Contribution of the Herzberg-Teller mechanism to the surface-enhanced Raman scattering of iron phthalocyanine adsorbed on a silver electrode

The SERS (surface-enhanced Raman scattering) and SERRS (surface-enhanced resonant Raman scattering) spectra of iron(II) phthalocyanine (FePc) adsorbed onto a silver electrode have been investigated. The electrochemical SERRS has shown that the reduction of FePc molecule is reversible at negative potentials below -1.0 V vs Ag/AgCl. The SERS spectra obtained by excitation off resonance contain mainly a(1g) and a(2g) vibrational modes. The excitation profiles of these modes show a maximum of SERS intensity that shifts to more positive values as the laser energy is decreased. We discuss the results of the at, modes as being enhanced by a potential modulated adsorbate to metal charge-transfer mechanism, in which two electronic states of the MPc are coupled by a Herzberg-Teller term. In this case the acceptor state is localized on the Ag-SERS active site and the MPc coupling electronic excited states are of A(2u) and A(1u) symmetry, resulting in the enhancement of a(2g) vibrational modes.