Stratospheric temperature profile from balloon-borne measurements of the 10.4-μm band of CO2

The technique of nonlinear least squares spectral curve fitting has been used to derive the stratospheric vertical temperature profile from balloon-borne measurements of the 10.4-μm band of CO₂. The spectral data were obtained at sunset with the ?0.02 cm^-1 resolution University of Denver interferometer system from a float altitude of 33.5 km near Alamogordo, New Mexico, on 23 March 1981. The r.m.s. deviation between the retrieved temperature profile and correlative radiosonde measurements is 2.2 K.     

Photovoltaic effect in cuprous oxide-copper junctions in relation to the optical absorption spectrum of cuprous oxide

The spectral dependence of the photovoltaic effect (PVE) of Cu₂O-Cu cells, annealed in three distinct ways, was measured. The copper contact was formed by high temperature reduction of the surface of the oxide. The measurements were made for different thicknesses of oxide, and with front wall and back wall illumination. Two types of absorption constant were calculated from PVE measurements and compared with directly measured absorption constants from transmission data. The results allow us to distinguish between electronically active absorption near the bandgap and electronically inactive absorption at longer wavelengths.     

Collision-induced absorption of O2 in the Herzberg continuum

Using a one meter normal incidence grating monochromator, the total absorption of radiation for O₂ has been reinvestigated between 2000–2500 Å. The absorption cross sections of O₂ and O₄ are then derived from total absorption measurements and compared with other available data. Additional measurements of O₂ absorption have been made in the presence of Ar which does not absorb in this spectral region and is chemically inactive with O₂. It is found that the absorption of oxygen increases in the presence of Ar, which is interpreted as due to the formation of O₂-Ar dimers. The significance of these measurements is discussed.     

Periodic variations of plasma optical emission during repetitive pulsed-laser irradiation of aluminum in ambient air

Optical emission from plasma produced in ambient air by focusing a Nd:YAG laser beam on an aluminum surface was spectrally analysed. A periodic behaviour was observed in spectral line intensities associated with series of laser pulses. Based on simultaneous measurements of diffuse surface reflectivity and complementary measurements in other gases (He, O₂, N₂), this behaviour is ascribed to a competition between thermally-assisted surface oxidation and nitridation, and laser ablation. PACS 52.38.Mf; 52.50.Jm; 81.65; 78.68.+m; 52.70.Kz     

Photoabsorption cross section measurements of CO2 between 106.1 and 118.7 nm at 295 and 195 K

High-resolution vacuum ultraviolet carbon dioxide photoabsorption cross sections are required for modeling airglow emissions from the Martian and Venusian atmospheres and for photochemical models of those atmospheres. We report cross section measurements on CO₂ at 295 and 195 K between 106.1 and 118.7 nm at a spectral resolution of 0.005 nm. Significant deviations from the results of previous, lower-resolution measurements are apparent, particularly in regions of sharp spectral structure. Temperature dependence in the photoabsorption cross sections is evident throughout the region, being especially pronounced at wavelengths greater than 113 nm.     

A method for retrieval of the profiles of atmospheric backscattering coefficients by three-wavelength lidar and nephelometric measurements

A method for retrieval of the profiles of atmospheric aerosol backscattering coefficients β _a along horizontal and inclined paths from the results of three-wavelength probing by laser location systems based on a Nd: YAG laser is proposed. The method uses a power dependence between the spectral values of β _a and includes spectral nephelometric measurements of the scattering coefficients by an angle of 33°, which characterize the relative spectral dependence of aerosol extinction. The sensitivity of this method to spatial variations in the above coefficients, to the deviation of β _a from the dependence used, and to errors in calibration and measurement of signals is studied. The method is demonstrated to be stable both upon using the absolute calibration of the lidar and at an arbitrary position of the reference point.     

Overlapping spectral features and new assignment of 2‐propanol in the C–H stretching region

The vibrational spectra of gaseous and liquid 2‐propanol in the C–H stretching region of 2800 ~ 3100 cm⁻¹ were investigated by polarized photoacoustic Raman spectroscopy and conventional Raman spectroscopy, respectively. Using two deuterated samples, that is, CH₃CDOHCH₃ and CD₃CHOHCD₃, the overlapping spectral features between the CH and CH₃ groups were identified. With the aid of depolarization ratio measurements and density functional theory calculations, a new spectral assignment was presented. In the gas phase, the band at 2884 cm⁻¹ was assigned to the overlapping of one CH₃ Fermi resonance mode and a CH stretching of gauche conformer. The bands at 2917 and 2933 cm⁻¹ were assigned to another two CH₃ Fermi resonance modes, but the latter includes weak contribution from CH stretching of trans conformer. The bands at 2950 and 2983 cm⁻¹ were assigned to CH₃ symmetric and antisymmetric stretching, respectively. The spectral features of liquid 2‐propanol are similar to those in the gas phase except for the blue shift of CH and the red shift of CH₃ band positions, which can be attributed to the intermolecular interaction in the liquid state. The new assignments not only clarify the confusions in previous studies from different spectral methods but also provide the reliable groundwork on spectral application of 2‐propanol in the futures. Copyright © 2014 John Wiley & Sons, Ltd.     

Measurements of the electronic transition moment of the A2Π−X2Π system of ClO

The electronic transition moment of the A²Π?X²Π band system of ClO has been determined from spectral emission measurements behind incident shock waves in Cl₂-O₂ mixtures. The temperature of the radiating gas was typically 3500°K. Narrow bandpass radiometers were used to obtain absolute measurements of the ClO emission from which electronic transition moments were derived by a synthetic spectrum analysis. Absolute measurements of the O₂ Schumann-Runge system and the photo-dissociation cross section of ClO are also reported.     

Shock-tube measurements of the band strengths of HBO2 and OBF in the short wavelength infrared

Shock-tube measurements are described for the quantitative spectral measurement of the i.r. signatures and band strengths of molecular boron species. The boron species were effectively isolated under conditions of optimum concentration and calculated thermodynamic conditions using the shock tube technique. Spectral measurements were performed using a scanning i.r. spectrometer which was specially designed for these experiments. System calibration experiments were performed using shock heated CO and CO₂ mixtures in diluent argon, and the measured band strengths were in excellent agreement with the accepted values for these molecules. Quantitative spectral measurements were performed on the HBO₂ fundamentals at 2.7 and 5.0 μm and OBF at 5.0 μm. The corresponding band strength values measured for these band systems were 650, 1375, and 1760 cm^-2 amagat^-1, respectively.     

Performance of photon drag and photon drag-optical rectification detectors in the 2 to 33 μm range

Previous studies of the spectral response of p-Ge photon drag and n-Gap photon drag-optical rectification detectors have been extended. Measurements have also been made of the spectral response of p-Si photon drag, p-GaP photon drag-optical rectification and p-GaAs photon drag-optical rectification detectors. The wavelength range of main interest is between 2 and 11 μm, (HF, HCl, HBr, H₂O vapour and CO₂ lasers), but some measurements have also been made at 28 and 33 μm, (H₂O vapour laser).     

The influence of coherence effects on measurements of saturable absorption with broadband radiation

For a gaussian broadband radiation field (multimode laser without phase-correlation) the bleaching behaviour of a homogeneously broadened two level system is investigated in dependence on the spectral width (δυ) of the field. Taking into consideration the coherent mode-interaction, originating from the nonstationary reaction of population inversion on the fluctuations of the radiation field, a decrease of the bleaching effect results as soon as the spectral width δυ becomes larger than ⊥₁ = 1/T₁ with T₁ the longitudinal relaxation time of the molecular transition concerned. Neglecting this effect in the analysis of broadband bleaching measurements can result in the unjustified assumption of an inhomogeneous broadening of the spectral band under consideration, which has consequences on the magnitude of the molecular absorption cross-section derived from these measurements.     

Photovoltage measurements of the energy gap in Cd1−xMnxTe

Surface photovoltage spectroscopy measurements in the spectral regions close to the energy gap have been performed for Cd_1?xMn_xTe single crystals with different compositions, in the range between room and liquid nitrogen temperatures. The dependence of the energy gap on the composition and temperature is linear.     

Effect of changes of the HITRAN database on transmittance calculations in the near-infrared region

In order to retrieve from high spectral resolution measurements with high accuracy, it is necessary to be able to evaluate the transmittance precisely. However, the uncertainty of the spectroscopic parameters is one of the most important contributions that affect the accuracy of transmittance. HITRAN is a compilation of spectroscopic parameters which has been updated several times. The transmittance calculations using the line parameters from the HITRAN’2000 database and the HITRAN’2004 database have been compared over the near infrared range from 4200 to 10,000 cm^?1. The differences between calculated transmittances over this spectral range are mainly caused by changes of the line parameters for H₂O, CO₂ and CH₄. For the tropical atmosphere, the differences are very prominent. Transmittance calculations for the sub-arctic winter atmosphere are less sensitive to the changes in the HITRAN database than those for the tropical atmosphere; but, the changes of line parameters still can not be ignored when considering the relative differences. For example, the relative difference is ～35% at 5073.3 cm^?1 with 0.2 cm^?1 spectral resolution. The comparisons have shown that it is important to pay attention to the changes of line parameters of the HITRAN database or to use the latest edition so as to improve the accuracy of atmospheric sounding with high spectral resolution.     

Vibrational moments of fundamental transitions in low-temperature molecular liquids: Determination from the band shape of vibrations in the overtone spectral range

It is shown that the integrated absorption coefficients of strong fundamental bands of some gaseous substances can be determined with an accuracy of 5–10% using experimental data on the spectral moments of bands in the overtone spectral range of these substances in the liquid phase near the melting point. The absorption coefficients are calculated within the framework of the cell model of the liquid state from the contributions of the resonance dipole-dipole interaction to the second spectral moment. The combination and overtone absorption bands of the SF₆, CF₄, SiF₄, NF₃, CHF₃, CC1F₃, CBrF₃, OCS, N₂O, and CO₂ molecules in the liquid state and in solutions in liquefied Xe, Kr, and O₂ are recorded. The data in the literature on the intensity of strong IR bands in the gas phase are analyzed. Using an improved experimental technique, additional measurements are performed. It is found that the absorption coefficients determined from the spectral characteristics of the liquids agree well with the coefficients measured in the gas phase.     

Light transmission through polyethylene samples

A spectral calibration Hg(Ar) lamp and a pulsed Nd:YAG laser were employed to study the UV, VIS and IR light transmission through polyethylene samples, as pure and doped with Fe₂O₃ and carbon nanotubes. Optical measurements were performed in the spectral range between 200 nm and 800 nm. Measurements with 0.1% and 1% in weight of filler concentrations were investigated. The polymers' optical absorption coefficients, calculated from the experimental transmitted component, were tabulated at different wavelengths as a function of the analyzed samples. The radiation depth penetration was also evaluated.     

A quantitative approach to evaluate the problem of coherence of spectral components of the third‐order susceptibility generating coherent anti‐Stokes Raman signals

Precise interpretation of spectral measurements is central to the development of the full extent of the applicative potential of coherent anti‐Stokes Raman spectroscopy (CARS). One recognized problem that jeopardizes the achievement of high precision is the determination of the best spectral convolution over the relevant bandwidths when degeneracy of laser frequencies is involved. Although the analytical solutions of CARS signals generated by pump and Stokes lasers with standard (i.e. Gaussian or Lorentzian) lineshapes are well known, research in this field has overlooked the criterion on how to discern coherence between spectral components of the third‐order nonlinear susceptibility. Understandably, the ordinary approach is based on an intuitive comparison between the spectral width σ₁ of the pump laser with respect to the width Γ of the relevant Raman transitions. More precisely, if σ₁ ≪ Γ, then the spectral synthesis can be obtained in the limit of narrowband pump; otherwise spectral coherence has to be included in the calculation leading to problematic spectral analysis. In an attempt to clarify this qualitative criterion better, the present work demonstrates that the limit between the two opposite regimes can have a clearer and neater definition than that accepted so far. In this case, this paper shows that for nonoverlapping Raman transitions determined by a Lorentzian susceptibility, the issue is governed by the analytic function erfc(Γ/σ₁)/σ₁, which depends uniquely on the ratio Γ/σ₁. The unitary limit of this function for σ₁ ≪ Γ justifies the incoherent or the narrowband‐pump approach. Copyright © 2006 John Wiley & Sons, Ltd.     

Investigation on infrared laser absorption spectroscopy measurement of acetylene trace quantities

A laser-based infrared spectrometer was developed for use in high-resolution spectroscopic analysis of trace gases in the atmosphere. Continuous wave, broadly tunable coherent infrared radiation was generated from 8 to 19 μm in a gallium selenide crystal by laser difference-frequency mixing. Measurements of acetylene trace concentrations using laser absorption spectroscopy are reported. The measurements have been performed by using the P(21), Q(11), and R(9) lines of the ν₅ band, respectively, in order to investigate optimal detection conditions: a trade-off choice between higher line absorption strength for sensitive detection and better spectral discrimination from lines overlapping for open path trace-gas monitoring applications. Minimum detectable concentrations are compared under different experimental conditions. The ν₅ R(9) line seems to be close to optimum in terms of absorption strength and freedom from spectral interference for spectroscopic detection of acetylene trace concentration at atmospheric pressure.     

Foreign gas broadening of an IR absorption line of formaldehyde

The theory of nuclear spin state relaxation of symmetrical molecules like formaldehyde contains a collision time t_c that is interpreted as time between rotationally inelastic molecular collisions. This time traditionally is determined from measurements of pressure broadening of spectral lines. In order to test whether these collision times, which determine spin relaxation rate constants and line broadening coefficients, respectively, are the same or at least related to another, we have performed systematic measurements of the broadening of an IR line of formaldehyde by other gases of different pressures.     

Far infrared and millimetre dielectric response of incommensurate and ferroelectric K2SeO4

Dielectric response of K₂SeO₄ in the spectral region 5–460cm^–1 was determined using transmissivity and reflectivity measurements as a function of temperature between 80 and 300K. The spectral features above 20cm^–1 are interpreted using results of lattice vibrational analysis in three known commensurate phases. The low-frequency dielectric anomaly in the incommensurate phase can be roughly described by critical slowing-down of a Debye relaxation given rise to by the overdamped infrared active phason mode which softens at the incommensurate-commensurate transition.