Fundamental Molecular Vibrations Research Articles

Fourier-transform infrared spectroscopy with undetected photons from high-gain spontaneous parametric down-conversion

Fourier-transform infrared spectroscopy (FTIR) is an indispensable analytical method that allows label-free identification of substances via fundamental molecular vibrations. However, traditional FTIR spectrometers require mid-infrared (MIR) elements, including low-efficiency MIR photodetectors. SU(1,1) interferometry has previously enabled FTIR with undetected MIR photons via spontaneous parametric down-conversion in the low-parametric-gain regime, where the number of photons per mode is much less than one and sensitive photodetectors are needed. In this work, we develop a high-parametric-gain SU(1,1) interferometer for MIR-range FTIR with undetected photons. Using our method, we demonstrate three major advantages: a high photon number at the interferometer output, a considerably lower photon number at the sample, and improved interference contrast. In addition, we broaden the spectral range of the interferometer by aperiodic poling in the gain medium. Exploiting the broadband SU(1,1) interferometer, we measure and evaluate the MIR absorption spectra of polymers in the 3-μm region.

Bacterial DNA Recognition by SERS Active Plasma-Coupled Nanogold.

It is shown that surface-enhanced Raman spectroscopy (SERS) can identify bacteria based on their genomic DNA composition, acting as a "sample-distinguishing marker". Successful spectral differentiation of bacterial species was accomplished with nanogold aggregates synthesized through single-step plasma reduction of the ionic gold-containing vapored precursor. A high enhancement factor (EF = 107) in truncated coupled plasmonic particulates allowed SERS-probing at nanogram sample quantities. Simulations confirmed the occurrence of the strongest electric field confinement within nanometric gaps between gold dimers/chains from where the molecular fingerprints of bacterial DNA fragments gained photon scattering enhancement. The most prominent Raman modes linked to fundamental base-pair molecular vibrations were deconvoluted and used to proceed with nitrogenous base content estimation. The genomic composition (percentage of guanine-cytosine and adenine-thymine) was successfully validated by third-generation sequencing using nanopore technology, further proving that the SERS technique can be employed to swiftly specify bioentities by the discriminative principal-component statistical approach.

Broadband complementary vibrational spectroscopy with cascaded intra-pulse difference frequency generation.

One of the essential goals of molecular spectroscopy is to measure all fundamental molecular vibrations simultaneously. To this end, one needs to measure broadband infrared (IR) absorption and Raman scattering spectra, which provide complementary vibrational information. A recently demonstrated technique called complementary vibrational spectroscopy (CVS) enables simultaneous measurements of IR and Raman spectra with a single device based on a single laser source. However, the spectral coverage was limited to ∼1000cm-1, which partially covers the spectral regions of the fundamental vibrations. In this work, we demonstrate a simple method to expand the spectral bandwidth of the CVS with a cascaded intra-pulse difference-frequency generation (IDFG). Using the system, we measure broadband CVS spectra of organic liquids spanning over 2000cm-1, more than double the previous study.

Thermal and infrared spectral analysis of TiCo doped NiZn Y-type strontium hexaferrite synthesized by sol gel auto-combustion

TiCo substituted polycrystalline NiZn Y-type strontium nano hexa-ferrites, Sr2NiZnFe12-X(TiCo)X/2O22 (x = 0, 0.5, 1.0, 1.5, 2.0 & 2.5) is synthesized by sol–gel auto combustion route via microwave irradiation. The thermal behaviour of the prepared sample was investigated by TGA and DTC. The exothermic peak on TGA reveals the temperature of Sr2NiZn -Y ferrite phase formation at 905 °C. Seebeck Coefficient is obtained from DTC. Infrared spectrum of this solid phase inorganic materials in the mid infra-red region 4000–400 cm−1 are taken to study fundamental molecular vibration. The fundamental molecular vibration frequencies are absolutely related with respective force constants of atomic group environment. The presence of higher vibrational frequency at 590 cm−1 (589–596 cm−1) (υ1) as calculated for tetrahedral and lower vibration frequency at 430 cm−1 (427–437 cm−1) (υ2) as calculated for octahedral confirm that the higher force field present in tetrahedral surrounding atom than octahedral.

Evaluation of near-infrared (NIR) and Fourier transform mid-infrared (ATR-FT/MIR) spectroscopy techniques combined with chemometrics for the determination of crude protein and intestinal protein digestibility of wheat

The potential of using the near-infrared (NIR) and Fourier transform mid-infrared (ATR-FT/MIR) spectroscopy for the determination of intestinal crude protein (CP) digestibility (IPD) of wheat was evaluated. For CP, the best NIR model showed an excellent prediction performance (R2 = 0.98); the best MIR model also gave an excellent prediction performance (R2 = 0.96). Regarding to IPD, the best model obtained by NIR technique showed approximate quantitative predictive ability (R2 = 0.68), and the best model generated by MIR technique obtained similar prediction performance (R2 = 0.67). NIR models generally showed better predictive abilities than MIR models, which may be due to the MIR spectra record fundamental molecular vibrations and can be more easily affected by multiple interferences. The amide I and II bands played important roles in the development of PLS models for CP and IPD. Results from this study demonstrated the potential of using IR spectroscopy for the prediction of nutrient digestibility while more efforts are required to improve the performance of NIR and ATR-FT/MIR spectroscopy in predicting the IPD of wheat.

Identifying leaf traits that signal stress in TIR spectra

Plants under constant water and temperature stress experience a chain of reactions that in the long term alter their leaf traits (morphology, anatomy and chemistry). The use of these traits as proxies for assessing plant stress was so far mainly based on conventional laboratory methods, which are expensive and time-consuming. Remote sensing methods based on spectral changes can detect changes in pigments and productivity using the visible and near infrared. However, the use of infrared spectra, where changes in the spectra are associated with physical changes of the leaf, is still incipient.In this study plants of Rhododendron cf. catawbiense, were exposed to low temperatures and low soil water content during a six months experiment. The spectral response in the infrared region 1.4–16μm, microstructural variables, leaf water content, leaf area and leaf molecules such as lignin and cellulose concentrations were measured in individual leaves after the period of stress. This study revealed that under cold conditions plants have most changes in leaf water content, lignin and cellulose concentrations and leaf area, while under drought conditions the most striking change is water loss. These leaf trait modifications are also correlated with changes in thermal infrared spectra, showing their potential as proxies for detecting plant stress in this species. A multinomial model allows the estimation of the stress treatments imposed on these plants from their infrared spectra. This model reveals a group of 15 bands in the SWIR and MWIR between 2.23 and 7.77μm, which show relatively large changes, and had an overall accuracy of 87%.Finally, individual partial least squares regression models show that lignin, cellulose, leaf water content and leaf area are the leaf traits reacting significantly to long-term stress and that are also generating measurable changes in the infrared spectra. Although these models are based on laboratory data, the congruence of the identified bands with the fundamental molecular vibrations used in remote sensing, shows the potential of these findings in the assessment of plant stress.

Perdeuterated liquid crystals for near infrared applications

For majority of Liquid Crystalline compounds the absorption occurs at two spectral regions: ultraviolet UV (due to electronic excitations) and infrared IR (caused by molecular vibrations). Both cause the absorption which deteriorates electro-optical modulation abilities of LC. In the MWIR and LWIR regions, there are many fundamental molecular vibration bands. The most intense are the ones with high anharmonicity, which in the case of LCs corresponds to the CH bonds, especially present in the aliphatic chains. In the NIR region, overtone molecular vibration bands derived from IR region begin to appear. In the case of CH bond system, the first overtones are present at 1.6–1.7 μm. To reduce NIR absorptions, perdeuterated Liquid crystal has been proposed. In this paper, we report the physical and optical properties of liquid crystals based on polarimetry measurements method. We also provide a polar decomposition of experimentally measured Mueller matrix in order to determine polarization properties of the device such as depolarization and diattenuation which cannot be obtained from absorption spectra.

Femtosecond time-resolved impulsive stimulated Raman spectroscopy using sub-7-fs pulses: Apparatus and applications.

We describe details of the setup for time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS). In this method, snapshot molecular vibrational spectra of the photoreaction transients are captured via time-domain Raman probing using ultrashort pulses. Our instrument features transform-limited sub-7-fs pulses to impulsively excite and probe coherent nuclear wavepacket motions, allowing us to observe vibrational fingerprints of transient species from the terahertz to 3000-cm(-1) region with high sensitivity. Key optical components for the best spectroscopic performance are discussed. The TR-ISRS measurements for the excited states of diphenylacetylene in cyclohexane are demonstrated, highlighting the capability of our setup to track femtosecond dynamics of all the Raman-active fundamental molecular vibrations.

Evaluation of Lignocellulosic Biomass Degradation by Combining Mid- and Near-Infrared Spectra by the Outer Product and Selecting Discriminant Wavenumbers Using a Genetic Algorithm.

Mid-infrared (MIR) and near-infrared (NIR) spectroscopy provide useful information on the molecular composition of biological systems. Because they are sensitive to organic and mineral components, there is a growing interest in these techniques for the development of biomarkers that reflect intrinsic characteristics of plants and their mode of degradation. Due to their complexity and complementary nature, an important challenge is the combining of MIR and NIR information to identify discriminating wavenumbers in each wavenumber region, with the ultimate goal of assessing the biodegradation process of a lignocellulosic biomass at different time scales. This work investigates the potential of using the outer product to combine MIR and NIR spectra to highlight the connections between fundamental molecular vibrations and their combinations and bonds. Because this operation yields high-dimensional spectra, we propose to use a genetic algorithm to select the most discriminant wavenumbers within the degradation process. The results from two lignocellulosic biomasses with different biodegradation kinetics, miscanthus aerial parts and maize roots, confirm that the outer product combination of MIR and NIR spectral information allows a better discrimination of the biodegradation kinetic compared with the simple concatenation of MIR and NIR spectra or with the use of MIR or MIR spectral information separately. We show that the genetic algorithm selects wavenumbers that correspond to principal vibrations of chemical functional groups of compounds that undergo degradation/conversion during the biodegradation of the lignocellulosic biomass.

Relative intensity correction of Raman optical activity spectra facilitates extending the spectral region

Using home‐built Raman optical activity (ROA) spectrometer and a relative intensity correction for different instrument responses, we report ROA spectra beyond the traditionally utilized spectral region of 200–2500 cm−1. With three different interchangeable gratings covering partially overlapping spectral regions, we can study ROA in the whole region of fundamental molecular vibrations (150–4000 cm−1). Complete panoramic spectra are assembled from subparts collected with different gratings after a relative intensity correction based on the National Institute of Science and Technology standards known from the analogous application in Raman and fluorescence spectroscopy. Using this setup, we report the still little known ROA from C–H and C–D stretching region of the testing substances α‐pinene and a tricyclic spirodilactam. The intensity‐corrected experimental data were compared with calculated ROA and Raman spectra of these substances both with and without anharmonic corrections. A comparison revealed that above 1200 cm−1, the anharmonic correction provides a clear improvement of the agreement. While the calculation of Raman spectra achieves already good accuracy, the analogous ROA calculations still need further development. Copyright © 2014 John Wiley & Sons, Ltd.

Spectroscopic characterization of oils yielded from Brazilian offshore basins: Potential applications of remote sensing

Spectroscopy is the basis to detect and characterize offshore hydrocarbon (HC) seeps through optical remote sensing. Diagnostic spectral features of HCs are linked to their chemical composition and fundamental molecular vibrations (SWIR-TIR features), as well as overtones and combinations of these vibrations (VNIR-SWIR). These features allow for the characterization of oil, oil on water and emulsified oil. This work shows the results of lab and field spectral measurements of 17 petroleum samples yielded from key, oil-rich sedimentary basins in Brazil. Measurements comprised reflectance data (VNIR- SWIR), Attenuated Total Reflectance (ATR), Directional Hemispherical Reflectance (DHR), and emissivity data (TIR). These spectra were analyzed by multivariate techniques, such as Principal Components Analysis (PCA) and Partial Least-Square analysis (PLS). The experimental results indicate that for the VNIR-SWIR range: (i) spectral features can be recognized for crude oil, emulsified oil and oil on ocean water; (ii) different oil types can be qualitatively distinguished based on these features (i.e. light or heavy), even considering oil on water; (iii) the same applies for oil measurements simulated at the spectral resolution of hyperspectral (357-bands/ProSpecTIR) and multispectral (9-bands/ASTER) sensors. Within TIR wavelengths (3–14 μm), typical HC spectral features can also be resolved and oil types qualitatively discriminated using PCA/PLS, including both full-resolution spectra and spectra resampled to hyperspectral sensor (128-bands/SEBASS). However, despite the fact that oil emissivity is always lower than that of water, such separation seems unfeasible using 8–12 μm TIR features only; emissivity spectra are essentially flat for all samples in this interval. This research demonstrated that oil can be qualitatively distinguished based on both VNIR-SWIR and TIR spectroscopy data, with important implications for remote off-shore oil exploration and classification of oil leakages.

Single-crystal, polarized, Raman scattering study of the molecular and lattice vibrations for the energetic material cyclotrimethylene trinitramine

The single-crystal, polarized, Raman spectra for cyclotrimethylene trinitramine (RDX) have been studied at room temperature and pressure over the region of 5–3400cm−1. While the observed Raman bands agree well with a previous Raman study, this work provides a consistent interpretation of the Raman spectra by assignment of fundamental modes, overtones, and combination bands. A total of 56 internal vibrations were observed, 36 of which were labeled as fundamental molecular vibrations. In addition, all 24 lattice optical phonons are identified. All six possible polarizations were probed to obtain the Ag, B1g, B2g, and B3g symmetry modes for both the lattice and molecular modes. Finally, the importance of these phonons for coupling with “doorway” Raman-active modes of RDX is discussed in relation to proposed mechanisms for detonation initiation.

Use of the Extraordinary Infrared Transmission of Metallic Subwavelength Arrays To Study the Catalyzed Reaction of Methanol to Formaldehyde on Copper Oxide

Ordered arrays of subwavelength holes in thin metallic films have been produced that exhibit extraordinary transmission resonances throughout the infrared including the range of wavelengths that excite fundamental molecular vibrations. This phenomenon is attributed to the excitation of surface plasmons which propagate along the surface and tunnel through the holes without being scattered out of an incident beam. Commercial nickel mesh has been coated with copper to reduce the hole size enhancing surface plasmon lifetimes (narrowing resonances) and enabling the study of catalytic processes on the Cu surface. The copper oxide catalyzed reaction of methanol has been studied by recording enhanced zero-order FTIR transmission spectra of the copper mesh activated with a drop of water and then exposed to a drop of methanol. Transformation of CH3OH(ads) to CH3O(ads) and then to the product CH2O(ads) is observed at room temperature and in air with a very simple procedure. The spectrum of adsorbed formaldehyde is analyzed with regard to its orientation on the copper oxide surface.

Infrared optical sensors for water quality monitoring

In-situ monitoring of water quality with particular emphasis on organic pollutants is a global priority topic in water analysis. Recent developments in optical sensor technology provide advanced analytical tools for continuous assessment of pollution levels in the liquid phase and in the gas phase. Infrared sensing schemes are among the most promising concepts due to inherent molecular specificity provided by absorption patterns of fundamental molecular vibrations of organic molecules. The advent of mid-infrared transparent optical fibers and waveguides, appropriate light source technology, such as quantum cascade lasers, and the potential for the development of highly integrated analytical devices based on microfabrication technology substantiates the trend towards spectroscopic sensing techniques. Chemical modification of the waveguide surface leads to enhanced analyte recognition based on tunable properties of enrichment or (bio)chemical recognition layers. Discussion of fundamental sensing technology is complemented by recent examples, highlighting the state-of-the-art in this dynamic research field.

Vibrational spectra of halophthalonitriles

The fundamental vibrational modes of a series of six halophthalonitriles have been studied using Raman and infrared spectroscopy. The vibrational assignment of experimental wave numbers obtained from solid samples was aided using quantum chemical computations. Semi-empirical methods and the local SVWN functional were used to obtain vibrational wave numbers and atomic displacement representations of the fundamental molecular vibrations. The study of a series of molecules with similar structure permits the identification of characteristic wave numbers and the effect of the halosubstitution in the molecular structure.

Quantitative estimation of SiO/sub 2/ content in igneous rocks using thermal infrared spectra with a neural network approach

Develops a method for estimating the SiO/sub 2/ content of igneous rocks using thermal infrared reflectance spectra, aiming to utilize it in the remote sensing of thermal spectral emission. Silicate minerals, which are the major components of the Earth's surface, display their strongest fundamental molecular vibration in the thermal infrared region (8-12 /spl mu/m). The wavelengths of these so-called reststrahlen bands are systematically related to the SiO/sub 2/ content of rocks. Pattern matching (using a back-propagating neural network approach) between simulated remotely sensed data and the SiO/sub 2/ content was performed. This approach was evaluated by comparing the spectrally estimated SiO/sub 2/ content with the chemically determined SiO/sub 2/ content for a separate set of rock samples. The estimated error between the spectrally estimated and chemically determined SiO/sub 2/ contents for most samples was within 7%. Future multiband satellite sensors of the Earth's thermal emission will have much higher spectral and spatial resolution than existing ones, and should be able to detect these spectral trends. >

Thermal infrared (2.5–13.5 μm) spectroscopic remote sensing of igneous rock types on particulate planetary surfaces

Fundamental molecular vibration bands are significantly diminished by scattering. Thus such bands in spectra of fine particulate regoliths (i.e., dominated by <5‐μm particles), or regoliths displaying a similar scale of porosity, are difficult to use for mineralogical or rock type identification. Consequently, other spectral features have been sought that may be more useful in spectroscopic remote sensing of composition. We find that mineralogical information is retained in overtones and combination tones of the fundamental molecular vibrations in the 3.0‐ to 7.0‐μm region, but that relatively few minerals have a sufficiently distinctive band structure to be unambiguously identified with currently available techniques. More significantly, identification of general rock type, as defined by the SCFM chemical index (SCFM = SiO2/SiO2 + CaO + FeO + MgO), is possible using spectral features associated with the principal Christiansen frequency and with a region of relative transparency between the Si‐O stretching and bending bands. However, environmental factors may affect the appearance and wavelengths of these features. Finally, prominent absorption bands may result from the presence of relatively small amounts of water, hydroxyl or carbonate, because absorption bands exhibited by these materials in the 2.7‐ to 4.0‐μm region, where silicate spectra are otherwise featureless, increase strongly in spectral contrast with decreasing particle size. Such materials are thus detectable in very small amounts in a particulate regolith composed predominantly of silicate minerals.

Usefulness of weak bands in midinfrared remote sensing of particulate planetary surfaces

Midinfrared (2.5–25 μm) reflectance spectra of minerals are often used to predict emittance qualitatively. These spectra display weak overtone and combination tone bands, which may be as diagnostic of composition as the strong fundamental molecular vibration bands usually considered for remote sensing applications, but which have been widely ignored. However, unlike the strong bands, the contrast of weak bands relative to the continuum in the midinfrared usually does not decrease with decreasing particle size, but typically increases. To illustrate this behavior, transmittance and reflectance spectra of calcite and quartz are presented for the wavelength range from 4000 to 400 cm−1 (2.5–25 μm). It is the purpose of this paper to point out that these weak bands are potentially useful for compositional remote sensing of particulate planetary surfaces; this will require completion of supporting laboratory studies to document the occurrence of weak bands.

Molecular Vibration Frequency Assignment of Dimethylsilanediol

The fundamental molecular vibration frequencies of dimethylsilanediol were studied and assigned based upon data collected by infrared and Raman spectroscopy, and upon the literature of analogous compounds. Dimethylsilanediol and dimethylsilane-d2-diol were prepared by the hydrolysis of dimethoxydimethylsilane with water and deuterium oxide, respectively. The purity of dimethylsilanediol was verified by a gas chromatographic technique. This research found the SiOH deformations of dimethylsilanediol to absorb at 1175 cm−1 and 1060 cm−1.

The effect of particle size and porosity on spectral contrast in the mid-infrared

Contrary to previous work, we find that the decreasing intensity of fundamental molecular vibration bands with decreasing particle size is due primarily to increasing porosity of the finer particle size ranges, rather than to particle size per se. This implies that laser reflectance measurements from orbiting spacecraft should avoid loss of spectral contrast for fine particulate surfaces, because such measurements near zero phase angle will benefit from the opposition effect.