The Optics Laboratory

Group of Hans Hallen, North Carolina State University Physics Department

GFR in SERS?

GFR should be observable in a Raman experiment when the molecule studied is near a metal surface. Most of the Raman work (except NSOM) performed near metal surfaces has been concerned with SERS. We do not comment on SERS models here, but note which peaks not normally seen in Raman are expressed.

In particular, when the strong new (not normally Raman allowed) lines involve strong IR vibrations, it suggests that GFR is important. Surface effects, for the rough surfaces typically used for SERS, must also be included in the analysis of the selection rules. When the spectra is minimally impacted by the surface, indicated by the normally allowed Raman energies unperturbed from the bulk material, new active modes probably result from GFR or the quadrupole term rather than symmetry changes or bonding. The presence of GFR induced lines cannot be proved, since particular scenarios could cause these 'GFR preferred' lines to be strong even if another mechanism altered the selection rules. However, strong circumstantial evidence for GFR results from the presence of normally forbidden modes and their relative strength of compared to bulk IR strength, corrected for orientation with respect to the surface.

SERS of Benzene

Many normally Raman forbidden lines observed.

Substrate interaction weak (small shifts in Raman allowed lines).

Intensities of new lines follow those of gas or liquid phase IR spectroscopy: 3 of 5 out-of-plane modes selectively enhanced (other 2 not Raman or IR active).

• M. Moskovits and D.P. DiLella, J. Chem. Phys 73, 6068 (1980).

• P.A. Lund, R.R. Smardzewski and D.E. Tevault, Chem. Phys. Lett 89, 508 (1982).

•P.A. Lund, D.E. Tevault and R.R. Smardzewski, J. Phys. Chem 88, 1731 (1984).

• NIST Chemistry WebBook, http://webbook.nist.gov.

SERS of Pyrazine

Many normally Raman forbidden lines observed.

Substrate interaction weak on Ag surface (small shifts in Raman allowed lines).

The strong IR modes are usually strong.

Irreproducibility of these in-plane modes could result from strong GFR dependence on orientation (which has been shown to vary).

• Q.J. Huang, J.L. Yao, B.W. Mao, R.A. Gu and Z.Q. Tian, Chem. Phys. Lett 271, 101 (1997).

• Gayle R. Erdheim, Ranald L. Birke and John R. Lombardi, Chem. Phys. Lett 69, 495 (1980).

• Ralf Dornhaus, Marshall B. Long, Robert E. Benner and Richard K. Chang, Surface Science 93, 240 (1980).

SERS of Other Molecules

Ru(CN)₆^4- on Ag and Cu show strong lines from the strong IR vibrations near 550 cm^-1 (several CN's move ^ surface) and 2048 cm^-1.

C₆₀ on Ag and Au with weak surface interactions show modes in the spectra that are only IR active.

2-butene on silver has shown the presence of several out-of-plane modes, but not in-plane modes, when it lies on the surface.

• Craig S. Allen and Richard P. van Duyne, J. Am. Chem. Soc 103, 7497 (1981).

• Kelly L. Akers, Lisa M. Cousins and Martin Moskovits, Chem. Phys. Lett 190, 614 (1992).

• Yun Zhang, Yuhua Du, J.R. Shapley and M. J. Weaver, Chem. Phys. Lett 205, 508 (1993).

• Daniel P. DiLella and Martin Moskovits, J. Phys. Chem 85, 2042 (1981).

For a more detailed comparison to GFR, see our papers.

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