Ground-based solar absorption studies for the Carbon Cycle science by Fourier Transform Spectroscopy (CC-FTS) mission

Carbon cycle science by Fourier transform spectroscopy (CC-FTS) is an advanced study for a future satellite mission. The goal of the mission is to obtain a better understanding of the carbon cycle in the Earth's atmosphere by monitoring total and partial columns of CO₂, CH₄, N₂O, and CO in the near infrared. CO₂, CH₄, and N₂O are important greenhouse gases, and CO is produced by incomplete combustion. The molecular O₂ column is also needed to obtain the effective optical path of the reflected sunlight and is used to normalize the column densities of the other gases. As part of this advanced study, ground-based Fourier transform spectra are used to evaluate the spectral region and resolution needed. Spectra in the 3950–7140 cm^?1 region with a spectral resolution of 0.0042 cm^?1 recorded at Kiruna (67.84°N, 20.41°E, and 419 m above sea level), Sweden, on 1 April 1998, were degraded to the resolutions of 0.01, 0.1, and 0.3 cm^?1. The effect of spectral resolution on the retrievals has been investigated with these four Kiruna spectra. To obtain further information on the spectral resolution, optical components and spectroscopic parameters required by the future mission, high-resolution solar absorption spectra between 2000 and 15000 cm^?1 were recorded using Fourier transform spectrometers at Kitt Peak (31.9°N, 111.6°W, and 2.1 km above sea level), Arizona, on 25 July 2005 and Waterloo (43.5°N, 80.6°W, and 0.3 km above sea level), Ontario, on 22 November 2006 with spectral resolutions of 0.01 and 0.1 cm^?1, respectively. Dry air volume mixing ratios (VMRs) of CO₂ and CH₄ were retrieved from these ground-based observations. The HITRAN 2004 spectroscopic parameters are used with the SFIT2 package for the spectral analysis. The measurement precisions for CO₂ and CH₄ total columns are better than 1.07% and 1.13%, respectively, for our observations. Based on these results, a Fourier transform spectrometer (maximum spectral resolution of 0.1 cm^?1 or 5 cm maximum optical path difference (MOPD)) operating between 2000 and 15000 cm^?1 is suggested as the primary instrument for the mission. Further progress in improving the atmospheric retrievals for CO₂, CH₄, and O₂ requires new laboratory measurements of the spectroscopic line parameters.     

Some improvements of the HNO3 spectroscopic parameters in the spectral region from 600 to 950 cm−1

In an earlier study [Flaud JM, Brizzi G, Carlotti M, Perrin A, Ridolfi M. MIPAS database: validation of HNO₃ line parameters using MIPAS satellite measurements. Atmos Chem Phys 2006;6:1–12] the HNO₃ line parameters in the HITRAN spectroscopic database have recently been revisited in the interval corresponding to the MIPAS spectral range. For the 11.3 μm region these updates involve the line positions for the {ν₅, 2ν₉} bands, and the intensities. However, comparisons between observed spectra recorded by MIPAS and the calculated ones in the 600–950 cm^?1 region still show significant disagreements. The goal of the present paper is to solve some of these problems. Based on the analysis of N₂-broadened HNO₃ laboratory spectra recorded with a Fourier transform instrument, a new HNO₃ database was generated, including the following improvements: (i) approximate parameters for the ν₅+ν₇?ν₇ and ν₅+ν₆?ν₆ hot bands have been added, and (ii) the intensities for the ν₆ and ν₈ bands have been updated. (iii) The air broadening and its temperature dependence, calculated using a semi-classical approach, have been added for each HNO₃ transition. (iv) Finally, line-mixing effects have been pointed out, particularly in the ν₅+ν₉?ν₉ Q branch, and an approach correctly modeling the resulting spectral shape is proposed.     

Infrared absorption cross sections for methanol

Infrared absorption cross sections for methanol, CH₃OH, have been determined near 3.4 and 10 μm from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125HR) and a multipass cell with a maximum optical path length of 19.3 m. Methanol/dry synthetic air mixtures were prepared and spectra were recorded at 0.015 cm^?1 resolution (calculated as 0.9/MOPD) at a number of temperatures and pressures (50–760 Torr and 204–296 K) appropriate for atmospheric conditions. Intensities were calibrated using composite methanol spectra taken from the Pacific Northwest National Laboratory (PNNL) IR database. The new measurements in the 10 μm region indicate problems with the existing methanol spectroscopic line parameters in the HITRAN database, which will impact the accuracy of satellite retrievals.     

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.     

Absolute line intensities measurements and calculations for the 5.7 and 3.6 μm bands of formaldehyde

The goal of this study is to achieve absolute line intensities for the strong 5.7 and 3.6 μm bands of formaldehyde and to generate, for both spectral regions, an accurate list of line positions and intensities. Both bands are now used for the infrared measurements of this molecule in the atmosphere. However, in the common access spectroscopic databases there exists, up to now, no line parameters for the 5.7 μm region, while, at 3.6 μm, the quality of the line parameters is quite unsatisfactory. High-resolution Fourier transform spectra were recorded for the whole 1600–3200 cm^?1 spectral range and for different path-length-pressure products conditions. Using these spectra, a large set of H₂CO individual line intensities was measured simultaneously in both the 5.7 and 3.6 μm spectral regions. From this set of experimental line strength which involve, at 5.7 μm the ν₂ band and, at 3.6 μm, the ν₁ and ν₅ bands together with nine dark bands, it has been possible to derive a consistent set of line intensity parameters for both the 5.7 and 3.6 μm spectral regions. These parameters were used to generate a line list in both regions. For this task, we used the line positions generated in [Margulés L, Perrin A, Janeckovà R, Bailleux S, Endres CP, Giesen TF, et al. Can J Phys, accepted] and [Perrin A, Valentin A, Daumont L, J Mol Struct 2006;780–782:28–42] for the 5.7 and 3.6 μm, respectively. The calculated band intensities derived for the 5.7 and 3.6 μm bands are in excellent agreement with the values achieved recently by medium resolution band intensity measurements. It has to be mentioned that intensities in the 3.6 μm achieved in this work are on the average about 28% stronger than those quoted in the HITRAN or GEISA databases. Finally, at 3.6 μm the quality of the intensities was significantly improved even on the relative scale, as compared to our previous study performed in 2006.     

Mid- and long-wave infrared absorption cross sections for acetonitrile

Infrared absorption cross sections for acetonitrile (methyl cyanide; CH₃CN) have been determined in the 880–1700 cm^?1 spectral region from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125 HR) and a multipass cell with a maximum optical pathlength of 19.3 m. Spectra of acetonitrile/dry synthetic air mixtures were recorded at 0.015 cm^?1 resolution (calculated as the Bruker instrument resolution of 0.9/MOPD) at a number of temperatures between 203 and 297 K and pressures appropriate for atmospheric conditions. Intensities were calibrated using three composite acetonitrile spectra recorded at the Pacific Northwest National Laboratory. These absorption cross sections will provide an accurate basis for upper tropospheric/lower stratospheric retrievals of acetonitrile in the mid-infrared spectral region from ACE satellite data.     

Enhanced 2.0 μm emission in Ho3+/Yb3+ co-doped silica-germanate glass

A detailed investigation on thermal and spectroscopic properties of different Ho³⁺/Yb³⁺ concentration ratios in silica-germanate glasses is displayed. According to the measurement of thermal properties, the host glass possesses high transition temperature (585 °C) as well as the large ΔT(155 °C). The 2.0 μm fluorescence can be obtained from all the samples. Maximum stimulated emission cross-section of around 2.0 μm is 0.56 × 10⁻²⁰ cm² of Ho³⁺ as calculated by McCumber theory. Besides, the underlying mechanism is analyzed by means of fluorescence spectra. Thus, desirable thermal properties and spectroscopic characteristics of Ho³⁺/Yb³⁺ co-doped silica-germanate glass is a promising material in 2.0 μm emission.     

Refinements of the WKMC empirical line lists (5852–7919 cm−1) for methane between 80 K and 296 K

During the last 4 years, empirical line lists for methane at room temperature and at 80 K were constructed from spectra recorded by (i) differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5852?6195 cm^?1) and in the icosad (6717–7589 cm^?1) and (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 μm and 1.28 μm transparency windows (6165–6750 cm^?1 and 7541–7919 cm^?1, respectively). We have recently constructed the global line lists for methane in “natural” isotopic abundance, covering the spectral region from 5854 to 7919 cm^?1 (Campargue A, Wang L, Kassi S, Mondelain D, Bézard B, Lellouch E, et al., An empirical line list for methane in the 1.26–1.71 μm region for planetary investigations (T=80–300 K). Application to Titan, Icarus 219 (2012) 110–128). These WKMC (Wang, Kassi, Mondelain, Campargue) empirical lists include about 43,000 and 46,420 lines at 80±3 K and 296±3 K, respectively. The “two temperature method” provided lower state energy values, E_emp, for about 24,000 transitions allowing us to account satisfactorily for the temperature dependence of the methane absorption over the considered region. The obtained lists have been already successfully applied in a large range of temperature conditions existing on Titan, Uranus, Pluto, Saturn and Jupiter.In the present contribution, we provide some improvements to our lists by using literature data to extend the set of lower state energy values and by correcting the distortion of the high E_emp values (J>10) due to the temperature gradient existing in the cryogenic cell used for the recordings. The proposed refinements are found to have an overall limited impact but they may be significant in some spectral intervals below 6500 cm^?1.The new version of our lists at 80 K and 296 K is provided as Supplementary Material: the WKMC@80K+ and WKMC@296K lists are adapted for planetary and atmospheric applications, respectively. The WKMC@80K+ list is made applicable over a wider range of temperatures and shows satisfactory extrapolation capabilities up to room temperature. It was obtained by transferring to the 80 K list the 27,580 single lines present only in the 296 K list, with corresponding lower state energy values chosen to make them below the detectivity limit at 80 K.In the discussion, the different line lists and databases available for methane in the near infrared are compared and some suggestions are given.     

Positive influence of Tm3+ on effective Er3+: 3 μm emission in fluoride glass under 980 nm excitation

Er³⁺ and Tm³⁺ singly doped and codoped new fluoride glasses were prepared by traditional melt-quenching method. Efficient 3 μm emission was obtained under 980 nm laser excitation. It is worthy to notice that one of the two ions can be the sensitizer to the other one by depressing the Er³⁺: 1.5 μm emission through the energy transfer process from Er³⁺:⁴I_13/2 level to Tm³⁺:³F₄ level. On the basis of measured absorption spectra, the Judd-Ofelt intensity parameters and radiation emission probability were calculated to evaluate the spectroscopic properties. Additionally, the micro-parameters together with the phonon assistance of Er³⁺:⁴I_13/2 → Tm³⁺:³F₄ and Er³⁺:⁴I_11/2 → Tm³⁺:³H₅ processes were quantitatively analyzed by using Dexter model. The theoretical micro-parameters results meet well with the experiments which indicates that Er³⁺/Tm³⁺ codoped fluoride glass is a potential kind laser glass for 3 μm laser.     

Broadband 2.84 μm luminescence properties and Judd–Ofelt analysis in Dy3+ doped ZrF4–BaF2–LaF3–AlF3–YF3 glass

2.84 μm luminescence with a bandwidth of 213 nm is obtained in Dy³⁺ doped (ZrF₄–BaF₂–LaF₃–AlF₃–YF₃) ZBLAY glass. Three intensity parameters and radiative properties have been determined from the absorption spectrum based on the Judd–Ofelt theory. The 2.84 μm emission characteristics and energy transfer mechanism upon excitation of a conventional 808 nm laser diode are investigated. The prepared Dy³⁺ doped ZBLAY glass possessing high predicted spontaneous transition probability (45.92 s^?1) along with large calculated emission cross section (1.17×10^?20 cm²) has potential applications in 2.8 μm laser.     

Infrared fluorescence,energy transfer process and quantitative analysis of thulium-doped niobium silicate-germanate glass

Infrared fluorescence, energy transfer process, thermal stability and quantitative analysis of Tm³⁺ doped novel niobium silicate-germanate glasses have been investigated. The thermal stability changes indicate that the introduction of La₂O₃ to substitute for Nb₂O₅ can improve the anti-crystallization of present glass system. Intense 1.8 μm fluorescence has been achieved and the value of emission cross section can reach as high as 12.2 × 10⁻²¹ cm². Besides, the microparameters of energy transfer were analyzed by the extended overlap integral method. Hence, the results indicate that the excellent spectroscopic characteristics of this kind of silicate-germanate glass together with the good thermal properties may become a promising matrix applied for 1.8 μm band near-infrared laser.     

Saturable absorber at 940 nm using single wall carbon nanotubes deposited by vertical evaporation technique

The vertical evaporation technique allows us to fabricate aligned single wall carbon saturable absorbers. The nonlinear parameters of the absorber at the wavelength of 940 nm were measured. The measured bi-exponential lifetimes of the absorber are 330 fs and 850 fs, respectively. The saturation intensity and modulation depth were found to be 2000 μJ/cm² and 10% for SWCNT absorber at the direction of alignment, in comparison to 950 μJ/cm² and 7% for the SWCNT solution.     

High resolution emission spectroscopy of the E2Π–X2Σ+ transition of SrH and SrD

Emission spectra of SrH and SrD have been studied at high resolution using a Fourier transform spectrometer. The molecules have been produced in a high temperature furnace from the reaction of strontium metal vapor with H₂/D₂ in the presence of a slow flow of Ar gas. The spectra observed in the 18 000–19 500 cm^?1 region consist of the 0–0 and 1–1 bands of the E²Π–X²Σ⁺ transition of the two isotopologues. A rotational analysis of these bands has been obtained by combining the present measurements with previously available pure rotation and vibration–rotation measurements for the ground state, and improved spectroscopic constants have been obtained for the E²Π state. The present analysis provides spectroscopic constants for the E²Π state as ΔG(½) = 1166.1011(15) cm^?1, B_e = 3.805503(32) cm^?1, α_e = 0.098880(47) cm^?1, r_e = 2.1083727(89) Å for SrH, and ΔG(½) = 839.1283(23) cm^?1, B_e = 1.918564(15) cm^?1, α_e = 0.034719(23) cm^?1, r_e = 2.1121943(83) Å for SrD.     

Spectroscopic analysis of Cr,Er:Gd3Ga5O12 crystals as candidates of potential xenon lamp pumped ~ 2.7 μm laser

A series of Cr,Er:Gd₃Ga₅O₁₂ crystals with high concentrations of Er^3 + were grown by Czochralski method. The absorption spectra, the up-conversion, near infrared (NIR) and mid-infrared (Mid-IR) luminescence spectra as well as the luminescence decay curves of Er: ⁴I_13/2 and ⁴I_11/2 levels were measured at room temperature. The spectroscopic properties of Cr,Er:Gd₃Ga₅O₁₂ crystals and Cr–Er energy transfer processes were investigated. The spectroscopy of the Er^3 +:⁴I_11/2 → ⁴I_13/2 transition was centralized to discuss, and the important optical parameters including luminescence lifetimes and the Cr–Er energy transfer efficiency are presented. Based on the comprehensive spectral analyses, 0.6 at.%Cr/50 at.%Er:GGG crystal is preferred as candidate of potential xenon lamp pumped ~ 2.7 μm laser in this work.     

Optical spectroscopy and laser parameters of Zn2+/Er3+/Yb3+-tridoped LiNbO3 crystal

The Zn/Er/Yb:LiNbO₃ and Er/Yb:LiNbO₃ crystals were grown by the Czochralski technique. The laser characteristics of 1.54 μm emission were predicted based on the Judd–Ofelt theory, and the intensity parameters Ω_t (Ω₂=7.23×10^?20 cm², Ω₄=3.15×10^?20 cm² and Ω₆=1.43×10^?20 cm²) were obtained. The stimulated emission cross sections (σ_em) at 1.54 μm emission in Zn/Er/Yb:LiNbO₃ were calculated based on the McCumber theory and the Füchtbauer–Ladenburg theory. The gain cross section spectrum of Zn/Er/Yb:LiNbO₃ crystal was also investigated. Under 980 nm excitation, a lenghthening lifetime of 1.54 μm emission and an enhancement of green upconversion emission were observed for Zn/Er/Yb:LiNbO₃ crystal. The studies on the power pump dependence and the upconversion mechanism suggested that both green and red upconversion emissions were populated via the three-photon process, and Zn²⁺ ion tridoping increases the probability of cross relaxation process between the two neighboring Er³⁺ ions.     

Critical evaluation of measured rotation–vibration transitions and an experimental dataset of energy levels of HD18O

All available transitions from microwave to visible region (0.2–12 105 cm^?1) of the HD¹⁸O molecule were collected and tested using the RITZ computer code. Literature data were completed by transitions assigned to HD¹⁸O in long path Fourier transform absorption spectra of the H₂O, HDO and D₂O gas mixtures with natural abundance of oxygen-18. In addition about 40 unassigned lines between 4200 and 6600 cm^?1 of our previous water study associated with the HD¹⁸O molecule have been found and assigned. The new long path absorption spectra of the HDO and D₂O mixtures allow us to observe about 1000 transitions of HD¹⁸O in the 6125–10 720 cm^?1 spectral region. These data have been critically analyzed and used to obtain the most complete and precise set of the experimental energy levels of this molecule.     

Spectroscopic properties of Co2+: ZnAl2O4 nanocrystals in sol–gel derived glass–ceramics

Transparent glass–ceramics containing zinc–aluminum spinel (ZnAl₂O₄) nanocrystals doped with tetrahedrally coordinated Co²⁺ ions were obtained by the sol–gel method for the first time. The gels of composition SiO₂–Al₂O₃–ZnO–CoO were prepared at room temperature and heat-treated at temperature ranging 800–950 °C. When the gel samples were heated up to 900 °C, ZnAl₂O₄ nanocrystals were precipitated. Co²⁺ ions were located in tetrahedral sites in ZnAl₂O₄ nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAl₂O₄ crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10–15 nm. The dependence of the absorption and emission spectra of the samples on heat-treatment temperature were presented. The difference in the luminescence between Co²⁺ doped glass–ceramic and Co²⁺ doped bulk crystal was analysed. The crystal field parameter D_q of 423 cm⁻¹ and the Racah parameters B of 773 cm⁻¹ and C of 3478.5 cm⁻¹ were calculated for tetrahedral Co²⁺ ions.     

Investigation of new band parameters with temperature dependence for self-broadened methane gas in the range 9000 to 14,000 cm−1 (0.71 to 1.1 μm)

This paper describes new measurements and modelling of the absorption of methane gas, one of the most important gases observed in the atmospheres of the outer planets and Titan, between 9000 and 14,000 cm^?1 (0.7 to 1.1 μm) and compares them with current best available spectral models.A series of methane spectra were measured at the UK's Natural Environment Research Council (NERC) Molecular Spectroscopy Facility (based at the Rutherford Appleton Laboratory, Oxfordshire, UK) using a Brüker 125HR Fourier transform spectrometer. To approximate the conditions found in outer planet atmospheres, the spectra were measured over a wide range of pressures (5 bar to 38 mbar) and temperatures (290–100 K) with path lengths of 19.3, 17.6, 16.0 and 14.4 m. The spectra were recorded at a moderate resolution of 0.12 cm^?1 and then averaged to 10 cm^?1 resolution prior to fitting a series of increasingly complex band-models including temperature dependence. Using the most complex model, a Goody line distribution with a Voigt line shape and two lower energy state levels, the typical rms residual error in the fit is between 0.01 and 0.02 in the wings of the main absorption bands.The new spectral parameters were then compared with the measured spectra and spectra calculated using existing data and shown to be able to accurately reproduce the measured absorption. The improvement in the temperature dependence included in the model is demonstrated by comparison with existing cold methane spectral data for a typical Jovian path.     

Upconversion luminescence dynamics of Er-doped fluoride crystals for optical converters

Erbium (Er)-doped fluoride crystals (YLF, BYF, CaF₂, etc.) are well-known as active media for solid-state lasers emitting in IR and VIS spectral domains, and as materials for efficient near-IR to VIS upconversion. In this paper, we report on the study of conversion of IR light from an ～1.5 μm spectral region to an ～1 μm spectral domain in low-phonon RE-doped fluoride crystals CaF₂ (RE=Er³⁺ Yb³⁺). Energy transfer processes taking place at selective pulsed and CW laser excitation are investigated experimentally. It is shown that in the CaF₂:RE crystals efficient conversion of IR radiation from the ～1.5 μm region to the ～1 μm region occurs, and these crystals are perspective for using in spectral converters for enhancing solar cell efficiency.     

Structural and luminescence properties of Nd3+-doped PbO–B2O3–TiO2–AlF3 glass for 1.07 μm laser applications

Trivalent neodymium doped multi-component lead borate titanate aluminumfluoride (LBTAFNd) glasses were prepared and characterized as a function of Nd³⁺ ions concentration through optical absorption, NIR luminescence and decay measurements. The intensity (Ω_2,4,6) and other radiative parameters were determined within the frame work of Judd–Ofelt theory. The intensities of absorption bands were expressed in terms of experimental oscillator strengths. Reasonably small root mean square deviation of ±0.384×10^?6 obtained between the experimental and calculated oscillator strengths indicates the validity of intensity parameters. Upon 805 nm laser excitation, the NIR emissions at 0.92 μm (⁴F_3/2→⁴I_9/2), 1.07 μm (⁴F_3/2→⁴I_11/2) and 1.35 μm (⁴F_3/2→⁴I_13/2) were observed. The spectroscopic quality factor has been determined from the Ω₄ and Ω₆ intensity parameters as well as the intensities of emission bands centered at 1.07 and 1.35 μm. The decay curves of the ⁴F_3/2 excited state were recorded by monitoring the emission and excitation wavelengths at 1.07 μm and 805 nm, respectively. The decay curves exhibit single exponential behavior for all the glasses. The laser characteristic parameters of ⁴F_3/2→⁴I_11/2 (1.07 μm) transition were determined and compared with other reported glasses.