IR Spectral Region Research Articles

Correlation of resistivity with zinc content in a vapor grown (Cd,Zn)Te:Se

We report the possibility to grow semi-insulating (Cd,Zn)Te:Se crystals by the modified Markov method (MMM) from the vapor phase. When oriented Cd(Te,Se) seed material was used for the growth, lattice matching and doping with Se resulted in an increase of resistivity up to several units of 109 Ω cm. A homogenous Zn distribution was also observed. The content of Se and Zn in the seed and grown crystals was determined by photoluminescence and energy dispersive analysis by x ray measurements. Resistivity and Zn concentration maps show a good spatial correlation in which the highest values of resistivity correspond to the areas of the highest Zn concentration. The main part of the crystals demonstrate a low extinction coefficient in the IR spectral region, except for the Zn inhomogeneity region.

Photodissociation from a manifold of rovibrational states and free–free absorptionby a diatomic molecule

An analytical approach for the description of photoabsorption by a gas or plasmamedium containing atomic and molecular components in thermodynamicequilibrium is developed. Continuous absorption of radiation is due tothe photodissociation of a diatomic molecule from a manifold of excitedrovibrational states and free–free transitions between the two electronicterms of a quasimolecule temporarily formed during a collision of atomicparticles. The formulae are obtained for individual photodissociation crosssections from a given rovibrational state and for the Boltzmann-averagedcross section. Particular attention is paid to the derivation of a generalanalytical expression for the total absorption coefficient including theintegral contribution of bound–free and free–free radiative transitions. Theconsideration is based on the theory of nonadiabatic transitions combinedwith the approximation of a quasicontinuum for rovibrational states. Thetheory is applied to the investigation of photoabsorption by the H2+ion in the IR, visible and UV spectral regions. It is shown that our results are ingood agreement with available ab initio quantal calculations of photodissociationcross sections and with semiclassical calculations of absorption coefficients. Specialattention is paid to the investigation of the relative contributions of the H2+and H− ions to the total absorption in a wide range of wavelengths andtemperatures.

Infrared absorption in dense sodium vapor

The absorption spectra of a dense resonance medium were experimentally studied for the example of thermally heated dense sodium vapor. Several mechanisms that might cause substantial absorption and enhanced intensity of emission in the IR spectral region, λ τ; 0.9 μm, were considered. For the first time, a detailed study of the structure of the absorption spectra of sodium vapor in the specified wavelength range was performed to determine the influence of the kind and pressure of the buffer gas. It was found that buffer gas characteristics had a substantial effect on the absorption coefficient of vapor. The presence of the molecular component (dimers and trimers) in sodium vapor could not explain the experimental dependences of absorption in the infrared region. Possible influence of microparticles formed in condensation of convective sodium vapor flows in heated cells on the optical properties of vapor was considered. Microparticles could contribute to the observed absorption, but were incapable of explaining the substantial intensity of vapor radiation reported earlier. Possible many-particle effects on the absorption in the far spectral line wing were discussed. For the first time, the method of molecular dynamics was used to show for the example of the distribution function of ionic microfields in a dense plasma that such effects were in principle capable of substantially raising the profile of the line and increasing absorption in the region of large detunings from the resonance compared with the simple quasi-static model in the nearest-neighbor approximation.

Retrieval of Heat Balance Components Using Remote Sensing Data

A method for retrieving heat balance (HB) components at the land surface is examined on the basis of data from simultaneous satellite measurements in the shortwave and IR-spectral regions ensuring determination of the temperature of the land surface and the shortwave radiation balance. A classification of the diurnal variation for the principal HB components is formulated on the basis of a fuzzy logic analysis of multiyear series of data from regularly scheduled heat balance observations made in the Russian Hydrometeorological Service network. The influence of wind speed and other meteorological factors on the accuracy in retrieving the evaporation rate is clarified.

Novel high-efficient materials for IR and red luminescence PbO–Bi 2O 3–Ga 2O 3–BaCl 2–Ho 3+ glasses

Large luminescence in the red and (IR) spectral region was found in the PbO–Bi 2O 3–Ga 2O 3–BaCl 2 (PBG) glasses doped by Ho 3+ ions at room temperature (RT). The mentioned glasses might be used as efficient materials for the luminescence in the red and IR spectral regions. The pulsed XeII laser light ( λ=714 nm) gives a substantial luminescence with quantum efficiency of about 16–25%. We have found that the electron–phonon subsystem plays a key role in the observed phenomena.

Effect of ruthenium doping on the optical and photorefractive properties of Bi 12TiO 20 single crystals

Bi 12TiO 20 (BTO) single crystals nominally pure and doped with ruthenium are grown by top-seeded solution growth method. The effect of ruthenium concentration on optical and photorefractive properties is studied. Strong influence of doping on these properties is observed. It is shown that optical transmission of crystal samples with higher ruthenium content is shifted to the near IR spectral region, while the absorption coefficient is considerably increased. The optical activity of Ru doped crystals is further reduced in comparison with undoped BTO. Photochromic effect is observed in strongly doped crystal. Photorefractive properties are experimentally investigated using two-beam coupling. Response time and diffraction efficiency are changed with ruthenium content.

Effective room-temperature broad-band recording of 3D dynamic holograms in CdF2:Ga crystals

Photochromic CdF2:Ga crystals with bistable impurity centers were effectively used for the dynamic recording of holograms and readout over the visible and near IR spectral regions at spatial frequencies of up to 5000 mm−1 at room temperature. The diffraction efficiency of the dynamic holograms was as high as 60% at maximum and exceeded 1% when the beams’ intensities were in the ratio 1:100. As one goes from the low temperatures (≤200 K) to 300 K, the peak diffraction efficiency of the dynamic holograms decreases approximately by a factor of 1.5, while the speed of their response and photosensitivity in the long-wavelength spectral region increases by more than an order of magnitude. For the sake of comparison, the dynamic holograms were recorded under the same conditions as the widely used electrooptical SBN crystals. Comparative analysis ascertained a unique combination of the useful features offered by CdF2:Ga crystals in holography.

Fluorescence and FT-IR spectroscopic studies of Suwannee river fulvic acid complexation with aluminum, terbium and calcium

In this study fluorescence emission and IR spectroscopy have been used to investigate the interaction of the class A (oxygen seeking ‘hard acid’) metal Al 3+, with Suwannee River fulvic acid. Addition of Al 3+ ion results in a significant enhancement in fulvic acid fluorescence emission (at λ em=424 nm) and significant red shift of the excitation wavelength (from λ ex=324 nm to λ ex=344 nm) at low pH values (pH∼4.0–5.0). At pH 4.0 (0.1 M ionic strength), where the predominant aluminum ion species is the ‘free’ (aquo) ion, the fulvic acid fluorescence reaches 142% of the value in the absence of added metal ion. Analysis of the pH 4.0 and pH 5.0 fluorescence enhancement data with the nonlinear (single site) model of Ryan and Weber indicated binding constants in the range of 4.67·10 4–2.87·10 6 M −1 and concentrations of ligand sites in the range of 18.6·10 −6–24.0·10 −6 M, both consistent with previous studies performed on both aquatic and soil fulvic acids. Companion fluorescence experiments performed on two other class A metal ions, Ca 2+ and Tb 3+, indicated no significant enhancement or quenching with Ca 2+ and only slight quenching with Tb 3+. Comparison of FT-IR spectra collected on fulvic acid alone and fulvic acid in the presence of the three class A metals (Al 3+, Ca 2+ and Tb 3+) provides strong evidence for the involvement of carboxyl carbonyl functions in the binding of all three metal ions, which is not unexpected. The spectra also reveal, however, a very pronounced difference in the 4000–2000 cm −1 IR spectral region between the Al 3+ spectrum and the Ca 2+ and Tb 3+ spectra. The –OH stretch spectral region in the Al 3+ spectrum has a major component shifted to higher energy (compared to fulvic acid alone or to fulvic acid in the presence of Ca 2+ or Tb 3+). Even more striking is the emergence of a pronounced IR band at 2407 cm −1, which is present only in the Al 3+ spectrum. The results of fluorescence and IR experiments with the model compounds salicylic acid and phthalic acid further confirm that both salicylic acid-like sites and phthalic acid-like sites are likely complexation sites for Al 3+ in fulvic acid and are major contributors to the observed spectroscopic changes associated with Al 3+ ion complexation. From a comparison of both the fluorescence and IR spectral results for all three class A metals, differing most strongly in the value of their ionic index, it seems clear that major sources of the deviation in spectral properties between Al 3+ and Ca 2+/Tb 3+ is the unusually high value of its charge density and relatively low propensity for involvement in covalent bonding interactions (very high ionic index and relatively low covalent index in the Nieboer and Richardson classification of environmental metals), as well as affinity for certain functional groups.

Characterization of highly absorbing black appliqués in the infrared

Angular and polarimetric directional hemispherical reflectance (DHR) measurements of three highly absorbing appliqués in the 2.5-14.3-microm IR spectral region are presented. In addition, scanning electron microscopy (SEM) images are presented to probe the surface morphology of these appliqués. DHR measurements of Energy Science Laboratory, Incorporated (ESLI), Rippey, and Rodel appliqués were obtained at incidence angles of 8 degrees, 30 degrees, and 50 degrees and as a function of S, P, and unpolarized incident light. The ESLI appliqué exhibited the lowest DHR for all angles and incident polarization states. SEM images revealed a fibrous structure for the ESLI appliqué with fiber diameters of the order of 7-8 microm, whereas the Rippey and the Rodel appliqués showed spongelike surfaces with pore diameters of 40-50 and 30-40 microm, respectively. All the appliqués use cavities or fibers in conjunction with carbon compounds to absorb IR radiation. The optical system designed to perform these measurements, a method for correcting off-normal DHR measurements in center-mount integrating spheres, and sources of measurement error are also discussed.

Role of Intermolecular Interactions in Forming the Vibrational Spectra of Carbohydrate Nitrate Crystals in the Region 1600–1700 cm−1

To elucidate the role of intermolecular interactions in forming the vibrational spectra of the crystals of carbohydrate nitrates in the region 1600–1700 cm−1, theoretical calculations are performed (in the approximation of an additive model of interatomic interactions with the use of the Green's functions) for the density of vibrational states and the intensity of absorption bands in the IR spectrum of a methyl-β-D-glucopyranoside tetranitrate molecule in a crystal. It is shown that the splitting of absorption bands in the IR spectral region under consideration relates to the crystal effect.

Fabrication of 3 D Photonic Crystals by Multiphoton Processes.

Photomodification of transparent dielectric materials, induced by tightly focused laser beams, can be utilized for the fabrication of 3 D photonic crystal (PhC) structures. The photomodification occurs in the focal region of the beam due to multiphoton absorption. Nonlinearity of this process allows to reduce the size of the photomodified region even below the diffractive limit of the focusing optics. We utilize two types of photomodification for the PhC fabrication: (i) damage in silica glass that leaves empty voxels, and (ii) polymerization in liquid resins that leaves solid voxels. By damaging/solidifying the initial materials at precisely controlled periodic locations, we have recorded 3 D PhC structures. Using the (i) process we have recorded PhCs in silica with 3 D fcc lattice types, which exhibit clear signatures of photonic bandgaps in the near IR spectral region. With (ii) process we have recorded log-pile PhC structures, which exhibit photonic bandgaps and microcavity effects in the same spectral region.

Sensitivity analysis of differential absorption lidar measurements in the mid-infrared region

The availability of new laser sources that are tunable in the IR spectral region opens new perspectives for differential absorption lidar (DIAL) measurements. A region of particular interest is located in the near IR, where some of the atmospheric pollutants have absorption lines that permit monitoring of emissions from industrial plants and in urban areas. In DIAL measurements, the absorption lines for the species to be measured must be carefully chosen to prevent interference from other molecules, to minimize the dependence of the absorption cross section on temperature, and to optimize the measurements with respect to the optical depth. We analyze the influence of these factors and discuss a set of criteria for selecting the best pairs of wavelengths (lambda(on) and lambda(off)) to be used in DIAL measurements of several molecular species (HCl, CO, CO(2), NO(2), CH(4), H(2)O, and O(2)). Moreover, a sensitivity study has been carried out for selected lines in three different regimes: clean air, urban polluted air, and emission from an incinerator stack.

Emission spectra of working mixtures of a HgBr/HgCl excimer lamp

A study of emission spectra of a gas-discharge plasma produced in a HgBr/HgCl excimer lamp, which is filled with multicomponent working mixtures at atmospheric pressure (HgBr2 and HgCl2 with additions of molecular nitrogen and xenon), are reported. A gas-discharge plasma was produced by high-frequency (pulses ∼100 ns long with a repetition rate of up to 2000 Hz) barrier and surface discharges, which took place simultaneously. Emission of HgBr* and HgCl* excimer molecules, the second positive system of molecular oxygen, and helium and xenon lines in the UV, visible, and IR spectral regions was observed. The strongest emission of HgBr* and HgCl* molecules (the emission intensities were in the ratio 10:1) was observed in the HgBr2: HgCl2: N2: He mixture. Regularities in spectral and integrated characteristics of gas-discharge plasma emission are discussed.

The effect of electron damage on silicon solar cells coated with diamond-like carbon films

The unique properties of the diamond-like carbon (a:DLC), such as high mechanical hardness and abrasive resistance, optical transparency in the visible and IR spectral regions and high thermal conductivity, provide this material with advantages over other types of protecting materials for solar cells. Furthermore, the a:DLC films are inert to corrosive gases and other corrosive agents. Resistance to radiation damage of the a:DLC films deposited on solar cells is very important for space application. In the study we investigate the effect of electron damage on silicon solar cells coated with a:DLC films. We measure the I – V characteristic and the spectral response and calculate the values of the seven parameters of the double exponential solar cell model (usually not investigated) as a function of electron fluence irradiation. In addition we obtain also the usual external parameters Isc, Voc, Im, Vm, FF, and efficiency) of the solar cells. We investigate solar cells with and without anti-reflecting coating coated with a:DLC films which were exposed to electron radiation. The main findings show that the solar cells with a:DLC films of thickness up to 500 nm degrade similarly to regular silicon cells exposed to electron irradiation. The degradation of the spectral response of the solar cell is mainly in the range of longer wavelengths and the irradiation affects the solar cell parameters (mainly the reverse saturation currents). Copyright © 2000 John Wiley & Sons, Ltd.

Ultrafast optical nonlinearity in the quasi-one-dimensional mott insulator Sr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching.

Photoinduced Charge Transfer between Tetracyano-Anthraquino-Dimethane Derivatives and Conjugated Polymers for Photovoltaics

The photoinduced charge transfer between tetracyano-anthraquino-dimethane (TCAQ) derivatives and poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO−PPV) has been studied by photoinduced absorption (PIA) spectroscopy in the VIS and IR spectral region and by light induced electron spin resonance (LESR). Studies on three different TCAQ derivatives with different side chain substitutions are reported. Among them a supermolecule with TCAQ attached to a fullerene have been synthesized to serve as tandem electron acceptor. The photoinduced absorption in the Vis/near-IR range shows a broad plateau around 1.8 eV followed by two peaks at 1.35 and 1.24 eV with an additional broad feature below 0.5 eV for all three acceptors in composite with MDMO−PPV. All PIA features have a power law excitation intensity dependence with an exponent close to 0.5 as expected for bimolecular kinetics. LESR studies show one absorption at a g-factor of 2.0028 with a width (peak-to-peak) of 3.5 G, originating from TCAQ anion radical and the polymer cation radical. Evidence for the formation of a C60 anion was found in the case of electron transfer from the MDMO−PPV polymer to the TCAQ−C60 acceptor dyade from these LESR studies. Finally bulk-heterojunction type thin film structures of MDMO−PPV as donor and the TCAQ derivatives as acceptor have been fabricated and tested as photovoltaic devices.

Formation and Reactions of Tetracarbonyl Intermediates of the Fischer Carbene Complex (CO)5WC(OMe)Ph. A Laser Flash Photolysis Study Using Time-Resolved Infrared and UV/Vis Spectroscopy

The results of nanosecond laser flash photolysis (XeCl excimer irradiation at 308 nm) of the Fischer carbene complex [(CO)5WC(OMe)Ph] (CZ) in di-n-butyl ether and n-hexane are reported. Spectrokinetical detection of the intermediates was based on transient absorptions simulaneously recorded in the UV/vis and the IR spectral regions. The primary photoproducts are the unstable isomer CE produced with a quantum yield of about 30% and a tetracarbonyl complex I−S, probably of structure [(CO)4(S)WC(OMe)Ph], where S is a solvent molecule. The quantum yield of CO photoelimination was estimated as 1.7%. The CE isomer was found to return to the stable CZ isomer within a few microseconds, confirming previous results of McGarvey and co-workers. The tetracarbonyl species reacts with cosolutes present in the solution such as N2, H2O, acetonitrile, and methyl trans-crotonate to form secondary complexes of cis-tetracarbonyl complex structure [(CO)4(L)WC(OMe)Ph] (I−L). The tetracarbonyl complexes react readily with CO. For I−N2 in n-hexane the rate constant is 3.5 × 108 M-1 s-1. The rate constants of the corresponding reactions with acetonitrile (MeCN) and the olefin are larger by a factor of 2. In the presence of CO the complexes I−MeCN and I−olefin revert to CZ in a dissociative reaction mechanism. The rate constants for the elimination of MeCN or olefin are 5 and 105 s-1, respectively. The I−olefin complex irreversibly decomposes with a rate constant of 8 × 103 s-1.

Study of optical constants of PbTe and GeTe films

Results of studying the absorption coefficients and refractive indices of PbTe and GeTe films in the spectral range from 1 to 15 μm are presented. The optical constants of films were found by a technique that combines calculation methods and the presence of strong absorption bands in the spectra. The spectral intervals were determined where the film-forming materials under consideration offer promise in the production of interference coatings for the IR spectral region.

Filtration of IR radiation by three-component polymer-crystalline systems

We have investigated the regularities in forming the spectral characteristic of three-component polymeric-crystalline polytetrafluorethylene-Ge-LiF interference systems in the wide IR spectral region 0.8–160 µm. The efficiency of suppressing short-wave radiation by structurally asymmetric imerference systems consisting of polytetrafluorethylene and quasihomogeneous layers with a high refractive index is shown. Two of this kind of three-layer systems are sufficient for suppressing background radiation within the wavelength interval from the visible range to ∼35 µm owing to multiple reflection at the boundaries of elementary Ge and LiF layers that form quasihomogeneous layers (λ ∼ 1.7–17.0 µm) and also due to absorption in the Ge (λ<1.7 µm) and Lif (λ ∼ 17–35 µm) layers.

Charge-transfer interactions in face-to-face porphyrin-fullerene systems: solvent-dependent luminescence in the infrared spectral region

The cyclophane-type molecular dyads 1 x 2H and 1 x Zn, in which a doubly bridged porphyrin donor adopts a close, tangential orientation relative to the surface of a fullerene acceptor, were prepared by Bingel macrocylization. The porphyrin derivatives 2 x 2H and 2 x Zn with two appended, singly linked C60 moieties were also formed as side products. NMR investigations revealed that the latter compounds strongly prefer conformations with one of the carbon spheres nesting on the porphyrin surface, thereby taking a similar orientation to that of the fullerene moiety in the doubly bridged systems. Cyclic voltammetric measurements showed that the mutual electronic effects exerted by the fullerene on the porphyrin and vice versa are only small in all four dyads, despite the close proximity of the donor and acceptor components. The steady-state and time-resolved absorption and luminescence properties of 1 x Zn and 2 x Zn were investigated in toluene solution and it was shown that, upon light excitation, both the porphyrin- and the fullerene-centered excited states are deactivated to a lower-lying CT state, emitting in the IR spectral region (lambda max = 890 and 800 nm at 298 and 77 K, respectively). In the more polar solvent benzonitrile, this CT state is still detected but, owing to its very low energy (below 1.4 eV), is not luminescent and shorter-lived than in toluene. The remarkable observation of similar photophysical behavior of 1 x Zn and 2 x Zn suggests that a tight donor-acceptor distance cannot only be established in doubly bridged cyclophane-type structures but also in singly bridged dyads, by taking advantage of favourable fullerene-porphyrin ground-state interactions.