Narrow Spectral Intervals Research Articles

NH 2 and NH spatial intensity distribution in the coma of Halley’s comet

In this work, the NH 2 and NH radial emission profiles in the sunward coma of Halley’s comet are studied. Spectra in the near UV and visible region, registered from the three-channel spectrometer (TKS) on board the VEGA-2 interplanetary station, on 9 March 1986, are used. The intensity of the observed species is determined from narrow spectral intervals, in which their emissions are predominant. The following wavelength intervals are used: for NH (0, 0) – λλ 3349–3374 Å (pixels 262–265); for NH 2 – (0, 9 ,0) 598 nm ( λλ 5975–5999 Å, pixels 699–702), (0, 9, 0) 606 nm ( λλ 6058–6064 Å, pixel 713), (0, 8, 0) 634 nm ( λλ 6330–6348 Å, pixels 759–761), (0, 8, 0) 631 nm ( λλ 6306–6312 Å, pixel 755), (0, 10, 0) 573 nm ( λλ 5702–5744 Å, pixels 653–659), (0, 7, 0) 662 nm ( λλ 6618–6630 Å, pixels 808–809). An improved method is applied to subtract the dust continuum in the UV part of the spectra. The intensities are corrected for the optical depth in the near-nucleus region. Theoretical curves using a Haser model are computed and the data and model are compared. The applicability of the recommended parameters to the TKS data is discussed and the most suitable ones are chosen. In the frames of the TKS experiment the Haser model is found to describe well the obtained dependences of the NH and NH 2 column intensities on the distance p of the line of sight to the nucleus, termed the projected distance or p-parameter. A comparison with the results for other neutrals by the TKS spectra, as well as with the results for NH 2 and NH published so far, is made, and a similarity is found. The scanning of TKS allowed for the construction of spatial intensity distributions. From the seven obtained distributions, composite intensity distributions for each examined emission are constructed, covering a larger space region. The NH and NH 2 intensity spatial distributions are studied, in comparison with other neutral compounds.

Frequency tuning of long-wavelength VCSELs

Tuning properties of long-wavelength VCSELs have been studied experimentally, for the first time to our knowledge. Injection current and temperature tuning rates of two VCSELs operating near 1512 and 1577 nm have been measured using a Fabry–Perot etalon with free spectral range 0.056 cm −1. A 100-Hz saw-tooth modulation with depths of modulation of ∼10% or less was superimposed on a direct injection current (dc bias) to tune lasers in narrow spectral intervals (0.3–1.2 cm −1) around a central frequency set by the dc bias. The lasers have been found to be capable of being tuned faster at higher levels of dc bias. The enhancement factors were up to ∼2 and ∼3 for the 1512- and 1577-nm lasers, respectively, as compared with their tuning rates measured at the levels of the dc bias close to the threshold of lasing. A linear dependence between injection current tuning rates and the levels of dc bias has been observed. Temperature tuning coefficients have been proved to be independent of the laser heat sink temperature and of the dc bias. Frequency tuning curves were approximated with a second-order polynomial. The frequencies of more than 40 absorption lines of CO, CO 2, H 2O and NH 3 known from spectral databases were compared with the calculated frequencies. The accuracy of the approximation was found to be within 0.2 cm −1 for spectral intervals up to 38 cm −1. The dependence of current tuning rates of the VCSELs on dc bias was shown to be taken into account for accurate analysis of absorption line profiles. The results obtained can be used for precise spectroscopic measurements with long-wavelength VCSELs.

Spectroscopic Detection of Sulfur Oxides in the Aircraft Wake

The absorption/emission spectral regions of SO, SO2, SO3, S2O and HSO are analyzed for the range from UV (λ ≥ 0.2 µm) to IR (λ < 30 µm) and are compared with the atmospheric transmission spectrum. It is shown that many vibrational bands of the compounds considered fall into atmospheric transmission windows. For the vibrational bands of SO, SO2, SO3, S2O, and HSO molecules there are some gases which hinder the absorption diagnostics of the indicated compounds. These interfering gases are natural components of atmospheric air as well as specific gases of aircraft engine exhaust. It is found that the least influence of the interference takes place in the 2400–2700 cm−1 IR region. The spectroscopic techniques used for the detection of aircraft engine exhaust compounds are briefly reviewed, with much consideration given to SO2. The IR absorption spectra of SO2 and other gases are calculated for the conditions of the aircraft engine nozzle exit. Narrow spectral intervals suitable for SO2 detection in a hot flow are determined. An analysis is made for the detection capabilities of CO2 lasers (including isotope CO2 lasers) and CO lasers (both fundamental band and first-overtone ones) as applied to SO2 detection in aircraft engine exhaust.

Daytime operation of a pure rotational Raman lidar by use of a Fabry–Perot interferometer

We propose to use a Fabry-Perot interferometer (FPI) in a pure rotational Raman lidar to isolate return signals that are due to pure rotational Raman scattering from atmospheric nitrogen against the sky background. The main idea of this instrumental approach is that a FPI is applied as a frequency comb filter with the transmission peaks accurately matched to a comb of practically equidistant lines of a pure rotational Raman spectrum (PRRS) of nitrogen molecules. Thus a matched FPI transmission comb cuts out the spectrally continuous sky background light from the spectral gaps between the PRRS lines of nitrogen molecules while it is transparent to light within narrow spectral intervals about these lines. As the width of the spectral gaps between the lines of the PRRS of nitrogen molecules is -114 times the width of an individual spectral line, cutting out of the sky background from these gaps drastically improves the signal-to-background ratio of the pure rotational Raman lidar returns. This application of the FPI enables one to achieve daytime temperature profiling in the atmosphere with a pure rotational Raman lidar in the visible and near-UV spectral regions. We present an analysis of application of the FPI to filtering out the pure rotational Raman lidar returns against the solar background. To demonstrate the feasibility of the approach proposed, we present temperature profiles acquired during a whole-day measurement session in which a Raman lidar equipped with a FPI was used. For comparison, temperature profiles acquired with Vaisala radiosondes launched from the measurement site are also presented.

Retrieval and characterization of ozone profiles from solar infrared spectra at the Jungfraujoch

Vertical distributions of ozone from June 1996 to November 2000 have been retrieved from high‐resolution Fourier transform infrared (FTIR) solar absorption spectra recorded at the primary Network for Detection of Stratospheric Change station of the Jungfraujoch in the Swiss Alps (46.5°N, 8°E, 3580 m above sea level (asl). The retrievals were performed using the Optimal Estimation Method (OEM), both in a narrow spectral interval (1002.567–1003.2 cm−1) and in a broad spectral interval (1000.0–1005.0 cm−1) in the O3 9.6‐μm band. A thorough characterization of the retrievals has been performed following the lines of OEM, including an information content analysis, a study of the correlations between retrieved instrumental parameters and retrieved ozone concentrations, and an evaluation of the O3 profile error budget. It is demonstrated that the information content is significantly higher for spectra in the broad microwindow, resulting in higher vertical resolutions, on the order of 8 km, of the retrieved profiles extending up to 40 km, and less correlations between retrieved parameters. An independent statistical verification of the retrieval results and their characterization has been performed by comparison of the FTIR ozone profiles with independent measurements. These are the ozone profile measurements from balloon soundings at Payerne, from the microwave radiometer at Bern and the lidar at Observatoire de Haute‐Provence (OHP), and the total column data from the Dobson spectrophotometer at Arosa. Applying the optimum retrieval procedure in the broad spectral interval, an excellent agreement has been found between the FTIR O3 profile data and the correlative data. The largest offset of the FTIR data in comparison with the correlative data is found with respect to the lidar data in the 24‐ to 40‐km layer, and is on the order of 5%. No systematic biases have been found in the troposphere, neither in the upper troposphere‐lower stratosphere (UTLS) up to 18 km. The dispersion of the relative differences between the data sets, if any, is never larger than half of the natural ozone variability.

Sensitive and selective spectrochemical analysis of metallic samples: the combination of laser-induced breakdown spectroscopy and laser-induced fluorescence spectroscopy

In order to improve on analytical selectivity and sensitivity, the technique of laser-induced fluorescence spectroscopy (LIFS) was combined with laser-induced breakdown spectroscopy (LIBS). The main thrust of this investigation was to address analytical scenarios in which the measurement site may be difficult to access. Hence, a remote LIBS+LIFS arrangement was set up, and the experiments were carried out on samples surrounded by air at atmospheric pressure, rather than in a controlled buffer gas environment at reduced pressure. Representative for proof of principle, the detection of aluminium, chromium, iron and silicon at trace level concentrations was pursued. These elements are of importance in numerous chemical, medical and industrial applications, and they exhibit suitable resonance transitions, accessible by radiation from a pulsed Ti:sapphire laser system (its 2nd and 3rd harmonic outputs). All investigated elements have an energy level structure in which the laser-excited level is a member of a group of closely-spaced energy levels; thus, this allowed for easy off-resonant fluorescence detection (collisional energy transfer processes). Since numerous of the relevant transition wavelengths are within a narrow spectral interval, this opens the possibility for multi-element analysis; this was demonstrated here for Cr and Fe which were accessed by rapidly changing the tuneable laser wavelength.

Hole burning spectroscopy with preliminary photoselection of impurity molecules

The photoselection of impurity molecules caused by spectral hole burning produces a specific spectral angular distribution of the molecules in low-temperature amorphous matrices: All the molecules, whose zero-phonon absorption lines lie within a selected narrow spectral interval, have a specified distribution of their orientations relative to the laboratory coordinate system. In this paper, the possibility of application of known methods of hole burning in this spectral interval is studied and the outlook for combination of these methods with preliminary photoselection is demonstrated by the example of the Stark effect.

A correlated‐k distribution scheme for overlapping gases suitable for retrieval of atmospheric constituents from moderate resolution radiance measurements in the visible/near‐infrared spectral region

An accurate and fast radiative transfer scheme for the simulation of the spectral radiance measurements of a new generation of multichannel moderate resolution satellite spectrometers, such as GOME/ERS‐2 (240–790 nm) and SCIAMACHY/ENVISAT‐1 (240–2385 nm), has been developed. Representative absorption cross sections of the individual molecular line‐absorbers CH4, CO, CO2; H2O, N2O, and O2 for relatively narrow spectral intervals (≤0.2 nm) have been determined using a combined correlated‐k (c‐k) distribution and exponential sum fitting of transmittances (ESFT) scheme. The speed of the c‐k calculations mainly depends on the channel‐dependent spectral resolution of the instruments. For the measurements to be made by SCIAMACHY, this may be up to 800 times faster than line‐by‐line calculations. The agreement between c‐ k radiances and line‐by‐line reference radiances is in the range 1–2%. The dominant error (∼1%) is attributed to the correlation assumption made when applying the k distribution method to inhomogeneous atmospheres. As the commonly used c‐k methods for overlapping gases are shown to result in large radiance errors at SCIAMACHY spectral resolution or do not permit individual representative absorption cross sections to be defined for each absorber separately, a new and novel method for the accurate simulation of overlapping absorption has been developed, verified, and validated. This technique enables overlapping line absorption to be considered for arbitrary spectral correlation of the monochromatic absorption cross sections of the individual gases. The new c‐k scheme is based on a linear mixing of two radiance terms determined assuming correlation and anticorrelation of the monochromatic absorption cross sections of the overlapping gases.

Dielectric barrier discharges with steep voltage rise: laser absorption spectroscopy of NO concentrations and temperatures

Laser absorption spectroscopy was applied to determine in situ particle concentrations and temperatures in an atmospheric pressure dielectric barrier discharge. The measurements were performed with a small model reactor. The electrical excitation was by fast voltage pulses rising to 20 kV in about 300 ns (in some later measurements in approximately 90 ns); the repetition rate was 50 kHz maximum. Effects of filament distribution and discharge homogeneity were investigated. The fast rising voltage has the consequence that all filaments are generated within a 50 ns interval for each half-cycle. Observation of the NO(0,0)-band allows the determination of NO concentrations, spatially resolved as a function of the distance from a single filament; gas temperatures can be inferred from a comparison of calculated and measured line absorptions in characteristic narrow spectral intervals.

Optimized forward model and retrieval scheme for MIPAS near-real-time data processing

An optimized code to perform the near-real-time retrieval of profiles of pressure, temperature, and volume mixing ratio (VMR) of five key species (O(3), H(2)O, HNO(3), CH(4), and N(2)O) from infrared limb spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment on board the European Space Agency (ESA) Environmental Satellite ENVISAT-1 was developed as part of a ESA-supported study. The implementation uses the global fit approach on selected narrow spectral intervals (microwindows) to retrieve each profile in sequence. The trade-off between run time and accuracy of the retrieval was optimized from both the physical and the mathematical points of view, with optimizations in the program structure, in the radiative transfer model, and in the computation of the retrieval Jacobian. The attained performances of the retrieval code are noise error on temperature <2 K at all the altitudes covered by the typical MIPAS scan (8-53 km with 3-km resolution), noise error on tangent pressure <3%, and noise error on VMR of the target species <5% at most of the altitudes covered by the standard MIPAS scan, with a total run time of less than 1 min on a modern workstation.

Turbulence decay in stratified and homogeneous marine layers

A spectral approach is applied to shear-induced turbulence in stratified layers. A system of spectral equations for stationary balance of turbulent energy and temperature variances was deduced in the vicinity of the local shear scale LU = (ϵ/UZ3)12. At wavenumbers between the inertial-convective (k−53) and wak turbulence (k−3) subranges, additional narrow spectral intervals—‘production’ subranges—may appear (E ∽ k−1, ET ∽ k−2). The upper boundary of these subranges is determined as LU, and the lower boundaries as LR ∽ (ϵ/UZN2)12(χ/TZ2). It is shown that the scale LU is a unique spectral scale that is uniform up to a constant value for every hydrophysical field. It appears that the spectral scale LU is equivalent to the Thorpe scale LTh for the active turbulence model. Therefore, if turbulent patches are generated in a background of permanent mean shear, a linear relation between temperature and mass diffusivities exists. In spectral terms, the fossil turbulence model corresponds to the regime of the Boldgiano-Obukhov buoyancy subrange (E ∽ k−115, ET ∽ k−75). During decay the buoyancy subrange is expanded to lower and higher wavenumbers. At lower wavenumbers the buoyancy subrange is bounded by L∗∗ = 3(χ12/N12TZ), which is equivalent to the Thorpe scale LTh. In such a transition regime only, when the viscous dissipation rate is removed from the set of main turbulence parameters, the Thorpe scale does not correlate with the buoyancy scale LN ∽ ϵ12/N32 and fossil turbulence is realized. Oceanic turbulence measurements in the equatorial Pacific near Baker Island confirm the main ideas of the active and fossil turbulence models.

Presence of terrestrial atmospheric gas absorption bands in standard extraterrestrial solar irradiance curves in the near-infrared spectral region

The solar irradiance curves compiled by Wehrli [Physikalisch-Meteorologisches Observatorium Publ. 615 (World Radiation Center, Davosdorf, Switzerland, 1985)] and by Neckel and Labs [Sol. Phys. 90, 205 (1984)] are widely used. These curves were obtained based on measurements of solar radiation from the ground and from aircraft platforms. Contaminations in these curves by atmospheric gaseous absorptions were inevitable. A technique for deriving the transmittance spectrum of the Sun's atmosphere from high-resolution (0.01 cm(-1)) solar occultation spectra measured above the Earth's atmosphere by the use of atmospheric trace molecule spectroscopy (ATMOS) aboard the space shuttle is described. The comparisons of the derived ATMOS solar transmittance spectrum with the two solar irradiance curves show that he curve derived by Wehrli contains many absorption features in the 2.0-2.5-µm region that are not of solar origin, whereas the curve obtained by Neckel and Labs is completely devoid of weak solar absorption features that should be there. An Earth atmospheric oxygen band at 1.268 µm and a water-vapor band near 0.94 µm are likely present in the curve obtained by Wehrli. It is shown that the solar irradiance measurement errors in some narrow spectral intervals can be as large as 20%. An improved solar irradiance spectrum is formed by the incorporation of the solar transmittance spectrum derived from the ATMOS data into the solar irradiance spectrum from Neckel and Labs. The availability of a new solar spectrum from 50 to 50 000 cm(-1) from the U.S. Air Force Phillips Laboratory is also discussed.

The Balmer jump in astrophysical hydrogen plasmas

It is shown that, according to accepted theoretical concepts, there should be no intensity jump at the edge of the Balmer continuum, nor at that of other hydrogen series. We point out three reasons for a fairly smooth increase of the intensity to occur at the continuum edge. As a result, ‘the jump’ sometimes spreads over hundreds of angstroms, due mainly to the electron density inhomogeneity of solar plasma. Furthermore, the Doppler and Stark effects also give rise to a considerable spread of the series edge. We point out that excess continuum emission over a relatively narrow spectral interval before the continuum edge can result from overlapping of line wings.

Temporal behavior of stimulated Kerr scattering in a CS_2 liquid-core hollow-fiber system

The temporal behavior of the stimulated Kerr scattering (SKS) from a 1.5-m-long CS2 liquid-core hollow-fiber system is studied by using nanosecond and subpicosecond pump-laser pulses. In both situations the duration of the SKS pulses is much shorter than that of the corresponding pump pulses. With ~20-ns pump pulses, the pulse width of a SKS component that is located within a narrow (~1.1 nm) spectral interval and is generated by the second- or third-order stimulated Raman scattering can be shorter than 2–3 ns. In order to measure the temporal behavior of SKS obtained with ultrashort (0.5 ps) pump pulses, a Michelson-interferometer-grating device is used. The pulse duration of the SKS component within a narrow (~1.2 nm) spectral interval is approximately 260–270 fs with ~0.5-ps pump pulses. Our experimental results are qualitatively consistent with the theoretical consideration of the SKS processes.

Influence of fluctuations of laser radiation on the formation of ultrashort-period gratings in inhomogeneously broadened media

A theoretical investigation is reported of the influence of laser fluctuations on the formation of ultrashort-period population-inversion gratings in the case of an inhomogeneously broadened resonant transition. It is shown that such gratings may be formed in a narrow spectral interval and that their amplitude always obeys the normal distribution for different spectral isochromats.

Energy threshold characteristics of electron-induced photon emission from Ag

Abstract The intensity of electron-induced photon emission from Ag films of various thicknesses on W(110) has been measured as a function of the energy of the incident electrons, the emitted photons being detected in two narrow spectral intervals at ℏω 1 = 3.83 eV and ℏω 2 = 3.08 eV. The threshold for photon emission was found to the E p = 0 eV for both wavelengths. The measured isochromat spectra are compared to the total current spectra. Minima at E 1 = 16.5 eV and E 2 = 20 eV above the Fermi level are thought to correspond to an increase of the electron reflection from the sample surface at these energies. At the same time the maximum in the ℏω = 3.08 eV isochromat spectrum at E = 6.5 eV above the Fermi level is due to the dominating radiative capture to states at E = 3.4 eV above the Fermi level. The spectral distribution ofintensity of the electron-induced photon emission was obtained for Ag films of various thicknesses at the electron energy E p = 0 eV above the vacuum level.

Determination of the parameters of an impulsive-discharge plasma form the relative intensities of the dielectronic satellites of Be+ ions

The lines emitted with radiative decay of doubly excited states near the resonance transitions of heliumlike ions are convenient for spectroscopic diagnostic plasma because of the fact that the satellite and the resonance line lie in a narrow spectral interval and therefore wavelength calibration of the spectrometer is unnecessary. In this paper the transitions from autoionized levels in a three-electron beryllium ion were employed for the diagnostics of the plasma of an impulsive discharge in a vacuum. The spectrum of dielectronic satellites was excited in the discharge between beryllium electrons. The line intensities were determined photographically. Atomic model calculations of configuration interaction and electron temperature along with effective charge and electric conductivity are presented.

The longwave radiation estimated from NOAA polar orbiting satellites: An update and comparison with Nimbus-7 ERB results

The planetary outgoing longwave radiation has been estimated since 1974 from two different series of NOAA operational polar spacecraft. The first series provided data from June 1974 through February 1978 and was designated “SR” for the scaning radiometers used at that time. This data set has been used in a variety of radiation budget and climate studies, such as that by Ohring and Gruber, 1983. The second satellite system is the currently operational TIROS-N series of satellites. Data from this series began in January 1979 and are continuing. In both systems, estimates of the outgoing longwave radiation are obtained from narrow spectral interval (10–12 μm) window radiances. A comparison is made of the estimates from the two different series of satellites in order to arrive at an assessment of their compatibility. This is important since the SR observations were taken at approximately 0900 and 2100 local times, while the TIROS-N data alternate between 0730-1930 and 0300-1500 local times. In addition, there is a period of overlap between the TIROS-N data and the broad band (5–50 μm) Nimbus 7 EArth radiation budget data. A comparison of those two data sets indiciate excellent agreement generally within about 1–2 Wm −2 on the monthly means on global and hemispherical scales. Comparisons of zonal averages indicate maximum differences as large as 9 Wm −2. Evidence is presented to suggest that observations taken at different local observing times may be biased by the diurnal variation of emitted flux, even on global scales.

Determination of surface albedo from satellites

The dynamical features of the climate system result from the interaction of the atmosphere with the surface. The hope for improving climate prediction, on seasonal and interannual time scales, is based on the premise that slowly varying boundary conditions force well defined predictable patterns of general circulation. It is now recognized that in models of surface climate over land, surface properties should not be regarded as constants and that it is important to specify a realistic value of the surface albedo ∗ ∗ Albedo is defined as the ratio of reflected to incident electromagnetic radiation in the total solar spectrum or in a certain wavelength interval. Reflectivity refers to a specific wavelength and direction. The term “albedo” is used rather freely in the literature. No attempt will be made to define it as appropriate in each reviewed case. in order to correctly estimate the amount of solar radiation absorbed at the surface. The albedo is also considered by some as an internal dependent parameter of climate, since it time integrates the effects of changes in more variable quantities, such as rainfall. The global nature of the links between forcing and response imply global monitoring of the parameters which control the transfer and feedback of the energy at the surface/atmosphere interface. Satellites can play an important role in resolving the difficult problem of properly representing the average surface albedo over large areas. However, satellites measure only the earth-atmosphere reflectance in narrow spectral intervals, narrow solid angles, and in most cases at a fixed local time. To derive from these observations the effective surface albedo, one has to compute the total reflected planetary flux, integrated over the whole solar spectrum, over all the viewing angles, and over time. A transformation from top of the atmosphere to the surface is also required. In this presentation the implications of atmospheric corrections, bidirectional reflectance, and transformations from narrow spectral bands to the total solar spectrum for albedo derivations will be addressed. Also, the various attempts to derive surface reflectivity and surface albedo from satellites will be reviewed.

Latitudinal distribution of ten stratospheric species deduced from simultaneous spectroscopic measurements

Solar infrared absorption spectra have been recorded from an aircraft during nine flights, in April–May 1980, at various latitudes between 62° north and 60° south. Using a solar occultation technique, the instrument is a spectrometer associated with a heliostat and designed for scanning automatically, in a repetitive sequency of measurements, a set of selected narrow spectral intervals within the spectral range 1000 to 4000 cm−1. Simultaneous measurements are presented concerning NO2, COS, H2O, CH4, N2O, O3, CO, HNO3, HF, and HCl. The results deduced from observations are the vertical column density above the flight altitude, near 11.5 km, the local concentration at the flight altitude, and qualitative information about the stratospheric vertical profile. A similar variation is observed for HNO3, HCl, and HF: for these three species, the vertical column density increases from the equator to the polar region in both hemispheres. The CO vertical column density decreases by a factor of 4 from the equator to high latitude in both hemispheres, with values slightly larger in the northern equatorial region than in the southern hemisphere at the same latitude. The latitudinal variation of H2O vertical column density is discussed in relation to the tropopause height. Some similarities are observed for latitudinal and seasonal variations of nitrogen dioxide and ozone.