Ν3 Band Research Articles

Effective Dipole Moment Model for Axially Symmetric C3v Molecules: Application to the Precise Study of Absolute Line Strengths of the ν6 Fundamental of CH335Cl.

The effective dipole moment model for molecules of axial C3v symmetry is derived on the basis of the symmetry properties of a molecule which, on the one hand, is of the same order of efficiency (but much simpler and clearer in applications) as the analogous models derived on the basis of the irreducible tensorial sets theory, and, on the other hand, mathematically more correct in comparison with concepts like the Herman-Walles function used in the models. As an application of the general results obtained, we discuss high-resolution infrared spectra of CH335Cl, recorded with the Zürich prototype ZP2001 (Bruker IFS125 HR) Fourier transform infrared spectrometer at a resolution of 0.001 cm-1 and analyzed in the region of 880-1190 cm-1 (ν6 bending fundamental centered at ν0 = 1018.070790 cm-1). Absolute strengths of more than 2800 transitions (2081 lines) were obtained from the fit of their shapes both with Voigt and Hartmann-Tran profiles, and parameters of the effective dipole moment of the ν6 band were determined by the computer code SYMTOMLIST (SYMmetric TOp Molecules: LIne STrengths), created on the basis of a derived theoretical model. As the first step of the analysis of the experimental data, assignments of the recorded lines were made. A total of 5124 transitions with Jmax = 68, Kmax = 21 were assigned to the ν6 band. The weighted fit of 2077 upper energy values obtained from the experimentally recorded transitions was made with a Hamiltonian which takes into account different types of ro-vibrational effects in doubly degenerate vibrational states of the C3v-symmetric molecule. As the result, a set of 25 fitted parameters was obtained which reproduces the initial 2077 upper "experimental" ro-vibrational energy values with a root mean square deviation drms=4.7×10-5 cm-1. At the second step of the analysis, the computer code SYMTOMLIST was used for determination of the parameters of the derived effective dipole moment model. Six effective dipole moment parameters were obtained from the weighted fit procedure which reproduces absolute experimental strengths of the 2804 initial experimental transitions with a relative drms=3.4%.

Effect of chemical composition on zircon radiation damage dating: Implications for low-temperature thermochronology

Zircon radiation damage dating is a low-temperature thermochronological method that can reveal the cooling histories of magmatic intrusions and discriminate sedimentary provenance in combination with other dating methods. This method has broad application prospects because of its advantages of non-destructive, high efficiency, and capable of double (or multiple) dating, involving only multiple measurements by Raman spectrometer and laser ablation-inductively coupled plasma-mass spectrometry. However, several factors, such as zircon chemical composition and the non-uniformity of radiation damage annealing kinetics, can cause poor precision when using this method and thus restricts its wide application. This study examined the effect of chemical composition (P, Ti, Dy, Th, U, and Hf) on Raman spectra using synthetic zircon crystals grown in a lithium-molybdate flux. The results show that the full width at half-maximum (FWHM) of the ν3(SiO4) band has positive linear correlations with the concentrations of P, Ti, Dy, Th, and U in decreasing order of influence, while the FWHM is unaffected by Hf at concentrations <1 wt.% but increases at concentrations >10 wt.%. Furthermore, the Raman shift is negatively correlated with Th, U, and Dy concentrations, positively correlated with Hf, and shows no obvious correlation with Ti and P. Thus, our study shows that chemical composition is a non-ignorable factor for calculating zircon radiation damage age using Raman spectroscopy, especially for zircon with relatively high concentrations of P, rare earth elements (REEs), Th, U, and Hf. The obtained multiple linear regression equation provides a potential means for estimating the FWHM at zero dose and implication for improving the dating precision of this method. In addition, the observed effects of REEs, Th, U, and Hf on the Raman shift of the ν3(SiO4) band indicate that chemical composition in zircon might affect the estimation of the P-T conditions of geological processes when using entrapped zircon inclusions in host minerals or the field of zircon as an in situ pressure sensor in hydrothermal experiments. Our study suggests that zircon radiation damage dating, excluding geochemical effects, will be more accurate for addressing lower-temperature geological processes.

The influence of line mixing on the j and k dependencies of halfwidths and temperature exponents in N2-broadening coefficients of CH3F spectral lines

The N2-broadening halfwidths of CH3F in the ν5 and ν6 perpendicular bands have been calculated, along with their temperature exponents. These calculations utilize a modified and refined version of the Robert-Bonamy formalism, developed by the current authors within a semi-classical line shape framework. Extensive comparisons between the predicted halfwidths from the model and experimental measurements at 296 K and 183 K are presented. Our latest model accurately predicts the dependencies of the halfwidths on both the j and k quantum numbers. Furthermore, by extending our calculations to two additional temperatures, namely 240 K and 350 K, the temperature exponent N is determined for various sub-branches. The dependencies of N on j and k are analyzed, and theoretical explanations are provided to elucidate the predicted behaviors of N.

Fourier transform infrared (FTIR) spectroscopy of formaldoxime isotopologue 12CD2NOH in the 300–3700 cm−1 region and its ν12 and ν9 bands: Vibrational and rovibrational analyses

The Fourier transform infrared (FTIR) spectrum of the formaldoxime isotopologue 12CD2NOH was recorded in the 500–3700 cm−1 region with a resolution of 0.50 cm−1 to identify its fundamental, overtone and combination bands and to measure their relative infrared (IR) band intensities. Furthermore, the high-resolution (0.00096 cm−1) FTIR spectrum of ν12 and ν9 bands of 12CD2NOH was recorded in Australian Synchrotron in the 300–510 cm−1 region for a rovibrational analysis. A total of 1060 IR transitions of the C-type ν12 band were fitted using the Watson's A-reduced Hamiltonian in the Ir representation with a root-mean-square (rms) deviation of 0.000524 cm−1. From the rovibrational analysis, the v12 = 1 state rovibrational constants up to all 5 quartic centrifugal distortion terms were derived for the first time. The band center of the ν12 band of 12CD2NOH was found to be 391.214740(46) cm−1. The ground state rovibrational constants up to all 5 quartic terms were determined for the first time by the fitting of 423 ground state combination differences (GSCDs) derived from the IR transitions of the ν12 band of 12CD2NOH of this work. The rms deviation of the GSCD fit was 0.000473 cm−1 using the Watson’s A-reduced Hamiltonian in the Ir representation. Furthermore, a total of 724 IR transitions of the predominantly B-type ν9 band of 12CD2NOH were fitted with a rms deviation of 0.000360 cm−1 to derive the band center at 465.151277(39) cm−1 and rovibrational constants of the v9 = 1 state up to 4 quartic terms for the first time. Additionally, all 3 rotational constants and 5 quartic centrifugal distortion terms of the ground state and 3 rotational constants of the v12 = 1 and v9 = 1 states of 12CD2NOH were computed from theoretical anharmonic calculations at 2 different levels of theory, B3LYP and MP2 with the cc-pVTZ basis set, for comparison with the experimental results. Close agreement was found for the calculated and experimental rotational constants of 12CD2NOH for the ground, v12 = 1 and v9 = 1 states. The vibrational frequencies (cm−1) of 12 fundamental bands of 12CD2NOH in the 300–3700 cm−1 region, and their IR band intensities (km/mol) were also calculated using B3LYP and MP2 with the cc-pVTZ basis set, and they were compared with the respective experimental data.

The pressure effect on the line profiles observed in the [formula omitted] band of acetylene: Revisited

In our previous paper [K. Iwakuni et al., Phys. Rev. Lett.117, 1439026 (2018)] the ortho-para dependence of the pressure effects was reported for the ν1+ν3 band of acetylene measured by dual-comb spectroscopy. Sears and coworkers [E.C. Gross et al., J. Chem. Phys.154, 054305 (2021)] reevaluated the profile parameters for some lines of this band by comb-referenced laser spectroscopy, and pointed out that the ortho-para phenomenon must be a consequence of the use of Voigt function as the profile function, with the Doppler widths fixed at their theoretical values. Thus, we have remeasured this band with two improved spectrometers, the comb-locked laser spectrometer of the National Metrology Institute of Japan and another dual-comb spectrometer of Yokohama National University, and analyzed the lines by using the speed-dependent Voigt function as the profile function with several options. By the present work, we confirm the fact pointed out by Sears and coworkers, and present some remaining problems in the experimental determination of the line profile parameters.

High resolution FTIR spectrum of CH2D37Cl: ν4 and ν8 fundamental bands

The high-resolution Fourier transform infrared spectrum of CH2D37Cl has been analyzed in the region of the ν4 and ν8 bands from 1170 to 1370 cm−1. The upper states of these fundamentals, separated by 0.6 cm−1, interact with each other by a-type as well as b-type Coriolis resonance, and a strong asymmetric distribution of the intensity of the rotational structure in each band is observed. The spectral analysis resulted in the assignment of 2774 and 1611 rovibrational transitions for ν4 and ν8 bands, respectively, which have been simultaneously fitted using Watson's A-reduction Hamiltonian in the Ir representation and the relevant perturbation operators. A set of spectroscopic constants for the two fundamentals as well as seven coupling terms have been determined. From spectral simulations, the dipole moment ratio |Δµa/Δµb| of ν4 has been determined to be 1.3 ± 0.1 while the intensity ratio between ν4 and ν8 has been estimated to be 5.0 ± 1.0.

S 2 − and S3− radicals and the S42− polysulfide ion in lazurite, haüyne, and synthetic ultramarine blue revealed by resonance Raman spectroscopy

Abstract Taking advantage of the Raman resonance effect, we employed 405 and 532 nm excitations to (1) identify sulfur species present in lazurite, haüyne, and synthetic ultramarine blue pigments and (2) investigate the enigmatic ~485 cm–1 band found previously in Raman spectra of lazurite and haüyne collected with 458 nm excitation. In spectra of lazurite and haüyne, bands of the sulfate ion and S2− and S3− radicals can be seen. Spectra collected using 405 nm excitation show the enhancement of the intensity of ν1(S2−) band and its nν1 (n ≤ 7) progression. Spectra collected using 532 nm incident light show the enhancement of intensity of ν1(S3−), ν2(S3−), and ν3(S3−) bands and the nν1 (n ≤ 9) and ν2 + nν1 progressions of the ν1(S3−) band. In spectra collected with 405 nm excitation, we also found features that we ascribe to the S42− polysulfide ion. These include the ν1 symmetric S-S stretching band at ~481 cm–1, the ν2 symmetric S-S stretching band at ~443 cm–1 (only present in spectra of some lazurite samples), the ν3 symmetric S-S bending at 223 cm–1 and the nν1 (n ≤ 5) and nν1+ν3 progressions of the ν1(S42−) band. We observed that under laser illumination, the S42− polysulfide ion rapidly decomposes into two S2− radicals in lazurite while it remains stable in haüyne. In spectra of synthetic ultramarine blue pigments, only features of S2− and S3− radicals were observed. Finally, we verified the identity of the radical and polysulfide ions with ab initio molecular dynamics calculations. We conclude that Raman resonance spectroscopy is a powerful qualitative method to detect polysulfide and sulfur radical species with concentrations below the detection limit of conventional analytical techniques. Owing to the high stability of S42− in haüyne, this mineral structure appears promising as a host material for S42− entrapment, making it potentially useful for applications in optoelectronics.

Line positions and intensities of the ν1 band of 12CH3I using mid-infrared optical frequency comb Fourier transform spectroscopy

We present a new spectral analysis of the ν1 and ν3+ν1−ν3 bands of 12CH3I around 2971 cm−1 based on a high-resolution spectrum spanning from 2800 cm–1 to 3160 cm–1, measured using an optical frequency comb Fourier transform spectrometer. From this spectrum, we previously assigned the ν4 and ν3+ν4−ν3 bands around 3060 cm–1 using PGOPHER, and the line list was incorporated in the HITRAN database. Here, we treat the two fundamental bands, ν1 and ν4, together with the perturbing states, 2ν2+ν3 and ν2+2ν6±2, as a four-level system connected via Coriolis and Fermi interactions. A similar four-level system is assumed to connect the two ν3+ν1−ν3 and ν3+ν4−ν3 hot bands, which appear due to the population of the low-lying ν3 state at room temperature, with the 2ν2+2ν3 and ν2+ν3+2ν6±2 perturbing states. This spectroscopic treatment provides a good global agreement of the simulated spectra with experiment, and hence accurate line lists and band parameters of the four connected vibrational states in each system. It also allows revisiting the analysis of the ν4 and ν3+ν4−ν3 bands, which were previously treated as separate bands, not connected to their ν1 and ν3+ν1−ν3 counterparts. Overall, we assign 4665 transitions in the fundamental band system, with an average error of 0.00071 cm–1, a factor of two better than earlier work on the ν1 band using conventional Fourier transform infrared spectroscopy. The ν1 band shows hyperfine splitting, resolvable for transitions with J ≤ 2 × K. Finally, the spectral intensities of 65 lines of the ν1 band and 7 lines of the ν3+ν1−ν3 band are reported for the first time using the Voigt line shape as a model in multispectral fitting. The reported line lists and intensities will serve as a reference for high-resolution molecular spectroscopic databases, and as a basis for line selection in future monitoring applications of CH3I.

The 2ν3/ν2/ν5/ν3 + ν6 band system of CH3Br revisited: Anharmonic and Coriolis interactions in a four-level system near 1400 cm−1

The infrared spectrum of gaseous methyl bromide (CH3Br) in natural abundance has been analyzed with high resolution, between 1150 and 1700 cm−1. In this spectral region, two fundamental bands, ν2 (A1) and ν5 (E), an overtone 2ν3 (A1) and a combination band ν3 + ν6 (E) occur. The novelty of the present study consists in considering, besides the strong Coriolis and α-interactions coupling the v2 = 1 and v5 = 1 levels, a large variety of anharmonic and rovibrational interactions involving also the v3 = 2 and v3 = v6 = 1 levels. For this latter level, the data set belonging to the ν3 + ν6 combination band has been extended with respect to previous work of Ouahman et al. [Spectrochim. Acta45A (1989) 175–179], to about thousand assigned transitions with -6≤K·ΔK≤+16. Thanks to the completeness of the theoretical model, the global standard deviation of the reproduction of the experimental wavenumbers in the ν2/ν5 system was greatly improved with respect to the previous high-resolution study of Kwabia Tchana et al. [J. Mol. Spectrosc.228 (2004) 441–452]. For this Coriolis-interacting band system, two different reduction schemes, according to the theory of Stříteská et al. [J. Mol. Spectrosc.256 (2009) 135–140], were applied and were proved to be equally successful.

Observation and analysis of ν1 + ν3 and ν1 + 2ν4 bands of CF4 molecule

Infrared spectra of CF4 molecule were measured for three different temperatures in the range 2160–2210 cm−1 using Fourier transform spectrometer. Non-empirical effective Hamiltonian obtained from the potential energy surface using the contact transformation method was used for the modeling of line positions. The origins of two bands ν1 + ν3 (F2) and ν1 + 2ν4 (F2) at 2186.97 and 2168.21 cm−1 were experimentally determined for the first time. The experimental origin of the weak ν1 + 2ν4 (E) sub-band was tentatively evaluated near 2169.84 cm−1. Ab initio line intensities were calculated from high order dipole moment surface by variational method using the TENSOR computational code in the normal mode representation. The analyses of experimental spectra were carried out using the MIRS program suite accounting for the full tetrahedral symmetry of the molecule.

Broadening of the ν2 Raman Band of CH4 by C3H8 and C4H10.

Raman spectroscopy is a promising method for the analysis of natural gas. It is necessary to account for the broadening effects on spectral lines to improve measurement accuracy. In this study, the broadening coefficients for methane lines in the region of the ν2 band perturbed by propane, n-butane, and isobutane at room temperature were measured. We estimated the measurement errors of the concentration of oxygen and carbon dioxide in the case of neglecting the broadening effects on the methane spectrum by the pressure of C2-C6 alkanes. The obtained data are suited for the correct simulation of the methane spectrum in the hydrocarbon-bearing gases and can be used to improve the accuracy of the analysis of natural gas by Raman spectroscopy.

Study of the R-Branch of the 3ν3 Band of 13CH4 in the 1-μm Region

Study of the R-Branch of the 3ν3 Band of 13CH4 in the 1-μm Region

Measurement of self-broadening coefficients and line intensities of ν1 + ν3 + ν4 ‒ ν4 band of acetylene in 1.5 μm spectral region: External cavity diode laser based absorption study

Acetylene is considered to be an important molecule for spectroscopic research due to its role in environmental chemistry, astrophysics, biochemistry and photochemistry of various planets. The high resolution spectroscopic studies of acetylene (12C2H2) in the 6474–6515 cm−1 spectral range have been performed using in-house developed external-cavity diode laser absorption spectroscopy technique. The study is focused on experimental measurement of self-broadening coefficients (γself) and line intensities of 27 l-doublet ro-vibrational lines belonging to the P-branch transition of ν1 + ν3 + ν4 ‒ ν4 hot band lying in the 6474–6515 cm−1 spectral range. The γself and line intensity have been retrieved by modelling of each experimentally observed rotational line with standard Voigt line profile using custom written python based computer program. The measured γself and line intensities were compared with the values present in HITRAN2020 database, showing mean difference of about -1.9% and 2.48%, respectively. In contrast to the line intensity, where rotational and symmetry dependency are readily visible, γself exhibits only rotational dependency. The l-type doubling constants of both ν1 + ν3 + ν4 and ν4 states of ν1 + ν3 + ν4 ‒ ν4 hot band have also been obtained. Detailed understanding of these molecular parameters will play an important role in modelling of planetary atmospheres.

First investigation of the ν1, ν1-ν9, ν1+ν9, and ν1+ν7 absorption bands of nitric acid (1H14N16O3) at 3551.766 cm−1, 3092.708 cm−1, 4006.974 cm−1, and 4127.782 cm−1, respectively

This paper is the first of two back-to-back articles describing the first high resolution study and analysis of the rovibrational structure of the (mainly) A-type ν1 band, the (mainly) B-type ν1+ν7 band, and of the C-type ν1-ν9 and ν1+ν9 bands of nitric acid (H14N16O3) centered at 3551.766 cm−1, 4127.782 cm−1, 3092.708 cm−1, and 4006.974 cm−1, respectively. For this, high resolution Fourier transform laboratory spectra in the 2.8 µm, 2.4 µm, 3.2 µm, and 2.5 µm spectral regions were recorded at the SOLEIL synchrotron. The ν1 band is highly congested and perturbed and the identifications were often uncertain because of line overlap, due to the large value of the Doppler linewidths (∼0.003 cm−1 HWHM at 296 K). However, combining the analysis of the ν1 band with that of the ν1-ν9 difference band (centered at 3092.708 cm−1) proved to be highly helpful. Surprisingly enough, it appeared that the 11 energy levels are affected by torsional splitting, probably due to anharmonic resonances coupling together the 11 levels with those of vibrational states involving high excitation in the ν9 mode. Using an effective approach, we have generated a list of positions and intensities for the ν1 band and for the ν1+ν9-ν9 and ν1+ν7-ν7 hot bands in the 2.8 µm region.

Etalon-Assisted Determination of the Complex Index of Refraction of a Solution for the Study of Strong Cavity-Vibrational Coupling of PF6- in Acetonitrile.

A new method is established using an etalon cavity to assist in the determination of the wavelength-dependent complex index of refraction of a solution throughout the mid-infrared range. The results are used to study the cavity-vibration polaritons of PF6- in acetonitrile. Mixed states are formed by placing solution inside a pair of parallel plate mirrors with a wavelength-scale spacing, i.e., within an etalon, such that there are cavity states that are angle-tuned into resonance with the strong P-F vibrations. The dominant ν3 vibrations of PF6- consist of nearly triply degenerate oscillations of the partial-positively charged phosphorous against antisymmetric concerted motions of different sets of fluorine atoms with partial negative charges. These vibrations are dominant even though the solute is 29 times less concentrated than the solvent on a molar basis. The first part of the paper describes the method of determining the complex index of refraction of the solution from a combination of etalon transmission maxima and the attenuated total reflection (ATR) absorption spectrum of the solution. The results are presented as an analytical function including a sum of 37 vibrational contributions. Absolute integrated isolated band intensities were determined to be 463 ± 4, 462 ± 7, and 266 ± 4 km/mol for the three ν3 PF6- vibrations at 841.4, 847.4, and 854.0 cm-1, respectively, which sum to 1191 ± 9 km/mol for the ν3 band. Then, the results are used to simulate the measured etalon transmission using the transfer matrix (TM) method with and without the ν3 target vibrations. The etalon transmission simulations reconstruct the position of cavity modes in the absence of target vibrations. They provide input data for the testing of simple quantum mechanical models for the interaction of vibrations with cavity modes and the interactions of vibrations with other vibrations within the molecule and between solute and solvent. The model shows that the nearly degenerate ν3 vibrations interact with each other with a vibration-vibration coupling of 33 ± 5 cm-1. This is comparable to the cavity-vibration coupling of 30.4 ± 2.9 cm-1 of the two strongest vibrations of PF6-.

Self-broadening and self-shift in the [formula omitted] band of ammonia from mid-infrared-frequency-comb spectroscopy

We report the broadband absorption spectrum of the 3ν2 band of 14NH3 near 4 μm. The data were recorded using a mid-infrared frequency comb coupled to a homebuilt Fourier-transform spectrometer with a resolution of 0.00501 cm−1. Line positions, line intensities, self-broadening, and self-shift parameters for six rovibrational lines were determined at room temperature (T=296 K). Comparison with HITRAN 2016 shows good agreement at improved precision. The high precision and the rapid tunability of our experiment enables advanced fast spectroscopy of molecular gases.

Pressure broadening in Raman spectra of CH4–N2, CH4–CO2, and CH4–C2H6 gas mixtures

Different molecular environments change the spectrum of a given gas sample involved in a mixture compared to the spectrum of a pure gas. It is necessary to account for this effect to improve the accuracy of the analysis of the natural gas composition by Raman spectroscopy. First, the change in the main components of natural gas (methane, nitrogen, carbon dioxide, and ethane) must be considered. This work is devoted to the mutual influence of CH4–N2, CH4–CO2, and CH4–C2H6 on their characteristic Raman bands in the range of 300–2500 cm−1. The half-width and asymmetry of the Q branches of N2, CO2, and C2H6 as a function of methane concentration were obtained in the range of 1–50 bar. The averaged broadening coefficients of the rotational-vibrational lines of the ν2 band of CH4 perturbed by N2, CO2, and C2H6 are measured. A high-sensitivity spectrometer with a resolution of 0.5 cm−1 based on spontaneous Raman scattering was used to obtain reliable results. The algorithm and all the necessary parameters for simulating the effect of various molecular environments on the Raman bands of the main components of natural gas are presented.

Part 2:Validation for line lists generated for nitric acid (H14N16O3) for the ν1 band and its first two associated hot bands (ν1+ν9-ν9, ν1+ν7-ν7) in the 2.8 μm region, the ν1-ν9, ν1+ν9 and ν1+ν7 bands at 3.2 μm, 2.5 μm and 2.4 μm, respectively

This is the second of two back-to-back works, whose goal is to generate line lists for the ν1 band (3551.766 cm−1) and for the ν1+ν9-ν9 and ν1+ν7-ν7 hot bands for HNO3 in the 2.8 μm region. Also, we computed line lists for the weaker ν1-ν9, ν1+ν9, and ν1+ν7 bands, centered at 3093.53 cm−1, 4006.97 cm−1 and 4127.78 cm−1, respectively. While the first paper (Ref.[1], the preceding article in this issue), here labeled as “Part-1”, was devoted to the description of the determination of the needed spectroscopic parameters in term of line positions and intensities, this paper describes the resulting line lists and the validation process. For this task, we use laboratory spectra recorded at high resolution and described in Part-1, together with the cross-sections established by the Pacific Northwest Laboratory at low resolution. Also, we use high resolution stratospheric solar occultation spectra recorded by the MkIV balloon-borne instrument of the NASA Jet Propulsion Laboratory (JPL). Similar inter-comparisons with laboratory or atmospheric spectra are also performed using the pseudo-line-list generated at JPL in 2005 and with the ab initio ExoMol list calculated by the University College of London. It proves that the line list generated during this work provides significant better agreements between observed and computed spectra.

Highly sensitive methane detection using a mid-infrared interband cascade laser and an anti-resonant hollow-core fiber.

For over a decade hollow-core fibers have been used in optical gas sensors in the role of gas cells. However, very few examples of actual real-life applications of those sensors have been demonstrated so far. In this paper, we present a highly-sensitive hollow-core fiber based methane sensor. Mid-infrared distributed feedback interband cascade laser operating near 3.27 µm is used to detect gas inside anti-resonant hollow-core fiber. R(3) line near 3057.71 cm-1 located in ν3 band of methane is targeted. Compact, lens-free optical setup with an all-silica negative curvature hollow-core fiber as the gas cell is demonstrated. Using wavelength modulation spectroscopy and 7.5-m-long fiber the detection limit as low as 1.54 ppbv (at 20 s) is obtained. The demonstrated system is applied for a week-long continuous monitoring of ambient methane and water vapor in atmospheric air at ground level. Diurnal cycles in methane concentrations are observed, what proves the sensor's usability in environmental monitoring.

Temperature dependence of the absorption of the R(6) manifold of the 2ν3 band of methane in air in support of the MERLIN mission

Fifty nine high sensitivity spectra of the R(6) manifold of the 2ν3 band of methane in air, near 1.64 µm, have been recorded in support of the MERLIN mission. For this purpose, a cavity ring down spectrometer (CRDS) with a spectrally narrowed and stable (sub-kHz) laser source was coupled to a temperature regulated high-finesse optical cavity. The frequency scale of each spectrum was accurately determined from measurements of the beat note between a part of the laser light and the closest tooth of a frequency comb referenced to a rubidium clock. Series of spectra were recorded between 243 and 313 K with a 10 K temperature step. For each series, total pressure values of 50, 100, 250, 500 and 750 Torr were adopted. A multi-spectrum fitting procedure with the Hartmann-Tran (HT) line profile, including the first-order line-mixing parameter, has been used to derive the spectroscopic parameters for each of the six R(6) components, along with the temperature dependence of the line-shape parameters. The results show that the fitted effective model is able to reproduce the experimental spectra with a relative precision better than 0.2% for the entire R(6) manifold spectral region and better than 0.05% at the ON-line position of the MERLIN mission for the 250, 500 and 750 Torr spectra. The relative precision increases to 0.3% and the residuals at the ON-line position to 0.1% when including the 50 and 100 Torr spectra. Comparisons with ground-based atmospheric measurements show that these data significantly improve the modeling of methane absorption in this spectral region. The complete line list of the methane spectrum in the region of the R(6) manifold allowing notable improvement of the modeling of the absorption cross-section at the ON-line position of the MERLIN mission is provided as Supplementary Material.