 and (1/m_ads + 1/m_metal)^1/2. (m_ads and m_metal are, respectively the mass of the adsorbate and a metal atom.) If a proportional relationship is obtained in the former case then the molecule is bonded to the bulk metal, if in the latter case then it is bonded to an adatom. We show using two model calculations that that proposal is incorrect. Both proportional and non-proportional dependences may be obtained for either mode of bonding.