Metal electrode-chemisorbate bonding: General influence of surface bond polarization on field-dependent binding energetics and vibrational frequencies

An analysis is outlined in which the dependence of the binding energy, E-b, stretching frequency, nu (M-A), and equilibrium bond length, r(eq), of met al-adsorbate bonds on the external interfacial field (F), and hence surface potential of relevance to electrochemical systems, are described in terms of potential-energy surface and bond-polarization parameters. Density Funct ional Theory (DFT) calculations for finite metal clusters with monoatomic a dsorbates forming polar surface bonds are utilized both to examine metal-, adsorbate-, and field-dependent trends in the required parameters and to ex amine the applicability of the analytic relations in comparison with numeri cal calculations. To a very good approximation, the E-b-F dependence is des cribed by the field-dependent static dipole moment, mu (S), of the metal-ad sorbate bond. As expected, the field-induced changes in the potential-energ y surface (PES) are determined by the r-dependent mu (S) values, i.e., the dynamic dipole moment, mu (D). The factors determining the nu (M-A)-F depen dence, however, are distinctly more intricate, involving the PES anharmonic ity coupled with mu (D), as well as being influenced significantly by the m u (D)-r dependence in some cases. While both mu (S) and mu (D) are influenc ed importantly by surface bond polarity, the role of bond polarizability ap pears to be quite different, so that only a crude correlation between mu (S ) and mu (D) is evident. These differences further underscore the disparate nature of the E-b-F and nu (M-A)-F behavior. Nevertheless, an approximate inverse correlation between the nu (M-A)-F and r(eq)-F dependence is usuall y anticipated. The broad-based utility of DFT for assessing as well as pred icting the role of surface bond polarization in controlling the field-depen dent electrode-adsorbate PES is pointed out. (C) 2001 American Institute of Physics.