Infrared Absorption Features Research Articles

Oxygen vacancy-riched NdVO4/BiVO4 heterojunction with infrared absorption feature for efficient water oxidation

Bismuth vanadate (BiVO4) has been recognized as a favorable photocatalyst for generating oxygen from water decomposition using solar energy. While, BiVO4’s optical activity is limited due to its low charge mobility, high photo-generated electron hole recombination rate and low infrared light response. Herein, an oxygen vacancy-riched NdVO4/BiVO4 heterojunction with infrared absorption feature is successfully prepared by a two-step cation-exchange method using Na5V12O32 nanowire as a precursor. The in-situ growth of BiVO4 on NdVO4 forms a close contact interface, forming a heterojunction structure and generating an internal electric field, and the oxygen vacancies generated during cation-exchange can serve as electron traps, promoting the separation of photogenerated electron-hole pairs at the heterojunction. Furthermore, the enhanced infrared absorption properties of NdVO4, which forms the heterojunction, broaden the spectral response range. As a result, the synergistic effect of improved heterojunctions due to oxygen vacancies and enhanced infrared absorption, the photocatalyst exhibits enhanced photocatalytic oxygen evolution activity under visible and near-infrared light irradiation, with an oxygen evolution rate of 1543.16 μmol h−1 g−1 under visible light, which is 3 times higher than that of bare BiVO4. Oxygen can be generated even under near-infrared light.

Decoupling absorption and radiative cooling in mid-wave infrared bolometric elements.

We present a spectrally selective, passively cooled mid-wave infrared bolometric absorber engineered to spatially and spectrally decouple infrared absorption and thermal emission. The structure leverages an antenna-coupled metal-insulator-metal resonance for mid-wave infrared normal incidence photon absorption and a long-wave infrared optical phonon absorption feature, aligned closer to peak room temperature thermal emission. The phonon-mediated resonant absorption enables a strong long-wave infrared thermal emission feature limited to grazing angles, leaving the mid-wave infrared absorption feature undisturbed. The two independently controlled absorption/emission phenomena demonstrate decoupling of the photon detection mechanism from radiative cooling and offer a new design approach enabling ultra-thin, passively cooled mid-wave infrared bolometers.

Atmospheric oxidation of unsaturated hydrofluoroethers initiated by OH radicals

Hydrofluoroethers (HFEs) are perfluorinated chemicals that were developed as alternative for chlorofluorocarbons (CFCs) in various applications. Among notable unsaturated HFEs are perfluoro methyl vinyl ether (PMVE, CF3OCFCF2) and perfluoro ethyl vinyl ether (PEVE, C2F5OCF = CF2). During the implementation of PMVE and PEVE, they spill and release to the atmosphere. Due to the tendency of perfluorinated compounds to have strong infrared absorption features and long atmospheric lifetimes, they are among the most potent greenhouse gases. Thus, it is important to accurately assess their atmospheric sink routes in order to assess their impact on climate change. Herein, we illustrated detailed mechanistic pathways and developed kinetic models with the underlying aim to comprehend the atmospheric fate of the title HFEs and the likely products of decomposition. It was found that the production of main products to stem from the addition of OH radical to the inner carbon-carbon double bond atoms. Atmospheric lifetimes of PMVE and PEVE were calculated to be ∼23 and ∼56 h; respectively. Time-dependent molar yields are acquired for the experimentally major products; namely carbonyl fluoride (CF2O), perfluorinated glyoxal (CFOCFO), perfluorinated methylformates (CF3OCFO and CF3CF2OCFO). Photochemical Ozone Creation Potentials (POCPs) of PMVE and PEVE were calculated to be 1.78 and 1.18, respectively. Obtained results tap into efforts that aim to understand the atmospheric chemistry cycles of perfluorinated chemicals in general.

Characterizing Seasonal and Residual Ices at the South Pole of Mars Using a Universal Set of CRISM Spectral Endmembers

AbstractCurrent maps of compositional variation across south polar ice exposures on Mars do not resolve the meter‐scales at which erosional processes are most active, ultimately limiting our understanding of how the deposits form and evolve and how they can be used to interpret long‐term climate records. In this study, we use k‐means clustering and random forest classification to identify and map a set of universal spectral endmembers across 167 high‐resolution observations acquired during southern summer by the Compact Reconnaissance Imaging Spectrometer for Mars. The 21 endmembers show distinct combinations and strengths of key infrared absorption features reflecting diverse mixtures of CO2 ice, H2O ice, and dust. The resulting compositional framework can be used to characterize the nature of both seasonal CO2 frost and the residual ices it overlies across a variety of terrains. Following the large dust event of Mars Year 28, the residual CO2‐ice deposits (RCD) were covered by an unusually thick or long‐lived deposit of seasonal frost. Within the RCD, low‐albedo material around erosional features display H2O ice absorptions consistent with exposures around the outer margins of the RCD. These peripheral water‐ice deposits show unexpected variation in CO2 ice and dust content. Most notably, regions within several km of the edge of the RCD display spectral contributions from CO2 ice even after seasonal frost has been removed. These results can inform investigations focused on the dynamics of seasonal CO2 deposition, the development of erosional morphologies, and the creation of climate records in south polar stratigraphy.

On the use of extended-wavelength FTIR spectra for the prediction of combustion properties of jet fuels and their constituent species

A Fourier transform infrared (FTIR) spectra-based strategy was developed for estimating three important combustion properties of jet fuels and their constituent hydrocarbon species: ignition delay time (IDT), net heat of combustion (NHC), and derived cetane number (DCN). This approach leverages the strong sensitivity of combustion behavior to the different infrared absorption features of hydrocarbon fuels. Gas-phase FTIR spectra of pure hydrocarbons and jet fuels in the 2–15.38 μm wavelength range were used to train elastic-net regularized linear models, with the model parameters optimized separately for each property. The results from these models were compared with the results from previous models, which utilized only a limited spectral region in the wavelength range 3.3–3.55 μm. The new, optimized models developed in this work show significant improvement in predictive performance compared to the previous models for all three properties. The models’ prediction errors on test data (blends of conventional and alternative jet fuels) are found to be comparable to the standard property measurement uncertainties, demonstrating the value of infrared spectral analysis as a low-volume prescreening tool for accurate property estimation of both conventional and alternative jet fuels.

Fast-sweeping continuous wave quantum cascade laser operating in an external cavity with polygon mirror.

We report a continuous wave room temperature quantum cascade laser operating in an external cavity in the Littrow configuration with a 10-facet polygon mirror rotating at 24,000 RPM. The quantum cascade laser emission is swept across ∼1520 - 1625 cm-1 wavenumber range in less than ∼45 µs with a sweep repetition rate of 4 kHz. The measured maximum output power at the laser gain maximum, 15°C and 0.86 A driving current is ∼90 mW; the estimated average output power across the 45 µs wavenumber sweep is ∼50 mW. Through its sweep, the laser emits on the sequential Fabry-Perot longitudinal modes of the laser chip cavity with the mode separation of ∼0.5 cm-1. The linewidth of the emitting modes is less than ∼0.05 cm-1. Spectral measurements of the infrared absorption features of a 10 µm thick layer of acetophenone and water vapor in the air have demonstrated the capability of obtaining spectral data in less than 45 µs.

Multiple spin-orbit excitons and the electronic structure of α−RuCl3

The honeycomb compound $\alpha$-RuCl$_3$ is widely discussed as a proximate Kitaev spin-liquid material. This scenario builds on spin-orbit entangled $j = 1/2$ moments arising for a $t_{2g}^5$ electron configuration with strong spin-orbit coupling $\lambda$ and a large cubic crystal field. The low-energy electronic structure of $\alpha$-RuCl$_3$, however, is still puzzling. In particular infrared absorption features at 0.30 eV, 0.53 eV, and 0.75 eV seem to be at odds with theory. Also the energy of the spin-orbit exciton, the excitation from $j = 1/2$ to 3/2, and thus the value of $\lambda$ are controversial. Combining infrared and Raman data, we show that the infrared features can be attributed to single, double, and triple spin-orbit excitons. We find $\lambda$ = 0.16 eV and $\Delta$ =42(4) meV for the observed non-cubic crystal-field splitting, supporting the validity of the $j= 1/2$ picture for $\alpha$-RuCl$_3$. The unusual strength of the double excitation is related to the underlying hopping interactions which form the basis for dominant Kitaev exchange.

Structural and mineralogical features of diamond from M. V. Lomonosov deposit (Arkhangelsk province): new data and their interpretation

In the deposit n.a. Y. M. Lomonosov three groups of diamond crystals were distinguished on the base of morphology, photoluminescence, infrared absorption features and the thermal history. The crystals of the first group are octahedrons with minor signs of dissolution. In the first group, crystals have a high proportion of nitrogen in the Bform and the high model temperature. The crystals of the second type is highly resorbed dodecahedroids, they has low proportion of nitrogen in B form. The third group consists of crystals with the low temperature C defects, they are cuboids and resorbed tetrahexahedroids. These patterns indicate the polygenicity of the diamond in the deposit after M.V. Lomonosov.

Structural and Mineralogical Features of Diamonds from the Lomonosov Deposit (Arkhangelsk Province): New Data and Interpretation

In the deposit n.a. Y. M. Lomonosov three groups of diamond crystals were distinguished on the base of morphology, photoluminescence, infrared absorption features and the thermal history. The crystals of the first group are octahedrons with minor signs of dissolution. In the first group, crystals have a high proportion of nitrogen in the B form and the high model temperature. The crystals of the second type is highly resorbed dodecahedroids, they has low proportion of nitrogen in B form. The third group consists of crystals with the low temperature C defects, they are cuboids and resorbed tetrahexahedroids. These patterns indicate the polygenicity of the diamond in the deposit after M.V. Lomonosov.

Terahertz and infrared characteristic absorption spectra of aqueous glucose and fructose solutions

In this paper, the terahertz (THz) and infrared (IR) characteristic absorption spectra of aqueous glucose solutions and aqueous fructose solutions with different concentrations were measured and studied. The absorption spectra of these two molecules in solid-state and in aqueous solutions were compared and analyzed, the significant effect of molecular adjacent environment on the molecular structure and vibrational mode was revealed. In addition, the THz and IR absorption spectra of these two isomers’ aqueous solutions were also compared and explored. No obvious differences were found from their IR absorption features measured at room temperature, while their THz absorption spectra do have the differences, indicating THz characteristic absorption spectra more suitable for the detection and identification of aqueous glucose and fructose solutions. The results are helpful to understand the influence of aqueous solutions environment on the molecular structures and vibrational modes of the materials, and also provide a theoretical reference for the quantum chemical calculation of biological macromolecules.

Modeling CO, CO2, and H2O Ice Abundances in the Envelopes of Young Stellar Objects in the Magellanic Clouds

Abstract Massive young stellar objects (MYSOs) in the Magellanic Clouds show infrared absorption features corresponding to significant abundances of CO, CO2, and H2O ice along the line of sight, with the relative abundances of these ices differing between the Magellanic Clouds and the Milky Way. CO ice is not detected toward sources in the Small Magellanic Cloud, and upper limits put its relative abundance well below sources in the Large Magellanic Cloud and the Milky Way. We use our gas-grain chemical code MAGICKAL, with multiple grain sizes and grain temperatures, and further expand it with a treatment for increased interstellar radiation field intensity to model the elevated dust temperatures observed in the MCs. We also adjust the elemental abundances used in the chemical models, guided by observations of H ii regions in these metal-poor satellite galaxies. With a grid of models, we are able to reproduce the relative ice fractions observed in MC MYSOs, indicating that metal depletion and elevated grain temperature are important drivers of the MYSO envelope ice composition. Magellanic Cloud elemental abundances have a subgalactic C/O ratio, increasing H2O ice abundances relative to the other ices; elevated grain temperatures favor CO2 production over H2O and CO. The observed shortfall in CO in the Small Magellanic Cloud can be explained by a combination of reduced carbon abundance and increased grain temperatures. The models indicate that a large variation in radiation field strength is required to match the range of observed LMC abundances. CH3OH abundance is found to be enhanced in low-metallicity models, providing seed material for complex organic molecule formation in the Magellanic Clouds.

An airborne remote sensing case study of synthetic hydrocarbon detection using short wave infrared absorption features identified from marine-harvested macro- and microplastics

The abundance and distribution of plastic debris in natural waters is largely unknown due to limited comprehensive monitoring. Here, optical properties of dry and wet marine-harvested plastic debris were quantified to explore the feasibility of plastic debris optical remote sensing in the natural environment. We measured the spectral reflectance of microplastics (<5mm) from the North Atlantic Ocean, macroplastics (>5mm) washed ashore along the USA west coast and virgin plastic pellets over a wavelength range from 350 to 2500nm. Compared to the spectral variability of multi-colored dry macroplastics, the measured dry marine-harvested microplastic reflectance spectra could be represented as a single bulk average spectrum with notable absorption features at ~931, 1215, 1417 and 1732nm. The wet marine-harvested microplastics had similar spectral features to the dry microplastics but the magnitude was lower over the measured spectrum. When spectrally matched to the reference library of typical dry virgin pellets, the mean dry marine-harvested microplastics reflectance had moderate similarities to low-density polyethylene, polyethylene terephthalate, polypropylene and polymethyl methacrylate. This composition was consistent with the subset sampled with the Fourier Transform Infrared (FTIR) spectrometer and what has been reported globally. The absorption features at 1215 and 1732nm were observable through an intervening atmosphere and used to map the distributions of synthetic hydrocarbons at a landfill and on man-made structures from airborne visible-infrared imaging spectrometer (AVIRIS) imagery, indicating the potential to remotely sense dry washed ashore and land-origin plastics. These same absorption features were identifiable on wet marine-harvested microplastics, but the ability to conduct remote sensing of microplastics at the ocean surface layer will require more detailed radiative transfer analysis and development of high signal-to-noise sensors. The spectral measurements presented here provide a foundation for such advances towards remote detection of plastics from various platforms.

Modeling CO, CO2and H2O Ice Abundances in the Envelopes of Young Stellar Objects in the Magellanic Clouds

AbstractMassive young stellar objects (MYSOs) in the Magellanic Clouds (MCs) show infrared absorption features corresponding to significant abundances of CO, CO2and H2O ice along the line of sight, with the relative abundances of these ices varying between sources in the Magellanic Clouds and the Milky Way. We use our gas-grain chemical code MAGICKAL, with multiple grain sizes and grain temperatures, and further expand it with a treatment for increased interstellar radiation field intensity to model the elevated dust temperatures observed in the MCs. We also adjust the elemental abundances used in the chemical models, guided by observations of HII regions in these metal-poor satellite galaxies. With a grid of models, we are able to reproduce the relative ice fractions observed in MC MYSOs, indicating that metal depletion and elevated grain temperature are important drivers of the MYSO envelope ice composition. The observed shortfall in CO in the Small Magellanic Cloud can be explained by a combination of reduced carbon abundance and increased grain temperatures. The models indicate that a large variation in radiation field strength is required to match the range of observed LMC abundances.

Spectroscopic and Antibacterial Studies of Anisotropic Gold Nanoparticles Synthesized Using Malva parviflora

Anisotropic gold nanoparticles (Au NPs) have been synthesized via a green route with the aid of Malva parviflora (M. parviflora) extract. Their UV-vis-NIR spectra exhibit a near infrared absorption feature, which is due to the one-dimensional growth of Au NPs. The images of transmission electron microscopy (TEM) show that the Au NPs are triangular in shape. The formation of Au anisotropic nanoparticles was found to depend on the extract quantity. The Au NPs show good antibacterial activity against the Gram-positive bacteria Bacillus subtilis and Enterococcus faecalis.

Vibrational mid-infrared photothermal spectroscopy using a fiber laser probe: asymptotic limit in signal-to-baseline contrast.

We report on a mid-infrared photothermal spectroscopy system with a near-infrared fiber probe laser and a tunable quantum cascade pump laser. Photothermal spectra of a 6μm-thick 4-octyl-4'-cyanobiphenyl liquid crystal sample are measured with a signal-to-baseline contrast above 103. As both the peak photothermal signal and the corresponding baseline increase linearly with probe power, the signal-to-baseline contrast converges to an asymptotic limit for a given pump power. This limit is independent of the probe power and characterizes the best contrast achievable for the system. This enables sensitive quantitative spectral characterization of linear infrared absorption features directly from photothermal spectroscopy measurements.

The anharmonic quartic force field infrared spectra of three polycyclic aromatic hydrocarbons: Naphthalene, anthracene, and tetracene.

Current efforts to characterize and study interstellar polycyclic aromatic hydrocarbons (PAHs) rely heavily on theoretically predicted infrared (IR) spectra. Generally, such studies use the scaled harmonic frequencies for band positions and double harmonic approximation for intensities of species, and then compare these calculated spectra with experimental spectra obtained under matrix isolation conditions. High-resolution gas-phase experimental spectroscopic studies have recently revealed that the double harmonic approximation is not sufficient for reliable spectra prediction. In this paper, we present the anharmonic theoretical spectra of three PAHs: naphthalene, anthracene, and tetracene, computed with a locally modified version of the SPECTRO program using Cartesian derivatives transformed from Gaussian 09 normal coordinate force constants. Proper treatments of Fermi resonances lead to an impressive improvement on the agreement between the observed and theoretical spectra, especially in the C-H stretching region. All major IR absorption features in the full-scale matrix-isolated spectra, the high-temperature gas-phase spectra, and the most recent high-resolution gas-phase spectra obtained under supersonically cooled molecular beam conditions in the CH-stretching region are assigned.

External cavity quantum cascade lasers with ultra rapid acousto-optic tuning

We report operation of tunable external cavity quantum cascade lasers with emission wavelength controlled by an acousto-optic modulator (AOM). A long-wave infrared quantum cascade laser wavelength tuned from ∼8.5 μm to ∼9.8 μm when the AOM frequency was changed from ∼41MHz to ∼49 MHz. The laser delivered over 350 mW of average power at the center of the tuning curve in a linewidth of ∼4.7 cm−1. Measured wavelength switching time between any two wavelengths within the tuning range of the QCL was less than 1 μs. Spectral measurements of infrared absorption features of Freon demonstrated a capability of obtaining complete spectral data in less than 20 μs.

Transmission spectral properties of clouds for hot Jupiter exoplanets

Clouds have an important role in the atmospheres of planetary bodies. It is expected that, like all the planetary bodies in our solar system, exoplanet atmospheres will also have substantial cloud coverage, and evidence is mounting for clouds in a number of hot Jupiters. In order to better characterise planetary atmospheres we need to consider the effects these clouds will have on the observed broadband transmission spectra. Here we examine the expected cloud condensate species for hot Jupiter exoplanets and the effects of various grain sizes and distributions on the resultant transmission spectra from the optical to infrared, which can be used as a broad framework when interpreting exoplanet spectra. We note that significant infrared absorption features appear in the computed transmission spectrum, the result of vibrational modes between the key species in each condensate, which can potentially be very constraining. While it may be hard to differentiate between individual condensates in the broad transmission spectra, it may be possible to discern different vibrational bonds, which can distinguish between cloud formation scenarios such as condensate clouds or photochemically generated species. Vibrational mode features are shown to be prominent when the clouds are composed of small sub-micron sized particles and can be associated with an accompanying optical scattering slope. These infrared features have potential implications for future exoplanetary atmosphere studies conducted with JWST, where such vibrational modes distinguishing condensate species can be probed at longer wavelengths.

Enhancement of time resolution in transient kinetics

A methodology for enhancement of time resolution in transient experiments has been formulated where the mathematical background of the deconvolution problem is outlined. The analysis leads to an algorithm based on the discrepancy principle for calculation of the regularization parameter, which exhibits satisfactory convergence and can successfully handle the noise that is present in the original measurements. Application of the algorithm for different synthetic datasets has shown that the extent of enhancement in time resolution depends on the broadening function and the sampling frequency of the detector. Consequently, shorter residence times and higher sampling frequencies are in general desirable.The impact of deviations from the Beer–Lambert law in Fourier-transform infra-red spectroscopy has also been evaluated using computational fluid dynamics simulations. The flow field and concentration distribution in a gas cell with ∼5m optical pathlength have been simulated, and the absorbance–concentration correlations have been implemented in the analysis. Carbon monoxide and nitric oxide have been chosen as species with significant nonlinear infra-red absorption features, and their mixtures with different concentrations have been considered in separate simulations. Due to the nonlinear characteristics of the problem, various levels of error have been observed in the results, which are dependent on the concentration at the inlet of the gas cell and the gas phase species as well.The variance of concentration distribution normalized with mean squared concentration has been defined as a measure of intensity of segregation, which displays strong similarity for the same flow field pattern and relatively close molecular diffusivities.

Mixing of the immiscible: hydrocarbons in water-ice near the ice crystallization temperature.

Structural changes in hydrocarbon-doped water-ice during amorphous to crystalline phase conversion are investigated using polycyclic aromatic hydrocarbons (PAHs) as probes. We show that aggregation of impurity molecules occurs due to the amorphous-crystalline transition in ice, especially when they are hydrophobic molecules such as PAHs. Using ultraviolet-visible (UV-vis), Fourier-transform Infrared (FTIR), and laser-induced-fluorescence (LIF) spectroscopic techniques, we show that, although ice infrared absorption features change from a broad structureless band corresponding to amorphous ice to a sharp structured crystalline ice bands, simultaneously, sharper isolated PAH UV absorption features measured in the amorphous ice host turn broad upon ice crystallization. A simultaneous decrease in the monomer fluorescence and increase in the excimer emission band is observed, a clear indication for the formation of PAH molecular aggregates when amorphous ice is converted to crystalline ice at higher temperatures. Similar to the irreversible amorphous-crystalline phase transitions, the UV, fluorescence, and excimer emissions indicate that PAHs undergo irreversible aggregation. Our studies suggest that organic impurities exist as aggregates rather than monomers trapped in crystalline water-ice when cycled through temperatures that convert amorphous ice to crystalline ice, rendering a better insight into phenomena such as the formation of cometary crust. This aggregate formation also may significantly change the secondary reaction pathways and rates in impurity-doped ices in the lab, on Earth, in the solar system, and in the interstellar medium.