Far-infrared Absorption Research Articles

Absolute Band Intensity of the Iodine Monochloride Fundamental Mode for Infrared Sensing and Quantitative Analysis.

Iodine monochloride (ICl) is a potential off-gas product of molten salt reactors; monitoring this heteronuclear diatomic molecule is of great interest for both environmental and safety purposes. In this paper, we investigate the possibility of infrared monitoring of ICl by measuring the far-infrared absorption cross section of its fundamental band near 381 cm-1. We have performed quantitative studies of the neat gas in a 20 cm cell at 25, 35, 50, and 70 °C at multiple pressures up to ∼9 Torr and investigated the temperature and pressure dependencies of the band's infrared cross section. Quantitative measurements were problematic due to sample adhesion to the cell walls and windows as well as reactions/possible hydrolysis of ICl to form HCl gas. Effects were mitigated by measuring only the neat gas, using short measurement times, and subtracting out the partial pressure of the HCl(g). The integrated band strength is shown to be temperature independent and was found to be equal to 9.1 × 10-19 (cm2/molecule) cm-1. As expected, the temperature dependence of the band profile showed only a small effect over this limited temperature range. We have also investigated using the absorption data along with inverse least squares multivariate methods for the quantitative monitoring of ICl effluent concentrations under different scenarios using infrared (standoff) sensing and compare these results with traditional Beer's law (univariate) techniques.

Far-infrared absorption of undoped and Br-doped carbon nanofiber powder in stacked-cup cone configuration

We performed room-temperature far-infrared ($40--650\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) transmission measurements on undoped and bromine-doped powder samples of carbon nanofibers in stacked-cup cone geometry. The transmission spectra show enhanced transmittance after bromine doping and all spectra were fit to a Drude-Lorentz (DL) model. A decreased metallic conductivity along with a redshift of the lowest semiconducting gap was found with doped samples. A significant decrease in the scattering rate upon heavy doping has been qualitatively explained as partial ordering of intercalated dopant ions. Absorption spectra were derived from the transmission spectra under the assumption of nondispersive reflectance and subsequently compared with DL-model-derived spectra. The free-carrier density of the $n$-type powder and the electronic mean free path were estimated and compared with reported values for single-walled nanotubes and pyrolytic graphite.

Synthesis, Structure, and Complexation of an S-Shaped Double Azahelicene with Inner-Edge Nitrogen Atoms.

An S-shaped double azahelicene (1) was synthesized in excellent yield by a palladium-catalyzed double dehydrogenative C-H coupling reaction. The stereochemistry of 1 was confirmed to be dl by single-crystal X-ray diffraction analysis. Selective formation of dl-1 was attributed to the isomerization of the kinetically controlled product (meso-1) into the more thermodynamically stable dl-1 under the applied reaction conditions. dl-1 can coordinate to palladium(II) in a bidentate trans-chelating fashion, which was confirmed by X-ray absorption fine structure (XAFS) as well as by X-ray photoelectron spectroscopy (XPS), diffuse reflectance (DR) UV/Vis, and far-infrared (FIR) absorption spectroscopy. Theoretical calculations of palladium complex 16 revealed a weak attractive interaction between palladium and carbon atoms on the central dimethoxynaphthalene core, which could facilitate a disproportionation between a trans-chelating (dl-1)⋅PdCl2 complex and PdCl2 to form 16.

Light absorption in semiconducting quantum dot clusters with fixed surface areas in an external tilted magnetic field

In the present study, we theoretically investigate the far infrared (FIR) spectrum of clusters formed by AlxGa1−xAs/GaAs, GaN/AlN, InSb/GaSb, and ZnSe/CdSe semiconductor hetero-structure quantum dots (QDs). The clusters are obtained by circle packing disc-shaped QDs in a square domain. The close spacing effect has previously been observed by experiment. For a given square area, we analyze the effects of the externally applied magnetic field (intensity and direction) on the FIR absorption coefficient for different QD packing values. The finite difference method is used to solve the two-dimensional Schrödinger equation describing the QD clusters in magnetic fields.

Quantum chemical verification of identification of three phenol-type antioxidants by far-infrared absorption spectroscopy

Three different kinds of phenol-type antioxidants were added in low-density polyethylene sheets. Far-infrared and mid-infrared absorption spectra were measured for these sheets and the sheet with no antioxidants to obtain spectra of antioxidants. Furthermore, chemical structures of the antioxidants were optimized by quantum chemical calculations and possible vibrational modes and their energies were calculated numerically. As a result, it was found that our calculations simulate absorption spectra of the antioxidants successfully and that their causal vibrations in the mid-infrared range are consistent with those reported in the literature. Furthermore, it has become clear that all the major far-infrared absorption peaks are due to skeletal vibrations. This clearly explains our previous results that the three antioxidants with very similar structures can be identified by the far-infrared absorption spectroscopy.

Li@C60]PF6: Infrared spectra from 90K to 523K; Determination of the molar extinction coefficients and integrated molar absorptivity

The molar extinction coefficient ε and the integrated molar absorptivity ψ values of all the mid-infrared and far-infrared absorption bands of [Li@C60]+PF6- were determined in comparison to those of C60. The dependence from temperature of all the infrared absorption bands of [Li@C60]+PF6- was measured in the temperature range comprised from 523 K and 90 K and extrapolated to 0 K. Once again, the results were discussed in comparison to the C60 analogous experimental results.

The O2 far-infrared absorption spectrum between 50 and 170 cm-1

The oxygen absorption spectrum in the 50–170 cm-1 spectral range is studied at the AILES beam line of the SOLEIL synchrotron with a Fourier transform spectrometer equipped with a 151-m multipass gas cell. The spectrum recorded at room temperature (23.15 °C) with a pressure of 19.76 Torr is formed by weak pure rotational magnetic dipole transitions. Line parameters of 26 lines are derived and compared to literature values. In particular, measured line intensities confirm the 45 years-old previous values reported by Boreiko et al. (J. Quant. Spectrosc. Radiat. Transfer 32 (1984)109–117). The agreement with HITRAN intensities (within 1% level for the strongest lines) indicates that the 20% HITRAN error estimate was overly cautious: we show that the error is more likely within 2%.

The Greenhouse Effect in Buried Galactic Nuclei and the Resonant HCN Vibrational Emission

Abstract Recent interferometric observations have shown bright HCN emission from the ν 2 = 1 vibrational state arising in buried nuclear regions of galaxies, indicating an efficient pumping of the ν 2 = 1 state through the absorption of 14 μm continuum photons. We modeled the continuum and HCN vibrational line emission in these regions, characterized by high column densities of dust and high luminosities, using a spherically symmetric approach, simulating both a central heating source (active galactic nucleus, AGN) and a compact nuclear starburst (SB). We find that when the H2 columns become very high, N H2 ≳ 1025 cm−2, trapping of continuum photons within the nuclear region dramatically enhances the dust temperature (T dust) in the inner regions, even though the predicted spectral energy distribution as seen from the outside becomes relatively cold. The models thus predict a bright continuum at millimeter wavelengths for a luminosity surface brightness (averaged over the model source) of ∼108 L ⊙ pc−2. This greenhouse effect significantly enhances the mean mid-infrared intensity within the dusty volume, populating the ν 2 = 1 state to the extent that the HCN vibrational lines become optically thick. AGN models yield higher T dust in the inner regions and higher peak (sub)millimeter continuum brightness than SB models, but similar HCN vibrational J = 3–2 and 4–3 emission owing to both optical depth effects and a moderate impact of high T dust on these low-J lines. The observed HCN vibrational emission in several galaxies can be accounted for with an HCN abundance of ∼10−6 (relative to H2) and luminosity surface brightness in the range (0.5–2) × 108 L ⊙ pc−2, predicting a far-infrared photosphere with T dust ∼ 80–150 K, in agreement with the values inferred from far-infrared molecular absorption.

Broadband FIR absorption spectra of low‐density polyethylene sheets containing six different antioxidants and estimation of their contents by chemometric analysis

Antioxidants are usually added to polymeric insulating materials to suppress oxidative degradation. However, the details of antioxidant added to a polymer are often not disclosed. On the other hand, compared to mid-infrared absorption spectroscopy, terahertz (THz) absorption spectroscopy, and far-infrared (FIR) absorption spectroscopy have not been performed so often. Based on the above backgrounds, six different kinds of antioxidants were added into low-density polyethylene sheets with four different contents, and their THz and FIR absorption spectra were measured. As a result, it has become clear that each antioxidant shows its specific absorption peaks, by which the antioxidants can be identified. Furthermore, the antioxidant content can be estimated by the absorption intensity of the specific peak. A chemometric analysis can make the estimation more accurately than a simple comparison of absorption intensity.

IR-range photoinduced diode effect in the manganese-substituted bismuth ferrite films

The photoinduced diode effect has been found in BiMnXFe1−XO3 (0 < Х < 0.15) films in the near-infrared and violet spectral ranges in a wide temperature interval. The dependences of the diode effect on temperature, wavelength, and illumination power have been established. The mechanism of the ferroelectric diode has been determined using its I–V characteristics at different illumination powers, optical spectra, far-infrared absorption spectra, and electric polarization hysteresis. The interplay between the photodiode effect and conductivity mechanism has been elucidated. The model of strong Coulomb potential fluctuations and quantum well asymmetry caused by the electric polarization in the film and difference between the times of carrier thermalization and recombination of electrons and holes is proposed.

Low-frequency surface-enhanced Raman scattering spectroscopy at metal electrode surfaces

Low-frequency surface-enhanced Raman scattering (SERS) spectroscopy is a versatile tool for studying surface phenomena under electrochemical conditions. This spectroscopy enables us to obtain rich information on extramolecular vibrations between substrate and adsorbates, which are sensitive to atomistic surface features of the substrate. Owing to recent advancements in optical filter technology, low-frequency SERS signals are now becoming easily detectable using conventional Raman systems equipped with holographic notch filters. In addition, SERS background signals, which have been simply ignored, can provide electronic information on the metal substrate. This allows us to observe both sides of electrode–electrolyte interfaces in situ and simultaneously, which is never expected in far-infrared or terahertz absorption spectroscopy. This advanced SERS spectroscopy can help our understanding of electrochemical and electrocatalytic reactions at the molecular scale.

Design and demonstration of antenna-coupled Schottky diodes in a foundry complementary metal-oxide semiconductor technology for electronic detection of far-infrared radiation

Electronic detection of far-infrared (FIR) radiation up to 9.74 THz is reported in a foundry complementary metal-oxide semiconductor (CMOS) technology. The detectors were fabricated with Schottky-barrier diodes (SBDs) formed in 130-nm CMOS without any process modifications. Direct-antenna matched detectors achieve a measured peak optical responsivity (RV) of 383 and 25 V/W at 4.92 and 9.74 THz, respectively, near the 5 and 10 THz fundamental frequency of the antennas. A significantly improved RV at 9.74 THz (25× compared to the MOSFET detectors and ∼2× compared to the SBD) ensures negligible impact on the system noise-equivalent power (NEP) due to the input-referred noise of the amplifier following the detector. This work also demonstrated that by incorporating the effects of plasma resonance, transit time, and FIR absorption behavior of SiO2, as well as the 3D electromagnetic simulations into the SBD model, good agreement between the measurements and simulations can be attained. The detector designed for a 10-THz operation achieves an optical NEP of 1.1 nW/√Hz at 9.74 THz in the shot-noise limit, which is comparable to that of commercially available pyro-detectors that are 50 000× larger.

Far-infrared continuum absorption of forsterite and enstatite at low temperatures

Context. The far-infrared continuum opacity of cold dust is an important quantity for the study of debris disks in planetary systems and of protoplanetary disks. Forsterite and enstatite are considered to be the most abundant crystalline dust species in such environments. Aims. The optical constants of these minerals at wavelengths above 80 μm, which govern the opacity, and their temperature dependence are poorly known. Our aim is to fill in this lack of information with new laboratory data. Methods. We present spectroscopic transmission measurements on forsterite and enstatite single crystals of up to 10 mm thickness at wavelengths between 45 and 500 μm and for temperatures down to 10 K. We compare our results to literature data originating from powder transmission and from reflection spectroscopy. Results. The imaginary parts of the refractive indices calculated from the measurements show very strong temperature dependences, which to that extent are not seen in reflection-based data or in powder measurement data. The temperature dependences can be described by a simple theoretical model taking the contributions of single-phonon absorption and phonon difference processes into account. We also observe, for the first time, enstatite absorption bands at 87.5 μm and 116.6 μm wavelengths. Conclusions. The single-crystal optical constants of forsterite and enstatite predict an extremely small submillimeter opacity of crystalline silicate dust at low temperatures, which would make these particles almost invisible in the thermal radiation of cold dust. Thus, it is important to understand why absorption measurements with mineral powders resulted in much higher opacity values.

Identification of vibrational modes in thymine bases by broadband far-infrared absorption spectroscopy

Far-infrared and THz absorption spectra were obtained for thymine bases. Quantum chemical calculations were also conducted to simulate the far-infrared spectrum using density functional theory. Three models were used for the simulations. In one model, a thymine monomer was assumed. The second model assumed a thymine dimer, where two pyrimidine rings were aligned antiparallel. The last model assumed a thymine trimer, in which four hydrogen bonds were formed in three pyrimidine rings placed on the same plane. By comparing the measurements and simulations, almost all the absorption peaks appearing from 100 to 600 cm−1 could be assigned to torsional or skeletal vibrations of a pyrimidine ring. On the other hand, absorption peaks appearing in the THz range from 16.7 to 167 cm−1 were assigned to intermolecular vibrations.

Determining small refractive index contrast in chalcogenide-glass pairs at mid-infrared wavelengths

A two-composition thin film (Ge20Sb10Se70/Ge20Sb10Se67S3 atomic % core/cladding glasses) was fabricated using a hot-fibre-pressing technique in which both glasses follow the same post-fibre processing. A simple approach is proposed that uses normal incidence transmission spectra to determine their refractive index contrast over the wavelength range from 2 to 25 µm with an error of less than ± 0.002. Using an improved Swanepoel method, the calculated numerical aperture of these two compositions was within ± 0.011 of that obtained from prism minimum deviation measurements. Results show that introducing 3 atomic % S into the Ge-Sb-Se glass system lowered the refractive index and blue-shifted the visible optical bandgap, the far-infrared fundamental vibrational absorption bands and the zero-dispersion wavelength.

Hidden or missing outflows in highly obscured galaxy nuclei?

Context. Understanding the nuclear growth and feedback processes in galaxies requires investigating their often obscured central regions. One way to do this is to use (sub)millimeter line emission from vibrationally excited HCN (HCN-vib), which is thought to trace warm and highly enshrouded galaxy nuclei. It has been suggested that the most intense HCN-vib emission from a galaxy is connected to a phase of nuclear growth that occurs before the nuclear feedback processes have been fully developed. Aims. We aim to investigate if there is a connection between the presence of strong HCN-vib emission and the development of feedback in (ultra)luminous infrared galaxies ((U)LIRGs). Methods. We collected literature and archival data to compare the luminosities of rotational lines of HCN-vib, normalized to the total infrared luminosity, to the median velocities of 119 μm OH absorption lines, potentially indicating outflows, in a total of 17 (U)LIRGs. Results. The most HCN-vib luminous systems all lack signatures of significant molecular outflows in the far-infrared OH absorption lines. However, at least some of the systems with bright HCN-vib emission have fast and collimated outflows that can be seen in spectral lines at longer wavelengths, including in millimeter emission lines of CO and HCN (in its vibrational ground state) and in radio absorption lines of OH. Conclusions. We conclude that the galaxy nuclei with the highest LHCN − vib/LIR do not drive wide-angle outflows that are detectable using the median velocities of far-infrared OH absorption lines. This is possibly because of an orientation effect in which sources oriented in such a way that their outflows are not along our line of sight also radiate a smaller proportion of their infrared luminosity in our direction. It could also be that massive wide-angle outflows destroy the deeply embedded regions responsible for bright HCN-vib emission, so that the two phenomena cannot coexist. This would strengthen the idea that vibrationally excited HCN traces a heavily obscured stage of evolution before nuclear feedback mechanisms are fully developed.

Far-infrared self-continuum absorption of H216O and H218O (15–500 cm−1)

The water vapor continuum absorption is studied in a spectral range covering most of the pure rotational spectrum of water molecule up to 500 cm−1. The continuum absorption was derived from the broadband water vapor spectra recorded by Fourier transform spectrometer equipped with the 151-m multipass gas cell at the AILES beam line of the SOLEIL synchrotron. The coherent (10–35 cm−1) and standard (40–500 cm−1) radiation modes of the synchrotron were used. In order to refine the magnitude and clarify the physical origin of the continuum, spectra of the two major water isotopologues, H216O and H218O, were considered. Recordings at several water vapor pressures were used to check the expected quadratic pressure dependence of the continuum.The new data extend and supplement previous measurements filling, in particular, the gap between 200 and 350 cm−1, which was never studied before. The H216O and H218O absorption continua in the range of 50–650 cm−1 show similar frequency dependence and magnitude. In particular, both continua exhibit a clear water dimer spectral signature near 15 cm−1, in good agreement with previous ab initio calculations. The present data confirm that the MT-CKD empirical continuum model widely used in atmospheric applications, overestimates importantly the continuum magnitude in the whole range of the rotational band. The observed irregular frequency dependence of the retrieved self-continuum cross-section values is tentatively interpreted as due to uncertainties on the resonance lines of the water monomer spectrum which is subtracted from the recorded spectra. On the basis of spectra simulations, the inadequate description of the line shapes in the range of the intermediate wings (detuning of 5–10 cm−1 from line center) and the uncertainties on the self-broadening coefficients of water monomer lines are identified as possible mechanisms responsible of the observed irregular fluctuations.

Realization of the Kohn's Theorem in Ge/Si Quantum Dots with Hole Gas: Theory and Experiment.

This article discusses specific quantum transitions in a few-particle hole gas, localized in a strongly oblate lens-shaped quantum dot. Based on the adiabatic method, the possibility of realizing the generalized Kohn theorem in such a system is shown. The criteria for the implementation of this theorem in a lens-shaped quantum dot, fulfilled in the experiment, is presented. An analytical expression is obtained for the frequencies of resonant absorption of far-infrared radiation by a gas of heavy holes, which depends on the geometric parameters of the quantum dot. The results of experiments on far-infrared absorption in the arrays of p-doped Ge/Si quantum dots grown by molecular beam epitaxy (MBE) with gradually increasing average number of holes in dot are presented. Experimental results show that the Coulomb interaction between the holes does not affect the resonant frequency of the transitions. A good agreement between the theoretical and experimental results is shown.

Transitions between Be acceptor levels in GaAs bulk

The doping is one of important means in the semiconductor manufacturing techniques, by which the optical and electric properties of semiconductor materials can be significantly improved. The doping level and energy level structure of dopants have a great influence on the operating performances of micro-electronic devices. Beryllium is one of acceptors, which is frequently used to be doped in GaAs bulk, because it is very stable with respect to diffusion at higher temperatures. Therefore, it is significant for the application to optoelectronic devices that the energy-state structure of Be acceptors in GaAs bulk can be investigated in detail. The sample GaAs:Be used in experiment is a 5-μm-thick epitaxial single layer doped uniformly by Be acceptors with a doping level of 2 × 10&lt;sup&gt;16&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt;, and grown by molecular beam epitaxy on 450-μm-thick semi-insulating (100) GaAs substrates in a VG V80 H reactor equipped with all solid sources. The transitions between the energy states of Be acceptors are studied experimentally by different spectroscopy techniques. The far-infrared absorption experiments are performed by using a Fourier-transform spectrometer equipped with a tungsten light source and a multilayer wide band beam splitter. Prior to the absorption spectrum measurement, the sample is thinned, polished and wedged to approximately a 5° angle to suppress optical interference between the front and back faces. Then, the sample is placed into the cryostat with liquid helium (4.2 K). The photoluminescence and Raman spectra are also measured at 4.2 K by a Renishaw Raman imaging microscope. The optical excitation to the sample is provided by an argon-ion laser with a wavelength of 514.5 nm, and the excited power is typically 5 mW. The odd-parity transitions from the Be acceptor ground state 1S&lt;sub&gt;3/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;8&lt;/sub&gt; to three excited states, i.e. 2P&lt;sub&gt;3/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;8&lt;/sub&gt;, 2P&lt;sub&gt;5/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;8&lt;/sub&gt; and 2P&lt;sub&gt;5/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;7&lt;/sub&gt; are clearly observed in the far-infrared absorption spectra, then the respective transition energy values are obtained, which are in excellent agreement with the experimental results reported previously. In the photoluminescence spectrum, the emission peak labelled two holetransition, originating from the two-hole transition of recombination of the neutral-accptor bound excitons, is seen obviously, thus the energy of the even-parity transition between 1S&lt;sub&gt;3/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;8&lt;/sub&gt; and 2S&lt;sub&gt;3/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;8&lt;/sub&gt; states is found indirectly. Furthermore, in the Raman spectrum measured, the transition peak between 1S&lt;sub&gt;3/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;8&lt;/sub&gt; and 2S&lt;sub&gt;3/2&lt;/sub&gt;&lt;i&gt;Γ&lt;/i&gt;&lt;sub&gt;8&lt;/sub&gt; states is well resolved, and the transition energy between them is gained directly. By comparison, the transition energy values gained directly and indirectly are found to be consistent with each other.

Far-Infrared Absorption Spectra in Hydrogen-Bonding Mixed-Valence Re Complexes

Far-Infrared Absorption Spectra in Hydrogen-Bonding Mixed-Valence Re Complexes