Surface-enhanced Raman spectroscopy of benzene and benzene-d6 adsorbed on silver

Abstract

The surface‐enhanced Raman spectrum of benzene and benzene‐d₆ adsorbed on a silver film vapor‐deposited onto a low‐temperature substrate is reported. Many lines which correlate with Raman‐inactive and even infrared and Raman‐silent modes of the free molecule are observed with high intensity in the spectrum. The local symmetry which would cause those particular, otherwise inactive, vibrations to appear is C_3v(σ_d), which corresponds to a benzene molecule lying flat atop an equilateral triangle of silver atoms so that the normals to three alternate edges of benzene point to the centers of the three atoms. An alternative explanation for the activity of these normally silent modes is presented which does not depend on the adsorption site symmetry but which postulates instead that an unusally steep electric field gradient exists near the surface of the metal which causes modes spanned by the same representations which span the elements of a third rank tensor A to become active. These, together with the vibrations which are normally Raman‐active in benzene, are the same vibrations which are Raman‐active in C_3v(σ_d). An argument in support of the latter explanation is presented based upon the different response to excitation wavelength of the intensities of those modes which are active as a result of the polarizability tensor, α, as contrasted with those which become active because of A. An unequivocal choice between the two models cannot be made, however, on the basis of the data available.