Ronald L. Birke

Charge‐transfer theory of surface enhanced Raman spectroscopy: Herzberg–Teller contributions

A comprehensive development of the charge‐transfer theory of surface enhanced Raman scattering (SERS) is presented. We incorporate the Herzberg–Teller mixing of zero‐order Born–Oppenheimer electronic states by means of vibronic interaction terms in the Hamiltonian. This is similar to the theory of Tang and Albrecht12 except that we include metal states as part of a molecule–metal system. When this is done we may no longer discard a term involving mixing of ground‐state vibrations. The theory is comprehensive in that both molecule‐to‐metal and metal‐to‐molecule transfer is considered. Furthermore, both Franck–Condon and Herzberg–Teller contributions to the intensity are obtained. The former, however, contribute only to the intensity of totally symmetric vibrations, while the latter contribute to nontotally symmetric vibrations as well. Since overtones are observed in SERS only weakly if at all, the Herzberg–Teller terms are most consistent with experimental findings. The resulting formulas may be interpreted as a type of resonance Raman effect in which intensity for the charge transfer transitions is borrowed from an allowed molecular transition. We may also carry out the sum over metal states. This procedure predicts a logarithmic resonance at the Fermi level of the metal. We thus predict intensity vs voltage profiles such as I ∝ ‖ln(ωFI−ω+iΓ)‖2 which fits the experimental curves quite well.

A Unified View of Surface-Enhanced Raman Scattering

In the late 1970s, signal intensity in Raman spectroscopy was found to be enormously enhanced, by a factor of 10(6) and more recently by as much as 10(14), when an analyte was placed in the vicinity of a metal nanoparticle (particularly Ag). The underlying source of this huge increase in signal in surface-enhanced Raman scattering (SERS) spectroscopy has since been characterized by considerable controversy. Three possible contributions to the enhancement factor have been identified: (i) the surface plasmon resonance in the metal nanoparticle, (ii) a charge-transfer resonance involving transfer of electrons between the molecule and the conduction band of the metal, and (iii) resonances within the molecule itself. These three components are often treated as independently contributing to the overall effect, with the implication that by properly choosing the experimental parameters, one or more can be ignored. Although varying experimental conditions can influence the relative degree to which each resonance influences the total enhancement, higher enhancements can often be obtained by combining two or more resonances. Each resonance has a somewhat different effect on the appearance of the resulting Raman spectrum, and it is necessary to invoke one or more of these resonances to completely describe a particular experiment. However, it is impossible to completely describe all observations of the SERS phenomenon without consideration of all three of these contributions. Furthermore, the relative enhancements of individual spectral lines, and therefore the appearance of the spectrum, depend crucially on the exact extent to which each resonance makes a contribution. In this Account, by examining breakdowns in the Born-Oppenheimer approximation, we have used Herzberg-Teller coupling to derive a single expression for SERS, which includes contributions from all three resonances. Moreover, we show that these three types of resonances are intimately linked by Herzberg-Teller vibronic coupling terms and cannot be considered separately. We also examine the differences between SERS and normal Raman spectra. Because of the various resonant contributions, SERS spectra vary with excitation wavelength considerably more than normal Raman spectra. The relative contributions of totally symmetric and non-totally symmetric lines are also quite different; these differences are due to several effects. The orientation of the molecule with respect to the surface and the inclusion of the metal Fermi level in the list of contributors to the accessible states of the molecule-metal system have a strong influence on the observed changes in the Raman spectrum.

Cardiolipin binds nonyl acridine orange by aggregating the dye at exposed hydrophobic domains on bilayer surfaces.

10‐N‐Nonyl acridine orange (NAO) has been used at low concentrations as a fluorescent indicator for cardiolipin (CL) in membranes and bilayers. The mechanism of its selective fluorescence in the presence of CL, and not any other phospholipids, is not understood. The dye might recognize CL by its high pK (pK 2>8.5). To investigate that, we established that NAO does not exhibit a pK in a pH range between 2.3 and 10.0. A second explanation is that the dye aggregates at hydrophobic domains on bilayers exposed by the CL. We found that a similar spectral shift occurs in the absence of CL in a concentrated solution of the dye in methanol and in the solid state. A model is proposed in which the nonyl group inserts in the bilayer at the hydrophobic surface generated by the presence of four chains on the phospholipid.

The effect of molecular structure on voltage induced shifts of charge transfer excitation in surface enhanced Raman scattering

The Raman intensity versus electrode voltage behavior of a series of substituted pyridines and saturated nitrogen heterocycUc compounds was studied at two laser excitation energies. The voltage (resonance) maximum shifts to more positive potential with increasing excitation energy for the substituted pyridines (case I) and to more negative potential with increasing excitation energy for the saturated nitrogen heterocyclic compounds (case II). The voltage maxima can be correlated with the Hammett sigma function for substituted pyridines and with pK a for the saturated nitrogen heterocycles. The results are consistent with charge transfer from the metal to molecule (adcluster-molecule complex) in the case I system and from the molecule (adcluster-molecule complex) to the metal in the case II system.

Time-dependent picture of the charge-transfer contributions to surface enhanced Raman spectroscopy

We reexamine the Herzberg-Teller theory of charge-transfer contributions to the theory of surface enhanced Raman scattering (SERS). In previous work, the Kramers-Heisenberg-Dirac framework was utilized to explain many of the observed features in SERS. However, recent experimental and theoretical developments suggest that we revise the theory to take advantage of the time-dependent picture of Raman scattering. Results are obtained for molecular adsorption on nanoparticles in both the strong confinement limit and the weak confinement limit. We show that the Herzberg-Teller contributions to the charge-transfer effect in SERS display a resonance at the molecule-to-metal or metal-to-molecule transition while retaining the selection rules associated with normal Raman spectroscopy (i.e., harmonic oscillator, as opposed to Franck-Condon overlaps). The charge-transfer contribution to the enhancement factor scales as Gamma(-4), where Gamma is the homogeneous linewidth of the charge-transfer transition, and thus is extremely sensitive to the magnitude of this parameter. We show that the Herzberg-Teller coupling term may be associated with the polaron-coupling constant of the surface phonon-electron interaction. A time-dependent expression for the Raman amplitude is developed, and we discuss the implications of these results for both metal and semiconductor nanoparticle surfaces.

The theory of surface-enhanced Raman scattering

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited.

Surface enhanced raman spectrum of pyrazine. Observation of forbidden lines at the electrode surface

Abstract The surface enhanced Raman spectrum of pyrazine is obtained. Several new lines are observed including 440,800,1317, and 1488 cm −1 which may be assigned to B 2u , B 2u , B 3u and B 1u species respectively. These indicate a lowering of the molecular symmetry from the free molecule D 2h most likely to C 2v .

A charge-transfer surface enhanced Raman scattering model from time-dependent density functional theory calculations on a Ag10-pyridine complex.

Vibrational frequency calculations were made for a Ag(10)-pyridine vertex complex with density functional theory (DFT) for static simulated spectra and with time-dependent DFT (TD-DFT) for preresonance and resonance simulated spectra using both B3LYP/LANL2DZ and BP86/TZP methodologies. In addition, 40 excited states of the complex were calculated and assigned symmetry based on a C(2v) symmetry of the optimized complex found with B3LYP/LANL2DZ. Molecular orbital isosurfaces show that the excited states involve both Ag(10) intercluster excitations and charge-transfer (CT) excitations between the Ag nanocluster and the pyridine molecule. An excitation around 500 nm involving CT from the Ag cluster to pyridine was found in both calculations. For free pyridine, the relative average deviations between unscaled calculated and experimental results were 1.5 cm(-1) for BP86 and 3.1 cm(-1) for the B3LYP calculations. For the complex, simulated spectra at a variety of excitation wavelengths were calculated. In the case of 514 nm excitation, the simulated Raman cross section from the TD-DFT calculations (near the CT resonance) was plotted versus Raman shift frequency and compared with an experimental surface enhanced Raman scattering (SERS) spectrum obtained on an oxidation-reduction cycle, ORC roughened Ag electrode. The BP86 TD-DFT calculation with finite damping term showed a better fit to experimental spectrum with respect to both relative intensities and frequencies. The average deviation of the unscaled BP86 calculations for 16 bands in the experimental spectrum was 13.0 cm(-1). The calculated spectrum in both cases shows many contributions from nontotally symmetric as well as totally symmetric modes, indicating the contribution of Herzberg-Teller (HT) scattering. The simulated intensities of the Raman modes of different symmetry from Ag(10)-pyridine can be correlated with HT intensity borrowing from excited states of given symmetry and decent oscillator strength. These results explain the appearance of the a(2) mode at 388 cm(-1) and the moderately strong b(2) mode at 1573 cm(-1) found in the SERS experimental spectrum. The good agreement between experimental and simulated results indicates that a small Ag nanocluster-pyridine vertex complex can be used to simulate spectra with good relative intensities, if not absolute intensities, and accurate frequencies for most of the SERS bands for adsorbed pyridine on roughened bulk Ag.

Evidence for a Ag adatom-molecule complex in surface enhanced raman scattering

Abstract The SERS spectrum of molecules adsorbed on Ag sols show, lines, in the 200–300 cm −1 region in the absence of Cl − ions These lines are attributed to a Ag-N complex and frequency calculations show that they correspond to the vibrations of the molecule bound to a single silver atom.

Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has been observed at solid/solution, solid/gas, solid/vacuum, and solid/solid interfaces, and it is possibly the most sensitive surface high-resolution vibrational spectrosopic technique available as an analytical probe. The great interest in SERS is evidenced by the virtual explosion in the number of publications dealing with this technique since 1977 from the physics, chemistry, and materials science research communities. One reason for this interest in SERS is the generality of the technique with regard to the nature of the material phase in contact with the solid surface. Another reason is that there are several mechanisms that contribute to SERS, and a considerable scientific challenge exists in determining the extent of the contributions of various mechanisms to the overall enhancement.