Optimal sub-millimeter bands for passive limb observations of stratospheric HBr, BrO, HOCl, and HO2 from space

Present and near future limb-sounding heterodyne instruments operating at millimeter or sub-millimeter wavelengths will be capable to gather profile information on a variety of stratospheric trace gases with relatively strong spectral features such as O₃, H₂O, O₂ (temperature), N₂O, HCl, ClO, HNO₃, CH₃Cl, and CO. Some species however, which are of particular interest for the understanding of stratospheric ozone chemistry, have only very weak spectral signatures, either due to their very small abundances and/or spectroscopic properties. These are amongst others the radical BrO, the potential bromine reservoir HBr, HOCl as a member of the chlorine family, and the hydroperoxyl radical HO₂. Often, these target species are very difficult to detect and profiles have not yet been measured by space-borne sensors at all, even considering other spectral ranges and observation techniques. A study was therefore dedicated to the investigation of the retrieval of species with spectral features close to the detection limit of a typical state-of-the-art space-borne heterodyne receiver. A survey of the millimeter- and sub-millimeter spectral range was performed in order to identify appropriate spectral bands, followed by retrieval simulations for a representative atmospheric and instrumental scenario in order to explore the relative merits of the pre-selected bands and to derive associated parameters as measurement precision, altitude range and resolution. The feasibility of retrieving profile information from limb observations at sub-millimeter wavelengths of the “weak line” species under investigation is demonstrated. Optimal dedicated bands are recommended for HBr , BrO , and HOCl . For HO₂ five more or less equivalent bands between 585 and are identified.     

Temperature-dependent absorption cross-sections of HCFC-142b

Following the recent detection of HCFC-142b (1-chloro-1,1-difluoroethane) from space, laboratory infrared absorption cross-section spectra of this molecule in a pure vapour phase have been recorded in the spectral region using Fourier transform spectroscopy. The spectra have been recorded at a resolution of and a range of temperatures from 223 to 283 K. The resulting data show good agreement with the harmonic frequencies and intensities calculated using density functional theory as well as with the integrated absorption intensities of the spectral bands available in the literature. The new cross-sections will allow more accurate retrieval of atmospheric HCFC-142b concentrations using infrared spectroscopic techniques.     

Heterodyne interferometer for displacement measurement with amplitude quadrature and noise suppression

The high precision displacement measurement in nanoscale is crucial to many applications. We present a heterodyne interferometry with differential phase to amplitude conversion scheme for displacement measurement in nanoscale. In this approach, the differential phase introduced by the displacement is converted into the amplitudes of heterodyne signals in quadrature. Meanwhile, the heterodyne signals in phase quadrature are also achieved so that the displacement can be determined from the amplitude ratio of the quadrature signals, and the direction of displacement can be determined from the phase quadrature. Since the differential phase to quadrature amplitude conversion is achieved through the optical addition and subtraction by polarization tuning, which are based on differential detection concept. Thus the proposed method benefits from the features of differential detection with common phase noise and correlated amplitude noise rejection and that of quadrature detection with real time and wide dynamic range of phase measurement. To demonstrate the capability of proposed method in differential phase measurement, we measure the displacement drove by a commercially available PZT pusher and found close agreement between the experiment and the theory. The experimental evidence of noise suppression is also found with spectral measurements, which demonstrates the resolution of displacement measurement at 60 pm and minimum detectable differential phase of 5.6 × 10⁻⁶ rad/ over 50 kHz.     

The N-N stretching band of hydrazine

For the first time a very weak N-N stretching band (ν₅) of hydrazine was unambiguously assigned in the Fourier-transform infrared spectrum. Almost 1500 transitions with a resolution of for K^′ from 0 to 6 and for all symmetry species have been analyzed. Despite some perturbations a global fit has been carried out successfully and the band center was determined at , much higher than previously expected.     

FTIR and diode laser spectroscopy of isobutylene: Analysis of the rotational structure in the v28 fundamental band

Rotational structure in the fundamental band of isobutylene has been examined at room temperature using a combination of FTIR and Pb-salt diode laser instruments. The highest spectral resolution for the FTIR measurements was 0.125 cm⁻¹. Even at this resolution however, rotational structure for the band could be observed and appeared to possess a very regular pattern. A preliminary spectral assignment was obtained using the Watson/Gora asymptotic approximation for a rigid oblate asymmetric rotor. Within this approximation, the band origin was determined to be 890.937 (4) cm⁻¹. Excited state rotational constants, without the inclusion of centrifugal distortions terms, are A = 0.3033(16), B = 0.2801(12) and C = 0.15362 (8) cm⁻¹ respectively. Finally, a full set of spectroscopic constants, including quartic centrifugal distortion constants, were obtained for the band by including the high resolution Pb-salt spectra.     

High resolution rovibrational spectroscopy of pyrimidine: Analysis of the B1 modes ν10b and ν4 and B2 mode ν6b

We report the first high resolution rovibrational analysis of the infrared spectrum of pyrimidine (C₄H₄N₂) based on measurements using our Fourier transform spectrometer, the Bruker IFS 125 HR Zürich Prototype (ZP) 2001. Measurements were conducted at room temperature in a White-type cell with effective optical path lengths between 3.2 and 9.6 m and with resolutions ranging from 0.0008 to 0.0018 cm⁻¹ in the region between 600 and 1000 cm⁻¹. The spectrum was analyzed in the ν₄ (), ν_10b () and ν_6b regions of pyrimidine () using an effective Hamiltonian. A total of about 15 000 rovibrational transitions were assigned. The root mean square deviations of the fitted data are in the ranges d_rms = 0.00018-0.00024 cm⁻¹, indicating an excellent agreement of experimental line data with the calculations. The results are discussed briefly in relation to possible extensions to spectra of DNA bases and to intramolecular vibrational redistribution at higher energy. The analysis of the ν_10b and ν₄ bands will also be useful in the interstellar search for pyrimidine in the infrared region.     

Maximising rovibrational assignments in the ν1 band of NSCl by spectral analysis by subtraction of simulated intensities (SASSI)

The ν₁ band of thiazyl chloride (NSCl) has been measured by high resolution FTIR spectroscopy, and studied using the spectral analysis by subtraction of simulated intensities (SASSI)-technique. This involves assignment and fitting of a component, creating a global simulation of the entire component to be subtracted from the experimental spectrum, and subsequent analysis of the resultant spectrum. Through this iterative procedure it has been possible to assign rovibrational transitions for seven components, with populations in the lower vibrational state as low as 2% of the total. The components are the fundamental and , , and hotbands of ¹⁴N³²S³⁵Cl, the fundamental and hotband of ¹⁴N³²S³⁷Cl, and the fundamental of ¹⁴N³⁴S³⁵Cl. Rotational and centrifugal distortion constants beyond the quartic level have been obtained by fitting to Watson’s A-reduced Hamiltonian in upper and lower states.     

Ultrahigh magnetic field diagnostic with spectral profile calculation

We report the theoretical results on use of neon as a tracer element to measure the multi-megagauss magnetic field, which is induced in the ultrahigh intense laser-matter interactions. The shape of Zeeman splitting of spectral line for transition of He-like neon are calculated for high-intensity laser produced quasi one-components plasma with the consideration of the electron collision broadening, electron collision shift and magnetic field splitting. The results show that all of the Zeeman splitting spectrum can be identified under Rayleigh criterion for the plasma with the electron temperature from 10 to , the magnetic field from 10⁶ to and the electron density . With both the electron temperature and magnetic field increasing, the requirement for the resolution power of the spectrometer decreases. If a spectrometer with the resolution power of 1/1000 is used, the measurement of the quasistatic magnetic field by Zeeman splitting of spectral lines is applicable when quasistatic magnetic field is larger than some tens of megaGauss.     

Diode laser slit-jet spectra and analysis of the ν14 fundamental of 1-chloro-1,1-difluoroethane (HCFC-142b)

High resolution infrared spectra (0.001 cm⁻¹ FWHM) have been measured for mixtures of 1-chloro-1,1-difluoroethane in Ne, expanded in a supersonic planar jet. The ν₁₄ fundamental has been analyzed for both isotopic species, and .     

Experimental results of the performance of a laser fiber as a remote sensor of temperature

We present experimental results demonstrating the performance of an erbium-doped silica fiber as a remote temperature sensor. The sensor is based on the fluorescence intensity-ratio change of two spectral bands as a function of temperature in the wavelength interval from 515 to 570 nm. We apply a radiometric analysis to the fluorescence spectrum that we have measured to determine the optimal spectral bands to use in the power ratio of the sensor. The spectral bands used in the power ratio, with best performance, are 525–535 nm/555–565 nm, with a signal–noise ratio of 57 and 56 dB, respectively. The sensor sensitivity is about , and the resolution is approximately .     

Near infrared emission spectrum of HCN

The emission spectrum of H¹³CN at 1370 K has been recorded with a hot gas high resolution FT-IR emission apparatus [1] in the wavenumber region of with a resolution of . This work reports the analysis of 50 subbands for the H¹³CN isotopologue of hydrogen cyanide in the 2ν₁ wavenumber region. 23 rovibronic states of H¹³CN including the rovibronic states at have been characterized for the first time and for seven other states it was possible to improve the existing spectroscopic constants substantially. The dense emission spectrum was analyzed with the spectrum analysis software SyMath™ implemented in the Mathematica™ computer algebra system [1].     

High resolution analysis of the ν7 and ν9 bands of DCOOH

The 7¹ and 9¹ vibrational states of deuterated species of formic acid molecule DCOOH have been recorded by a FTIR spectrometer in the region 450- at a resolution of and a millimeter wave spectrometer. In the analysis microwave transitions from literature were used in addition to 14 835 assigned IR and 114 millimeter wave lines in the 7¹ and 9¹ vibrational states. The analysis resulted in band origins, rotational, centrifugal distortion, and eight interaction parameters of the Coriolis coupled 7¹ and 9¹ vibrational states. RMS deviation of the fit was for the IR data and the maximum values of J and K_a quantum numbers in the fit were 64, 28 and 64, 30 for 7¹ and 9¹ states, respectively.     

Density functional theory study of the near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene on group IV semiconductor surfaces

The near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene adsorbed on C(1 0 0)-2 × 1, Si(1 0 0)-2 × 1 and Ge(1 0 0)-2 × 1 surfaces is studied with density functional theory calculations. Time dependent density functional theory calculations of the near edge X-ray absorption fine structure with a modified exchange-correlation functional agree well with experiment, and show that the spectral features arise from excitation to π^∗, and orbitals, where X represents C, Si or Ge. The excitation energies are dependent on the surface, and for acetylene, the location of the π^∗ band also varies with the surface. Calculations of the vibrational modes show the CH stretching frequencies for carbon atoms bonded directly to the surface vary significantly between the three surfaces, while those for carbon atoms not bonded to the surface do not change significantly.     

Near infrared emission spectrum of HCN

To record the infrared emission of hot molecular gases an optimized emission apparatus for the Bruker IFS 120 HR high-resolution FT-IR spectrometer has been constructed. Using this apparatus the hot gas emission spectrum of HCN at 1420 K has been recorded in the wavenumber region of 6000-6800 cm⁻¹ with a resolution of 0.044 cm⁻¹. This work reports the analysis of 33 bands with 58 subbands (9200 line positions). Thirty-seven rovibronic states of HCN including at 12 603 cm⁻¹ have been characterized for the first time and for 25 other states it was possible to improve the existing spectroscopic constants substantially. The very dense emission spectrum with many overlapping features was analyzed with new spectrum analysis software implemented using the Mathematica^TM computer algebra system. Spectroscopic constants have now been determined for 220 HCN rovibronic states. For 102 states the rovibrational spectroscopic constants have been determined for the first time or improved substantially using emission spectra measured in Giessen.     

Test of far-infrared atmospheric spectroscopy using wide-band balloon-borne measurements of the upwelling radiance

The spectroscopy of the constituents of the Earth's atmosphere that are active in the far infrared spectral region, among which the water vapour is the main one, has been validated through the analysis of wide-band nadir-looking spectra acquired with the Radiation Explorer in the Far Infrared—Prototype for Applications and Development (REFIR-PAD) Fourier transform spectroradiometer. The spectra, covering from 100 to with a unapodized resolution, were acquired during a balloon flight performed in a tropical region in 2005. Atmospheric variables, namely water vapour and temperature vertical profiles, were retrieved from the REFIR-PAD data, and the residuals of the fitting are here critically analysed for the search of systematic effects that can be ascribed to spectroscopic errors. In the spectral interval between 150 and nosignificant inconsistency is detected between the residuals and the measurement uncertainty, proving the good quality of the radiative transfer model and of the HITRAN 2004 spectroscopic database. Significant difference are instead observed when the HITRAN 2000 database is used.     

Effect of various alkaline-earth metal oxides on the broadband infrared luminescence from bismuth-doped silicate glasses

We report on the effect of various alkaline-earth metal oxides on the broadband infrared luminescence covering 1000-1600 nm wavelength region from bismuth-doped silicate glasses. The full width at half maximum (FWHM) of the infrared luminescence and the fluorescent lifetime is more than 200 nm and 400 μs, respectively. The fluorescent intensity decreases with increasing basicity of host glasses. Besides the broadband infrared luminescence, luminescence centered at 640 nm was also observed, which should be ascribed to Bi²⁺ rather than to the familiar Bi³⁺. We suggest that the infrared luminescence should be assigned to the transition of BiO molecules dispersed in the host glasses.     

Measurements of the isotopic composition of water vapor using a DFG-based spectrometer at

We report on absorption measurements of the isotopic composition of water in a demineralized sample by means of wavelength modulation spectroscopy of H₂O lines in the spectral region around . The mid-IR radiation was obtained by difference-frequency mixing of the radiation of an extended cavity diode laser () and that of a diode-pumped monolithic Nd–YAG laser. Preliminary tests indicate the possibility of obtaining a precision for the ¹⁸O/¹⁶O, ¹⁷O/¹⁶O and ²H/¹⁶O isotope ratio measurements of the order of 1 per mil.     

Determining the optimum wavelength pairs to use for molecular absorption thermometry based on the continuous-spectral lower-state energy

A novel technique for selecting wavelengths that optimize the temperature precision of absorption-based thermometry is developed. A single universal objective function considering continuous spectral lower-state energy , spectral absorbance intensities and an experimental noise level is derived and applied in a brute-force approach to analyze two-wavelength, Doppler-broadened H₂O vapor-absorbance thermometry in the 1.33-1.37 μm range. The results reveal the top wavelength pairs and their temperature precisions over a wide temperature range from 280 to 2800 K. Although the spectral database used includes over thousands of transitions in this spectral range, the top pairs are composed of only 12 unique wavelengths. This wavelength selection technique is able to help researchers choose key wavelengths that will perform well in practical applications.     

Fourier transform spectroscopy of chemiluminescence from the AΠ-XΣ system of SrO

The A^′1Π-X¹Σ⁺ near infrared system of strontium oxide (SrO) was observed at high spectral resolution by measuring the chemiluminescence from a Broida flow reactor using a Fourier transform spectrometer. In total, 32 bands from , , were measured within the spectral region at a resolution of . Vibrational levels of the upper state were observed up to v_A^′=4, and more than 5600 rotational lines were assigned. Incorporating previously published high resolution data for the A¹Σ⁺-X¹Σ⁺ system, a global fit to both data sets yields improved Dunham constants for the ground state and for the lower vibrational levels (v_A^′=0, 1, and 2) of the A^′1Π state. Because perturbations arising from interactions with the b³Σ⁺ and A¹Σ⁺ states affect the higher vibrational levels of the A^′1Π state more strongly, levels v_A^′=3 and 4 were represented by effective band constants in the fits. RKR potentials for the X¹Σ⁺,A^′1Π, and b³Σ⁺ states have been generated utilizing all the available data, Franck-Condon factors have been calculated for the A^′1Π-X¹Σ⁺ system, and A^′1Π∼b³Σ⁺ and A^′1Π∼A¹Σ⁺ perturbations are discussed.