This work presents a detailed analysis of enhanced Raman scattering of the pyridine-Ag(20) model system using time-dependent density functional theory. A consistent treatment of both the chemical and electromagnetic enhancements (EM) is achieved by employing a recently developed approach based on a short-time approximation for the Raman cross section. A strong dependence of the absolute and relative intensities on the binding site and excitation wavelength is found. The analysis of the Raman scattering cross sections shows the importance of different contributions to the enhancements, including static chemical enhancements (factor of 10), charge-transfer enhancements (10(3)), and EM enhancements (10(5)). The largest enhancement found (10(5)-10(6)) is due to the EM mechanism, with a small contribution from the chemical interaction. This suggests that the enhanced Raman scattering due to atomic clusters is comparable to findings on single nanoparticles. A combination of information about the vibrational motion and the local chemical environment provides a simple picture of why certain normal modes are enhanced more than others.

This critical review highlights recent advances in using electronic structure methods to study surface-enhanced Raman scattering. Examples showing how electronic structure methods, in particular time-dependent density functional theory, can be used to gain microscopic insights into the enhancement mechanism are presented (150 references).

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The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized using time-dependent density functional theory. This has been achieved with a systematical study of the chemical enhancement of meta- and para-substituted pyridines interacting with a small silver cluster (Ag(20)). Changing the functional groups on pyridine enabled us to modulate the direct chemical interactions between the pyridine ring and the metal cluster. Surprisingly, we find that the enhancement does not increase as more charge is transferred from the pyridine ring to the cluster. Instead, we find that the magnitude of chemical enhancement is governed to a large extent by the energy difference between the highest occupied energy level (HOMO) of the metal and the lowest unoccupied energy level (LUMO) of the molecule. The enhancement scales roughly as (omega(X)/omega(e))(4), where omega(e) is an average excitation energy between the HOMO of the metal and the LUMO of the molecule and omega(X) is the HOMO-LUMO gap of the free molecule. The trend was verified by considering substituted benzenethiols, small molecules, and silver clusters of varying sizes. The results imply that molecules that show significant stabilization of the HOMO-LUMO gaps (such as those that readily accept pi-backbonding) would be likely to have strong chemical enhancement. The findings presented here provide the framework for designing new molecules which exhibit high chemical enhancements. However, it remains a challenge to accurately describe the magnitude of the Raman enhancements using electronic structure methods, especially density functional theory, because they often underestimate the energy gap.

In this work, we present the first calculation of the resonance Raman scattering (RRS) spectrum of rhodamine 6G (R6G) which is a prototype molecule in surface-enhanced Raman scattering (SERS). The calculation is done using a recently developed time-dependent density functional theory (TDDFT) method, which uses a short-time approximation to evaluate the Raman scattering cross section. The normal Raman spectrum calculated with this method is in good agreement with experimental results. The calculated RRS spectrum shows qualitative agreement with SERS results at a wavelength that corresponds to excitation of the S(1) state, but there are significant differences with the measured RRS spectrum at wavelengths that correspond to excitation of the vibronic sideband of S(1). Although the agreement with the experiments is not perfect, the results provide insight into the RRS spectrum of R6G at wavelengths close to the absorption maximum where experiments are hindered due to strong fluorescence. The calculated resonance enhancements are found to be on the order of 10(5). This indicates that a surface enhancement factor of about 10(10) would be required in SERS in order to achieve single-molecule detection of R6G.