Resonant Vibrational Coupling Research Articles

Surface-Enhanced Infrared Absorption Spectroscopy by Resonant Vibrational Coupling with Plasmonic Metal Oxide Nanocrystals.

Coupling between plasmonic resonances and molecular vibrations in nanocrystals (NCs) offers a promising approach for detecting molecules at low concentrations and discerning their chemical identities. Metallic NC superlattices can enhance vibrational signals under far-field detection by generating a myriad of intensified electric field hot spots between the NCs. Yet, their effectiveness is limited by the fixed electron concentration dictated by the metal composition and inefficient hot spot creation due to the large mode volume. Doped metal oxide NCs, such as tin-doped indium oxide (ITO), could overcome these limitations by enabling broad tunability of resonance frequencies in the mid-infrared range through independent variation of size and doping concentration. This study investigates the potential of close-packed ITO NC monolayers for surface-enhanced infrared absorption by quantifying trends in the coupling between their plasmon modes and various molecular vibrations. We show that maximum vibrational signal intensity occurs in monolayers composed of larger, more highly doped NCs, where the plasmon resonance peak lies at higher frequency than the molecular vibration. Using finite element and mutual polarization methods, we establish that near-field enhancement is stronger on the low-frequency side of the plasmon resonance and for more strongly coupled plasmonic NCs, thus rationalizing the design rules we experimentally uncovered. Our results can guide the development of optimal metal oxide NC-based superstructures for sensing target molecules or modifying their chemical properties through vibrational coupling.

Molecular relaxation effects on vibrational water vapor photoacoustic spectroscopy in air

Photoacoustic spectroscopy is a highly sensitive technique, well suited for and used in applications targeting the accurate measurement of water vapor in a wide range of concentrations. This work demonstrates the nonlinear photoacoustic response obtained for water vapor in air at typical atmospheric concentration levels, which is a result of the resonant vibrational coupling of water and oxygen. Relevant processes in the relaxation path of water in a mixture with air, excited with near-infrared radiation, are identified and a physical model for the acoustic signal measured with a resonant photoacoustic cell is presented. The model is valid for modulation frequencies typical for conventional and quartz-enhanced photoacoustic spectroscopy and provides a simplified means of calibration for photoacoustic water vapor sensors. Estimated values for comprised model coefficients are evaluated from photoacoustic measurements of water vapor in synthetic air. Furthermore, it is shown experimentally that the process of vibrational excitation of nitrogen is of negligible importance in the relaxation path of water vapor and thus insignificant in the photoacoustic heat production in atmospheric measurement environments.

Merged and separate band profiles arising from resonantly coupled vibrational modes of liquid mixtures: theoretical study

AbstractVibrational band profiles of equimolar liquid mixtures in the presence of intermolecular resonant vibrational coupling according to the transition dipole coupling (TDC) mechanism are studied by performing calculations on a model liquid system. It is shown that the bands of the two species in the mixture merge into a single band as the extent of resonance (closeness of the wavenumber positions as compared with the magnitude of the coupling) between the vibrations of the two species becomes greater and the bandwidth becomes larger. In the separate‐band case, intensity redistribution is seen in the isotropic Raman spectra because of the intermolecular vibrational coupling. The merged/separate behavior of the band profiles observed for isotopic mixtures of liquid formamide is reasonably reproduced by the calculations on the model liquid with the values of parameters in realistic ranges. Ab initio molecular orbital (MO) calculations on a linearly hydrogen‐bonded cluster of formamide are also carried out to support the discussion. Copyright © 2008 John Wiley & Sons, Ltd.

The Raman non-coincidence effect of the 12C [dbnd6]O stretching mode of liquid acetone in chemical and in isotopic mixtures

The concentration dependences of the first moments of both the isotropic and anisotropic Raman bands of the C O stretching mode ν( 12C O) of acetone in chemical mixtures of type [(CH 3) 2CO] x –[CCl 4] 1− x are compared with their dependences in isotopic mixtures of type [(CH 3) 2 12CO] x –[(CH 3) 2 13CO] 1− x . The role of the intermolecular resonant vibrational coupling is discussed via the experimentally observable Raman non-coincidence effect, i.e. the difference in the frequency position between the anisotropic and isotropic first moments. The experimental results are in good agreement with (1) molecular dynamics simulations and Monte Carlo simulations and (2) analytical predictions within the framework of the mean-spherical approximation of the dipole–dipole interaction. With the help of [(CH 3) 2 13CO] occurring at natural abundance in the chemical mixture and of [(CH 3) 2 13CO] at close to infinite dilution in the isotopic mixture, we are able to separate the environmentally induced frequency shifts from the measured anisotropic and isotropic first moments, thereby isolating the frequency shifts induced just by the intermolecular resonant vibrational coupling.

Classical and quantum mechanical infrared echoes from resonantly coupled molecular vibrations

The nonlinear response function associated with the infrared vibrational echo is calculated for a quantum mechanical model of resonantly coupled, anharmonic oscillators at zero temperature. The classical mechanical response function is determined from the quantum response function by setting variant Planck's over 2pi-->0, permitting the comparison of the effects of resonant vibrational coupling among an arbitrary number of anharmonic oscillators on quantum and classical vibrational echoes. The quantum response function displays a time dependence that reflects both anharmonicity and resonant coupling, while the classical response function depends on anharmonicity only through a time-independent amplitude, and shows a time dependence controlled only by the resonant coupling. In addition, the classical response function grows without bound in time, a phenomenon associated with the nonlinearity of classical mechanics, and absent in quantum mechanics. This unbounded growth was previously identified in the response function for a system without resonant vibrational energy transfer, and is observed to persist in the presence of resonant coupling among vibrations. Quantitative agreement between classical and quantum response functions is limited to a time scale of duration inversely proportional to the anharmonicity.

Adsorbate aggregation and relaxation of low-frequency vibrations

We present a study of resonant vibrational coupling between adsorbates and an elastic substrate at low macroscopic coverages. In the first part of the paper we consider the situation in which adsorbates form aggregates with high local coverage. Based upon our previously published theory, we derive formulas describing the damping rate of adsorbate vibrations for two cases of such aggregation: (i) adsorbates attached to step edges and (ii) adsorbates forming two-dimensional islands. We have shown that damping is governed by local coverage. Particularly, for a wide range of resonant frequencies, the damping rate of adsorbates forming well-separated islands is described by the damping rate formula for a periodic overlayer with the coverage equal to the local coverage in the island. The second part of the paper is devoted to facilitating the evaluation of damping rates for a disordered overlayer. The formula describing the damping rate involves the parameter β, which is related to the local density of phonon states at the substrate surface and does not allow a closed-form representation. For substrates of isotropic and cubic symmetries, we have developed a good analytical approximation to this parameter. For a vast majority of cubic substrates the difference between the analytical approximation and numerical calculation does not exceed 4%.

Collective Motion and Structural Order in Adsorbate Vibrational Dynamics

A simple, broadly applicable theory is developed to describe resonant vibrational coupling between adsorbates and a substrate lattice. This is one of the principal mechanisms governing the relaxation of adsorbate vibrations. This theory can be applied to widely varying surface coverages and arbitrary overlayer structures, and it correctly incorporates collective adsorbate motion, which has been shown to have a critical impact on the relaxation dynamics. Vibrational lifetimes predicted by this theory are in excellent quantitative agreement with experiments on adsorbate systems ranging from a diffuse, disordered overlayer to a dense, periodic overlayer. {copyright} {ital 1998} {ital The American Physical Society }

Effects of Intermolecular Hydrogen-Bonding Interactions on the Amide I Mode of N-Methylacetamide: Matrix-Isolation Infrared Studies and ab Initio Molecular Orbital Calculations

Effects of intermolecular hydrogen-bonding interactions on the amide I mode of N-methylacetamide (NMA) are studied by matrix-isolation infrared (IR) spectroscopy and ab initio molecular orbital calculations. The wavenumbers of the amide I IR bands of NMA in Ar and N2 matrixes with various NMA/matrix gas mixing ratios are compared with those calculated for the monomer, dimers, and trimers of trans-NMA and the monomer and dimer of cis-NMA. The band at 1708 (1706) cm-1 in Ar (N2) matrixes is assigned to the amide I mode of the monomer of trans-NMA. The band observed at 1686 (1681) cm-1 in Ar (N2) matrixes with the NMA/matrix gas mixing ratio larger than 1/500 is assigned to the amide I band due to the dimers of trans-NMA. The bands observed at lower wavenumbers for samples with the NMA/matrix gas mixing ratio as large as 1/100 are assigned to the amide I bands of the trimers and larger clusters of trans-NMA. It is likely that the band observed at 1695 (1693) cm-1 in the Ar (N2) matrix arises from the dimer of cis-NMA, which is as stable as the trans-NMA dimers because of the formation of two hydrogen bonds in a cyclic form. Although there are two amide I modes in an NMA dimer and three in an NMA trimer, only one mode is strongly IR active in each species. The intrinsic amide I wavenumbers of individual peptide groups in NMA clusters, i.e., the amide I wavenumbers in the case where there is no resonant vibrational coupling between the peptide groups, are examined by calculating the amide I wavenumbers for the dimers and trimers whose constituent molecules other than the target molecule have the CO group(s) substituted with 13C and 18O. It is shown that the shifts of the amide I band to lower wavenumbers induced by hydrogen bonding to the CO group are 20−25 cm-1, while those induced by hydrogen bonding to the N−H group are 15−20 cm-1. These shifts are approximately in line with the changes in the CO bond lengths and are approximately additive if both the CO and N−H groups of a peptide group are hydrogen bonded.

Non-coincidence splitting of the 1505 cm−1 adenine base vibration is due to coupling to water via hydrogen bonding

We have observed a noncoincidence splitting of 1–2 cm−1 for the 1505 cm−1 adenine base vibration in poly(rA), poly(dA), 5rAMP, and 5dAMP. In double stranded polynucleotides, the noncoincidence splitting is smaller than 0.3 cm−1. Noncoincidence splitting is usually associated with a transition dipole–transition dipole coupling between identical vibrational modes on different molecules of the same species, but any angular dependent intermolecular coupling mechanism may cause this effect. It is argued that resonant vibrational coupling of 1505 cm−1 vibrations on different adenine bases cannot be the source of the noncoincidence splitting. As the 1505 cm−1 base vibration does not have a large infrared absorption, it cannot be caused by transition dipole–transition dipole coupling. We propose that a coupling via hydrogen bonding of the 1505 cm−1 adenine base vibration to water vibrations yields the angular dependent intermolecular coupling potential.

The effect of resonant vibrational coupling on the frequency of the ν 2 mode of liquid methyl iodide in Raman scattering and infrared absorption

The ν 2 mode of methyl iodide has been measured in isotropic and anisotropic Raman scattering, and in infrared absorption in the neat liquid and in isotopic dilution with the deuterated compound. The local field affecting the shape and position of the intense infrared profile has been taken into account by a continuum dielectric approximation. The isotopic dilution shifts observed indicate the action of resonant intermolecular vibrational coupling which presumably is caused by transition dipole interaction. The results are, in principle, in agreement with current theories. For a comparison, certain approximations are necessary (MSM, etc.), which seem to be responsible for the fact that quantitative agreement is not yet very good.