New assignments in the submillimeter emission spectrum of the stratosphere

New line assignments in the spectrum of the stratospheric submillimeter emission, measured with unapodized resolution of 0.0033 cm^–1, have been made. Positive evidence for the presence of symmetric and asymmetric ozone isotopes, water vapour excited to the (010) level, and HCN is given.     

Comparison of measured and calculated high-resolution spectra of far-infra-red stratospheric emission

Summary Far-infra-red stratospheric emission spectra obtained from a balloon-borne platform at 38 km altitude with a spectral resolution of 0.0033 cm⁻¹ are compared with calculated spectra obtained with a radiative-transfer model. The overall agreement between the model and the measurements shows that a good understanding of this spectral interval has been obtained. Differences larger than the measurement error are observed in a few cases. The possible causes of these differences and the work that is needed in order to improve the data base of the model are discussed. Paper presented at the 1^o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Measurements of the temperature and water vapor concentration in a hot zone by tunable diode laser absorption spectrometry

A tunable diode laser absorption spectroscopy (TDLAS) technique and appropriate instrumentation was developed for the measurement of temperature and water vapor concentrations in heated gases. The technique is based on the detection of the spectra of H₂O absorption lines with different energies of low levels. The following absorption lines of H₂O were used: 7189.344 cm⁻¹ (E″=142 cm⁻¹), 7189.541 cm⁻¹ (E″=1255 cm⁻¹), 7189.715 cm⁻¹ (E″=2005 cm⁻¹). Spectra were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. A unique differential scheme for the recording of the absorption spectra was developed. An optimal technique for fitting the experimental spectra was developed.     

On the effect of ultraviolet radiation on water vapor absorption of submillimeter waves

The submillimeter absorption spectra of pure water vapor and a water vapor + dry air mixture are experimentally studied under the conditions of illumination of the gas sample by ultraviolet (UV) radiation. The measurements were carried out by a vacuum echelette spectrometer in the wave number range 21.5–56 cm⁻¹ with spectral resolution 0.4–0.9 cm⁻¹, using a DRT-375 mercury-vapor discharge lamp as the source of UV radiation. In contrast to the results of similar experiments performed by other researchers, the data presented here demonstrate the absence of a noticeable effect of the UV radiation on the absorption spectra of the gas samples used. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 5, pp. 581–587, May, 1998.     

Dielectric spectra of Bi<Subscript>0.98</Subscript>Nd<Subscript>0.02</Subscript>FeO<Subscript>3.00</Subscript> multiferroic thin films in the terahertz frequency range

Transmission and reflection spectra of Bi_0.98Nd_0.02FeO_3.00 multiferroic thin films on MgO single-crystal substrates have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 8 to 1000 cm⁻¹ at room temperature. The complex permittivity spectra of the films have been calculated in terms of the layered medium model. It has been revealed that a decrease in the film thickness leads to a considerable increase in the losses in a range of 30 cm⁻¹ and the corresponding fivefold increase in the static permittivity (to 500 for a film 32 nm thick). This phenomenon has been discussed in the framework of the phenomenological theory of phase transitions.     

Polarised raman and infrared spectra and vibrational analysis forα-naphthylamine

The Raman spectrum of polycrystalline α-naphthylamine was recorded in the region 100–4000 cm⁻¹. Polarisation measurements were made in CS₂ and CHCl₃ solutions. The infrared spectrum was recorded in nujol mull in the region 200–4000 cm⁻¹. The resolution was better than 2 cm⁻¹ and the accuracy of the measurements was within ± 2 cm⁻¹ for all the spectra. Vibrational assignments have been proposed for the observed frequencies. Out of the 54 normal modes of vibrations, 51 modes could be observed experimentally.     

Optical properties of BiFeO<Subscript>3</Subscript> ceramics in the frequency range 0.3–30.0 THz

Reflection and transmission infrared spectra of BiFeO₃ ceramic samples have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 5 to 1000 cm⁻¹ at temperatures in the range from 10 to 500 K. New resonant modes (probably, magnetic in nature) with the eigenfrequencies decreasing with an increase in the temperature have been recorded in the range 10–30 cm⁻¹ by IR spectroscopy for the first time. An additional absorption with a fairly large dielectric contribution has been revealed in the range 30–60 cm⁻¹. It has been demonstrated that the corresponding oscillators couple with both the lowest frequency phonon mode and the magnetic subsystem.     

A spectroscopic database for water vapor adapted to spectral properties at high temperature, and moderate resolution

The prediction of infrared spectral radiance from high temperature media such as combustion gases requires spectroscopic data for triatomic molecules like water vapor and carbone dioxyde. At temperature above 2000 K, water vapor spectrum is composed of hundreds of thousands lines making practical computations uneasy. We have set up a spectroscopic database for water vapor, based on three existing lines compilations. This database is well suited to computation of remote sensing spectra where hot gases emission is seen through atmospheric paths. The database enables efficient computation of water vapor spectra between 600 and 6600 cm⁻¹ at moderate spectral resolution (5 cm⁻¹). It has been used to compute parameters of a statistical narrow band model which are used in practical applications.     

High resolution,rotation, vibration spectra of D2O in the region ofν 1 andν 3 bands

The infrared spectrum of D₂O from 2400 to 3000 cm⁻¹ has been analysed at a resolution better than 0·02 cm⁻¹. It was recorded at the Fourier transform spectrometer of the Kitt Peak National Observatory, Tucson. Ground state constants of the reduced Watson-Hamiltonian, ground state energies and transition for theν ₁ andν ₃ bands are reported. The effect of strong Coriolis resonances on the spectra is discussed. Work done at the Jet Propulsion Laboratory, Division of Stratospheric and Planetary Research California Institute of Technology, Pasadena, Ca (U.S.A.)     

Telecom-grade fiber laser-based difference-frequency generation and ppb-level detection of benzene vapor in air around 3 μm

We report on the development of a field deployable compact laser instrument tunable over ∼232 cm⁻¹ from 3.16 to 3.41 μm (2932.5–3164.5 cm⁻¹) for chemical species monitoring at the ppb-level. The laser instrument is based on widely tunable continuous-wave difference-frequency generation (DFG), pumped by two telecom-grade fiber lasers. DFG power of ∼0.3 mW near 3.3 μm with a spectral purity of ∼3.3 MHz was achieved by using moderate pumping powers: 408 mW at 1062 nm and 636 mW at 1570 nm. Spectroscopic performance of the developed DFG-based instrument was evaluated with direct absorption spectra of ethylene at 3.23 μm (∼3094.31 cm⁻¹). Absorption spectra of vapor-phase benzene near 3.28 μm (∼3043.82 cm⁻¹) were recorded with Doppler-limited resolution. Line intensities of the most intense absorption lines of the ν ₁₂ band near 3043.8 cm⁻¹ were determined to support development of sensitive mid-infrared trace gas detection of benzene vapor in the atmosphere. Detection of benzene vapor in air at different concentration levels has been performed for the first time using multi-pass cell enhanced direct absorption spectroscopy at ∼3.28 μm with a minimum detectable concentration of 50 ppb (1σ).     

Application of 940 nm high-power DFB lasers for line-broadening measurements at normal pressure using a robust and compact setup

High-power distributed-feedback (DFB) lasers for the wavelength range near 940 nm (i.e. about 10,600 cm⁻¹) were used for line-broadening measurements of individual rotational-vibrational absorption lines of water vapour at atmospheric pressure using a minimalist set-up. The laser has a maximum output power larger than 500 mW. Over the whole power range from threshold to maximum power, it operates in single mode operation with a tuning range of 4.7 nm, i.e. 50 cm⁻¹, at 20°C. With an emission line-width ≤2 MHz (0.66×10⁻⁴ cm⁻¹), the device is well suited for high-resolution spectroscopy.     

Twisted lines and small-scale structures in generation spectra of Yb-doped media excited by focused radiation of LiF:F 2 + color center laser

Spectral lines of Yb lasing in 1.03–1.05 μm region structured by 50–200 μm spots were found at focusing a pulsed LiF:F₂⁺ color center laser of 0.5–5.0 GW/cm² intensity on highly doped Yb:YAG or Yb:glass plates in a resonator. Small spots at the spectrograph located ≈ 1 m apart from the resonator indicated a “sub-diffraction” directivity of Yb generation, 1–2 orders better than the diffraction limit 10⁻³–10⁻² rad determined by the pumped volume dimensions. Observed features of Yb emission are explained assuming off-axis oscillations in Yb laser on phase-synchronized photons due to a strong spatial-angular selection of radiation in the resonator. Propagation of near diffraction free beams at angles to the axis built at the spectrograph slit for every 10–15 ns pulse of Yb generation a magnified “image” of a structure of generating channels in the active medium. This image projection brought a corresponding structure of spots in Yb spectra. It was found that channels may be formed due to a high-frequency spatial modulation (micrometers scale) of the refractive index profile in samples caused by the oscillating amplitude of thermoelastic stresses in the pumped medium. Obtained data demonstrate a possibility to study (with high spatial and temporal resolution) non-equilibrium stales of materials in small volumes using laser radiation emerging from these objects. This study results evidence for the novel concept of the spatial distribution of electromagnetic field of a photon: not in the form of a “travelling” wave but in the form of a wave with maxima and nodes located at fixed positions along the photon propagation direction.     

Synthesis of w-CdS quantum dots and discovery of intense sub band emission owing to longitudinal optical phonons

Cadmium sulfide (CdS) quantum dots (QDs), capped with cetyltrimethylammonium bromide (CTAB), and was synthesized as stable, aqueous, colloidal nanofluid. A series of nine intense, well-resolved emission lines between 400 and 750 nm were observed for the first time when exciting the CdS QDs nanofluid with a 355-nm high energy pulsed Nd:YAG laser radiation. The energy separation between any two successive emission lines equals to the characteristic overtone energy of 295 cm⁻¹ of the longitudinal optical phonon of CdS QDs. In addition, recording the PL spectrum by using a xenon broad band light source resulted in the observation of this characteristic overtone energy of 295 cm⁻¹. In agreement with this photoluminescence characteristic, Raman spectrum exhibited four prominent Stokes lines with Raman shift equal to and multiple of 295 cm⁻¹. Transmission electron microscopy investigation showed that the CdS QDs were spherical with hexagonal wurtzite structure and had a size in the range of 5–10 nm.     

Far infrared emission spectra from stratospheric hydrogen peroxide

Synthetic emission spectra from two stratospheric altitude observations have been analyzed for the presence of H₂O₂ in the far infrared region. The calculations are made with a high spectral resolution (10^–3 cm^–1 or 10^–4 cm^–1) greater than those in experimental measurements which are in the region of 3.10^–3 cm^–1. Spectra cover a spectral interval between 40 and 120 cm^–1 showing the best features of H₂O₂ susceptible to observation in a stratospheric spectrum. The optimum conditions for identification have been considered. Using the variations in H₂O₂ abundance in the measurement data and photochemical models, the H₂O₂ features detection limits have been studied.     

Raman scattering of light and IR absorption in carbon nanotubes

Raman light scattering and IR absorption spectra of samples containing multilayer carbon nanotubes in different stages of purification by the selective oxidation technique have been investigated. It was found that the Raman spectra of carbon nanotubes exhibit softening of the mode at 1582 cm⁻¹ corresponding to E _2g vibrations of graphite hexagons and a line at 120 cm⁻¹ due to the radial vibrations of nanotubes. In IR absorption spectra measured in the region of 0.07–0.3 eV, several sets of lines with a spacing of 15 meV (120 cm⁻¹) between lines of each group have been detected. We suggest that each group corresponds to electron transitions generating electron-hole pairs in semiconducting nanotubes and contains a phononless 00-line and its phonon replicas with spacing between them equal to the “breathing” mode energy of 120 cm⁻¹. Measurements of electric conductivity at a frequency of 9300 MHz indicate that, in addition to semiconducting nanotubes, the samples contain nanotubes with properties of a highly disordered semimetal. Zh. éksp. Teor. Fiz. 113, 1883–1891 (May 1998)     

Variation in the fluorescent background in Raman spectra of distilled water purified by different methods

The contribution of impurity fluorescence was determined in the water Raman spectra excited by the second harmonic (λ = 532 nm) of a pulsed Nd:YAG laser. Water samples prepared by different techniques (tap water (undistilled), distilled water, Milli-Q water, water for injections, and water subjected to cavitation treatment) were investigated. The Raman (bands at ν ₂ ∼ 1550 cm⁻¹ and ν ₃ ∼ 3400 cm⁻¹) and fluorescence (Stokes shift 2500 cm⁻¹) signals were separated spectrally and according to the differences in the emission kinetics. It was established that all investigated samples, including distilled and specially purified water for injections, exhibit afterglow. The highest sensitivity to the presence of impurities was revealed near ∼ 2500 cm⁻¹. The least contribution to the fluorescence signal was found in the water for injections.     

Analysis of trace impurities in semiconductor gas via cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) has demonstrated powerful potential for trace-gas detection based on its unique combination of high bandwidth, rapid data acquisition, high sensitivity, and high resolution, which is unavailable with conventional systems. However, previous demonstrations have been limited to proof-of-principle experiments or studies of fundamental laboratory science. Here, we present the development of CE-DFCS towards an industrial application—measuring impurities in arsine, an important process gas used in III–V semiconductor compound manufacturing. A strongly absorbing background gas with an extremely complex, congested, and broadband spectrum renders trace detection exceptionally difficult, but the capabilities of CE-DFCS overcome this challenge and make it possible to identify and quantify multiple spectral lines associated with water impurities. Further, frequency combs allow easy access to new spectral regions via efficient nonlinear optical processes. Here, we demonstrate detection of multiple potential impurities across 1.75–1.95 μm (5710–5130 cm⁻¹), with a single-channel detection sensitivity (simultaneously over 2000 channels) of ∼4×10⁻⁸ cm⁻¹ Hz^−1/2 in nitrogen and, specifically, an absorption sensitivity of ∼4×10⁻⁷ cm⁻¹ Hz^−1/2 for trace water doped in arsine.     

Optimally chirped CARS using fiber stretchers

High spectral resolution coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy are demonstrated with femtosecond laser systems. We perform optimal chirping in glass fibers and demonstrate a spectral resolution enhancement to better than 26 cm⁻¹, which is limited by the bandwidth of the measured resonances. Considering the convolution with the resonance bandwidth this corresponds to a spectral resolution of approximately 2.5 cm⁻¹, which is an enhancement by a factor of 165 with respect to the use of bandwidth-limited pulses. In microscopic imaging, a water background suppression of 81.5% is achieved.     

Fluorescent properties and spontaneous Raman spectroscopy of new ketocyanine probes in organic solvents

We have used fluorescence spectroscopy and spontaneous Raman spectroscopy to study the characteristics of two ketocyanine dyes: 2,5-di[(E)-1-(4-diethylaminophenyl)methylidene]-1-cyclopentanone (CPET) and 2-[(E)-1-(4-diethylaminophenyl)methylidene]-5-{(E)-1-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecalin) phenyl]methylidene}-1-cyclopentanone (CPMR) in organic solvents. The position of their electronic spectra depends strongly on the polarity of the solvent. We measured the dipole moments of the dyes in the equilibrium ground state and the Franck-Condon excited state. In mixtures of neutral nonpolar toluene with aprotic polar dimethylsulfoxide, we observe inhomogeneous broadening of the electronic spectra for the indicated compounds, due to fluctuations in solution of the intermolecular interaction energy. The time-resolved characteristics of fluorescence obtained suggest formation of an intermolecular hydrogen bond between the dye and the surrounding medium in a toluene-ethanol mixture. We measured the Raman spectra of CPET and CPMR in different organic solvents. The most intense lines in the 1582–1591 cm⁻¹ region can be assigned to stretching of the phenyl rings of the molecules; the lines in the 831–842 cm⁻¹ region can be assigned to a cyclopentanone ring mode; the lines at 1186–1195 cm⁻¹ can be assigned to stretching of the =C-C-bond of the phenyl ring and rocking of the H atoms of the phenyl ring. We have observed that the position and width of the lines for the stretching vibrations of the ketocyanines depend substantially on the polarity of the surrounding medium. The studied dyes can be used as probes for studying different biological systems by site-selective laser spectroscopy and Raman spectroscopy. The fact that these two methods can be used simultaneously for diagnostics of biosystems is an important advantage of ketocyanine dyes compared with other known probes. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 763–769, November–December, 2006.     

Coherent spectroscopy of Mandelstam-Brillouin scattering in spatially inhomogeneous media

Mandelstam-Brillouin (MB) steady-state scattering in an elastic medium with a dense local zone inhomogeneity is considered in the 1D approximation. It is shown that for a certain size of inhomogeneity, the scattered radiation spectrum contains individual resonances whose frequencies depend on the elastic properties of microscopic inclusions. Experiments were performed using coherent four-photon scattering spectroscopy in the range 0–1 cm⁻¹ with a resolution of 0.06 cm⁻¹ in specially processed distilled water and in an aqueous solution of α-chymotrypsin albumin. In both media, the presence of MB resonances displaced is detected relative to the water resonance (≈0.25 cm⁻¹) in different directions and corresponding to different types of microinclusions.