Using density functional theory, we have studied the interactions between pyridine (Py) and coinage metals
(silver, copper, and gold) as well as the transition metal platinum. We present here a detailed analysis of the
influence of chemical enhancement effects on the SERS signals. We analyze the differential Raman scattering
cross sections of the a1 vibrational modes related to Py. The results show that the relative Raman intensities
of SERS spectra depend strongly on the binding interaction between Py and the SERS active centers, the
electronic property of metal materials, and the incident wavelengths. When the bonding between Py and a
SERS site is very weak, analogous to physical adsorption, the Raman spectra of the adsorbed Py are similar
to that of free Py. For Py interacting strongly with copper, gold, and platinum clusters, we find that the
Raman intensities of the v
1, v
6a, v
9a, and v
8a modes of Py are enhanced, whereas the intensity of the v
12 mode
decreases. To check the enhancement effect of the charge-transfer mechanism, we calculate the preresonance
Raman spectra. For Py interacting with silver, copper, and gold clusters, the low-lying charge transfer states
are formed from the valence shell s orbital of metal to the unoccupied π*-type orbitals (b1 and a2) of the Py
ring; whereas for Py adsorbed on platinum clusters, the low-lying charge transfer states are formed due to the
transitions from some d orbitals of the metal clusters to the two unoccupied π*-type orbitals of Py. The
results show that the Raman spectral property depends strongly on the property of these excited states and
the electronic structures of the metal materials.
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