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).     

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.     

IR spectroscopic manifestations of hydration of poly(diphenyl sulfophthalide) during its storage

IR spectroscopy measurements show that films of poly(diphenyl sulfophthalide) (PDSP), a cardo polymer, interact with atmospheric moisture during storage at room conditions. A total of 15 absorption bands were isolated in spectra of PDSP hydrated during storage, which belong to sorbed water and hydrolysis products. A number of absorption bands (within 1500–1800 cm⁻¹ and 980–1100 cm⁻¹) were obtained by subtracting the spectrum of the film after heating from that of the initial hydrated film. At least six individual bands in the region of the O-H bond stretching vibration were isolated by decomposing a broad complex band (3700–2000 cm⁻¹) into Gaussian components. The isolated bands were tentatively assigned based on the available literature data and quantum-chemical calculations of the characteristics of a number of complexes of a diphenyl sulfophthalide model compound with water molecules. The IR spectra and energies of the hydrogen bonds formed were calculated at the B3LYP/6-311G(d, p) level. In particular, the absorption bands at 1010 and 1079 cm⁻¹ were assigned to the symmetric stretching vibrations of the S=O bonds in the −SO₃⁻ anion, the 1062-cm⁻¹ absorption band, to ν(C-OH), and the absorption bands at 3646, 3586, and 3475 cm⁻¹, to complexes of water with sulfophthalide cycles of the polymer. After a long storage, PDSP largely transforms into a polymeric oxonium salt, and its spectrum becomes similar to that of a polymeric salt prepared by alkaline hydrolysis. A general mechanism of the interaction of PDSP with water is proposed, according to which the hydrolysis of the sulfophthalide cycles (SPC) by sorbed water yields new hydrophilic groups, sulfoacid, and hydroxyl groups. A further sorption of water by the sulfoacid results in its ionization and the formation of various hydroxonium forms. Sorption and hydrolysis are reversible processes: water is desorbed and the SPC is recovered when the polymer is heated to 100–150°C, as can be judged from an increase in the intensity of the S=O bond vibrations of the sulfophthalide cycle at 1352 and 1192 cm⁻¹. The possibility of using strongly hydrated PDSP for manufacturing proton-conducting membranes is discussed.     

Effect of a low-frequency magnetic field on the structure of globular blood proteins

We used IR Fourier absorption spectra of blood to study changes in the structure of globular blood proteins with extracorporeal autohemomagnetotherapy, used to treat ischemic heart disease. We compare the spectra of blood before and after magnetotherapy in the regions: amide I (1655 cm⁻¹), amide II (1545 cm⁻¹), amide III (1230–1350 cm⁻¹), amide IV and amide V (400–700 cm⁻¹). We have shown that pronounced changes in the spectra in the indicated regions on direct exposure of blood in vivo to a low-frequency pulsed magnetic field are connected with conformational changes in the secondary structure of globular blood proteins, which are apparent in the increase in the contribution of the α-helix conformation. We discuss the magnetotherapy-initiated appearance of new IR absorption bands at 1018 and 1038 cm⁻¹ and an increase in the intensity of a number of other bands located in the 1000–1200 cm⁻¹ region, which suggests a change in the concentration of some blood components. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 665–669, September–October, 2007.     

Spectral components that form UV absorption spectrum of Ce3+ and Ce(IV) valence states in matrix of photothermorefractive glasses

We have measured the UV absorption spectra of photothermorefractive glasses of the system Na₂O-ZnO-Al₂O₃-NaF-SiO₂ doped by cerium oxide in the range of (2.8–5.0) × 10⁴ cm⁻¹ (360–200 nm). The spectra have been processed by the method of dispersion analysis based on the analytical convolution model for the complex dielectric function of glasses. We show that the absorption band centered at 3.3 × 10⁴ cm⁻¹ (∼303 nm) that is attributed to the transition 2F _5/2 → 5d in the Ce³⁺ ion, is an envelope of three spectral components. The broad absorption range (3.5–4.7) × 10⁴ cm⁻¹ (200–270 nm) that is commonly interpreted as a charge transfer band of the Ce(IV) valence state, is an envelope of at least three spectral components.     

First experiments on field photodesorption IR spectroscopy

The ion current of adsorbed water molecules is studied experimentally as a function of the frequency of near-IR radiation incident on a surface at frequencies in the intrinsic vibrational bands of the water molecule. The ions are produced by nonequilibrium field surface ionization. The observed band (near one of the combination frequencies) has a width of 100 cm⁻¹ and is shifted relative to the free molecular band by 130 cm⁻¹. Estimates show that the cross section for absorption of the radiation by the adsorbed molecules is 3–4 orders of magnitude larger than for free molecules, as is typical of surface processes. Zh. Tekh. Fiz. 69, 123–127 (September 1999)     

Detection of <Emphasis Type="Italic">trans</Emphasis>-isomers of hydrocarbon residues of lipid molecules by IR absorption

IR spectroscopy is used for a comparative analysis of the trans-isomerization of double bonds in hydrocarbon residuals of lactic and hydrogenated lipids. The maximum of the absorption band of the trans-isomers for all the lipid samples is found to lie at 965 cm⁻¹. An absorption band at 970 cm⁻¹ is discovered in the spectra of the lactic lipids near the analytic band of the trans-isomers at 965 cm⁻¹. Based on a gaussian approximation for their absorption spectral bands, the trans-isomer content in the lactic lipid samples is 10–11%. The absorption by lipid molecules at 970 cm⁻¹ has to be taken into account when determining the trans-isomer content of fat and oil products. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 138–142, January–February, 2009.     

Transmission spectra of epitaxial layers of Pb1 ? xEuxTe (0 ≤ x ≤ 0.37) solid solutions in the frequency range 7–4000 cm?1

The spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.37) solid solution layers grown on BaF₂ and Si substrates have been investigated over a wide frequency range 7–4000 cm⁻¹ at temperatures of 5–300 K. Apart from the phonon and impurity absorption lines, the absorption in a local mode in PbEuTe layers of substrates and buffer layers has been observed in the frequency range 110–114 cm⁻¹. As the temperature decreases from 300 to 5 K, the transverse phonon mode softens from 33.0 to 19.5 cm⁻¹.     

IR absorption anisotropy in perylene-3,4,9,10-tetracarboxylic acid dianhydride

Vacuum-deposited perylene-3,4,9,10-tetracarboxylic acid dianhydride thin films are investigated by IR-spectropolarimetry with variable incident angle of plane-polarized radiation. Anisotropy of absorption bands in the region 700–900 cm⁻¹ is detected. Absorption bands in the subregions 728–736 and 806–810 cm⁻¹ are shown to be of triplet nature with orthogonally oriented dipole moments within the triplet.     

Infrared absorption spectra of pure and doped YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals

Several weak absorption bands have been observed in the optical absorption spectra of pure and rare-earth-doped YAl₃(BO₃)₄ single crystals in the 3350– 3650 cm⁻¹ wave number region. Two of them, peaking at about 3377 cm⁻¹ and 3580 cm⁻¹ in the 8 K spectra, appear in most of the samples. They are tentatively attributed to the stretching mode of OH⁻ ions incorporated in the crystal during the growth. An additional absorption band at about 5250 cm⁻¹ at 8 K has also been detected in almost all samples. The temperature and polarization dependences of these bands, and their possible origin, are discussed.     

QEPAS detector for rapid spectral measurements

A quartz enhanced photoacoustic spectroscopy sensor designed for fast response was used in combination with a pulsed external cavity quantum cascade laser to rapidly acquire gas absorption data over the 1196–1281 cm⁻¹ spectral range. The system was used to measure concentrations of water vapor, pentafluoroethane (freon-125), acetone, and ethanol both individually and in combined mixtures. The precision achieved for freon-125 concentration in a single 1.1 s long spectral scan is 13 ppbv.     

Continuous frequency tuning of a picosecond neodymium-glass laser in the UV and VUV

The results of an investigation of continuous frequency tuning of a neodymium laser in the UV and VUV ranges are reported. Generation of the sum frequency of second harmonic radiation and the radiation from a parametric light generator in the UV region (338–366 nm) is achieved. The optimal conditions for tuning UV radiation in the range 113.5–117.0 nm in third-harmonic generation processes in xenon and its mixtures with other gases are investigated. A third-harmonic generation efficiency of ∼5×10⁻⁴ and a tuning range >2600 cm⁻¹ are obtained in the VUV range investigated. Zh. Tekh. Fiz. 68, 82–89 (May 1998)     

Rapid and sensitive trace gas detection with continuous wave Optical Parametric Oscillator-based Wavelength Modulation Spectroscopy

A fiber-amplified Distributed Bragg Reflector diode laser is used to pump a continuous wave, singly resonant Optical Parametric Oscillator (OPO). The output radiation covers the 3–4 μm with ability of rapid (100 THz/s) and broad mode-hop-free tuning (5 cm⁻¹). Wavelength Modulation Spectroscopy is combined with the OPO to take optimal advantage of the spectral scan speed. The sensitivity of the system was determined as 0.8 ppbv (parts-per-billion by volume) for ethane (C₂H₆) for the absorption peak at 2996.9 cm⁻¹ recorded in 1.3 seconds, corresponding to a noise equivalent absorption sensitivity (NEAS) of 1.2×10⁻⁹ cm⁻¹/Hz^1/2. A comparison between results using the 1^st, 2^nd and 4^th harmonic derivative signal from wavelength modulation was performed. The broad continuous tunability was demonstrated by covering 35 cm⁻¹ while recording absorption features of ethane, methane and water.     

Analysis of experimental spectra andab initio calculations of water vapor

The results of ab initio calculations of water vapor absorption spectra obtained by Partridge and Schwenke are compared in detail with new experimental data. It is shown that the agreement between the calculated spectra of vibrational-rotational energy levels is better than Partridge and Schwenke pointed out in their pioneer paper, and the results of their calculations can be used directly for identification of lines in atmospheric spectra. At the same time, differences reaching 5cm ⁻¹ demonstrate that the potential energy function should be further refined. Institute of Atmospheric Optics of the SB RAS. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 93–101, March, 1999.     

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)     

Multiphonon absorption in a MgO single crystal in the terahertz range

The reflection and transmission spectra of a MgO single crystal at frequencies of 10–1000 cm⁻¹ in the temperature range 10–300 K have been measured using submillimeter and infrared Fourier spectros-copy. The evolution of anharmonic absorption with variations in temperature has been investigated in the frequency range below the frequency of the transverse optical phonon. The parameters of the dispersion model that adequately describes the lattice absorption of terahertz radiation in single-crystal magnesium oxide have been determined.     

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.     

The effect of electron irradiation on the optical properties of the natural armenian zeolite-clinoptilolite

Infrared (IR) absorption and luminescence in chemically and radiation-modified natural Armenian Zeolite (clinoptilolite) samples have been studied. The luminescence was studied in 390–450 nm and 620–710 nm wavelength bands, and the IR measurements were carried out in the 400–5400 cm⁻¹ range. It is shown that the luminescence intensity depends on the content of pure clinoptilolite in the samples and, probably on the distribution of “passive” luminescence centers over Si and Al sites that became “active” under radiation or chemical treatment. The samples of electron irradiated clinoptilolite have higher luminescence intensity than the chemically and thermally treated ones. A decrease in the intensity of IR absorption bands at 3550 cm⁻¹ and 3650 cm⁻¹ was found after irradiation.     

Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters

The x-ray spectra of a plasma generated by heating CO₂ and Ar clusters with high-intensity femtosecond laser pulses with q _las≃10¹⁸ W/cm² are investigated. Spatially resolved x-ray spectra of a cluster plasma are obtained for the first time. Photoionization absorption is observed to influence the spectral line profiles. The recorded features of the x-ray emission spectra definitely indicate the existence of a large relative number of excited ions (≃10⁻²–10⁻³) with energies of 0.1–1 MeV in such a plasma. Possible mechanisms underlying the acceleration of ions to high energies are discussed. It is shown that the experimental results can be attributed to the influence of ponderomotive forces in standing waves generated by the reflection of laser radiation from the clusters. Zh. éksp. Teor. Fiz. 115, 2051–2066 (June 1999)     

Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10⁻⁸ cm⁻¹ W/Hz^1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.