This paper presents a combined experimental and theoretical study of the hyper-Raman spectrum of pyridine adsorbed onto roughened silver electrodes. The surface enhanced hyper-Raman spectra (SEHRS) were measured using a focused cw mode-locked Nd:YAG laser with a peak power density of approximately 10 7 W /cm 2. Dominant bands in the pyridine spectra are the same (totally symmetric) bands as have been seen in the corresponding Raman (SERS) spectrum, although the relative intensities are different. To interpret these spectra, we present a semiempirical molecular orbital method for determining excitation energies, polarizability derivatives, and hyperpolarizability derivatives that is based on the 1T-electron Pariser-Parr-Pople (PPP) method. An empirical molecular force field is used to derive vibrational information, and the accuracy of the spectra is assessed by comparison with normal Raman spectra for liquid pyridine and with SERS spectra. The resulting SEHRS spectra are in good agreement with the measured spectra, particularly with respect to the intensity changes in the dominant lines in going from SERS to SEHRS. In addition, the theoretical/experimental comparisons indicate that SEHRS is more sensitive to adsorbate orientation than is SERS since the nontotally symmetric modes are predicted to be comparable in SEHRS (but not SERS) intensity to the totally symmetric modes for orientations other than perpendicular. Most important, a comparison of theoretical and experimental SEHRS/SERS ratios suggests that the enhancement factor associated with SEHRS is on the order of 10\3 which is much larger than the 10 6 enhancement seen for SERS.

Ionization energies below 20 eV of 10 molecules calculated with electron propagator techniques employing Hartree-Fock orbitals and multiconfigurational self-consistent field orbitals are compared. Diagonal and nondiagonal self-energy approximations are used in the perturbative formalism. Three diagonal methods based on second-and third-order self-energy terms, all known as the outer valence Green's function, are discussed. A procedure for selecting the most reliable of these three versions for a given calculation is tested. Results with a polarized, triple < basis produce root mean square errors with respect to experiment of approximately 0.3 eV. Use of the selection procedure has a slight influence on the quality of the results. A related, nondiagonal method, known as ADC(3), performs infinite-order summations on several types of self-energy contributions, is complete through third-order, and produces similar accuracy. These results are compared to ionization energies calculated with the multiconfigurational spin-tensor electron propagator method. Complete active space wave functions or close approximations constitute the reference states. Simple field operators and transfer operators pertaining to the active space define the operator manifold. With the same basis sets, these methods produce ionization energies with accuracy that is comparable to that of the perturbative techniques.

In this work the effect of aggregation and oxidation on the optical absorption of eumelanin oligomeric sheets is investigated by applying quantum mechanics and atomistic simulation studies to a simplified eumelanin structural model that includes 1-3 sheets of hexameric oligomer sheets. The oligomeric hypothesis is supported by AFM characterizations of synthetic eumelanins, formed by auto-oxidation or electrochemical oxidation of dihydroxyindole (DHI). Comparison of calculated absorption spectra to experimental spectra demonstrates a red shift in absorption with oxidation and stacking of the eumelanin and validates the theoretical results.

Spectroscopic simulations of a leading structural model for melanin, the pigment responsible for coloration and photoprotection in humans and animals, were done. We performed density functional theory (DFT) calculations using both the local density approximation (LDA) and the generalized gradient approximation (GGA) on a recent structural model for eumelanin based on higher oligomers of the monomer of neutral 5,6-indolequinone and its reduced forms, semiquinone and hydroquinone. This paper reports on our semiempirical spectroscopic simulations for the monomer and dimer energy-minimized structures. Our second study (part II) extends this approach to higher oligomers (tetramers through hexamers) and points out that the known optical spectra of eumelanins can be adequately explained on this basis.

Spectroscopic simulations of a leading structural model for melanin, which is the pigment responsible for coloration and photo protection in humans and animals, were conducted. In direct continuation of an earlier study on possible monomer and dimer subunits of eumelanin, we have performed density functional theory (DFT) calculations on a recent structural model for eumelanin based on higher oligomers of neutral 5,6-indolequinone. This paper further reports on our semiempirical spectroscopic simulations for the higher oligomer (tetramers through hexamers) energy-minimized structures. A linear combination of the oligomeric spectra reproduces several features of the experimental spectrum. This result strongly supports the assumption that melanin is constituted of substructures such as those considered in this study.