Hydrofluorocarbons (HFCs) are used as substitutes for ozone-depleting substances regulated under the Montreal Protocol. While having zero ozone depletion potential, HFCs strongly absorb infrared (IR) radiation, making them potent greenhouse gases. Vibrational modes associated with C-F stretching absorb strongly within the atmospheric window (750-1250 cm(-1)), contributing substantially to radiative forcing. The low-frequency region (< 500 cm(-1)), which accounts for approximately 16% of the Earth's thermal emission, has however remained largely unexplored mainly due to instrumental challenges. Here, we present the first experimental measurements of IR absorption cross-sections in the 150-500 cm(-1) range for HFC-236fa, HFC-245fa, and HFC-43-10mee - three industrially relevant compounds with high global warming potentials (GWPs). The spectra were recorded at the Rutherford Appleton Laboratory using a high-resolution Fourier-transform infrared (FTIR) spectrometer in the temperature range between 225 and 298 K at resolution of 0.25 cm(-1). In addition, IR cross section spectra were simulated through quantum chemical (QC) calculations including a non-empirical treatment of anharmonic effects. From the experimental results, we derived effective radiative efficiencies (EREs) in the low-frequency region of 0.001, 0.005, and 0.003 W m(-2) ppb(-1) for HFC-236fa, HFC-245fa, and HFC-43-10mee, respectively, and revised global warming potentials over 20-, 100-, and 500-year time horizons. Comparison with values reported in the WMO Ozone Assessment Report 2022 reveals minor differences for HFC-245fa and HFC-43-10mee, whereas their value for HFC-236fa shows a significant overestimation, corresponding to a discrepancy of approximately 360 units in the 100-year GWP. Theoretical predictions reproduced experimental band strengths with an overall average deviation of 4%, confirming the reliability of the computational approach even in the low-frequency region. This indicates that the QC technique is likely to provide reliable estimates for RE and GWP for similar compounds where measurements are not available. These findings highlight that small variations in the treatment of low-frequency absorptions can propagate into substantial contributions in climate metrics, particularly for long-lived compounds. Overall, this study provides a consistent experimental-theoretical framework for quantifying the radiative forcing of HFCs and similar compounds and reduces current uncertainties in the estimation of their climate-relevant parameters.

The Harwell observatory, located in Oxfordshire, UK (51.571° N, 1.315° W), now part of the Total Carbon Column Observing Network (TCCON), has been performing ground-based remote sensing of averaged dry columns of atmospheric greenhouse gases since September 2020. Measurements are performed through near-infrared and shortwave infrared high-resolution spectroscopy of the atmosphere's transmission in direct sun viewing geometry, following the TCCON methodology. We report on the development, the measurements, and the performance of the observing system installed at Harwell. The hardware and software are described and characterized, as well as the outputted data quality, based on the 4-year data record collected so far. The Harwell site is demonstrated to produce data of high quality, well in line with the requirements for the TCCON infrastructure. The dataset is available at https://doi.org/10.14291/tccon.ggg2020.harwell01.R0 (Weidmann et al., 2023).

Ultrafast laser inscription (ULI) has previously been employed to fabricate volume diffraction gratings in chalcogenide glasses, which operate in transmission mode in the mid-infrared spectral region. Prior gratings were manufactured for applications in astrophotonics, at wavelengths around 2.5 μm. Rugged volume gratings also have potential use in remote atmospheric sensing and molecular spectroscopy; for these applications, longer wavelength operation is required to coincide with atmospheric transparency windows (3-5 μm) and intense ro-vibrational molecular absorption bands. We report on ULI gratings inscribed in IG2 chalcogenide glass, enabling access to the full 3-5 μm window. High-resolution broadband spectral characterization of fabricated gratings was performed using a Fourier transform spectrometer. The zeroth order transmission was characterized to derive the diffraction efficiency into higher orders, up to the fourth orders in the case of gratings optimized for first order diffraction at 3 μm. The outcomes imply that ULI in IG2 is well suited for the fabrication of volume gratings in the mid infrared, providing the impact of the ULI fabrication parameters on the grating properties are well understood. To develop this understanding, grating modeling was conducted. Parameters studied include grating thickness, refractive index modification, and aspect ratio of the modulation achieved by ULI. Knowledge of the contribution and sensitivity of these parameters was used to inform the design of a 4.3 μm grating expected to achieve > 95% first order efficiency. We will also present the characterization of these latest mid-infrared diffraction gratings in IG2.

The ultrafast laser inscription technique was used to fabricate mid-infrared volume gratings in the bulk of chalcogenide glass IG2 (Ge33As12Se55). These gratings are spectrally characterized using a Fourier transform infrared spectrometer, by measuring the transmission spectrum over the entire transmission band of the IG2 substrate. The gratings exhibit first order diffraction across the entire 2–5 μm spectral band, with the specific transmission band tuned by changing the angle of incidence. Higher diffraction orders are also observed. High-resolution spectral data is presented alongside detailed modelling and analysis. This work provides the basis for future design of mid-infrared transmission gratings; ULI provides a low-cost, robust alternative to mid-infrared reflection gratings, with the added capability to engineer the grating response to a specific application.

The ultrafast laser inscription technique was used to fabricate mid-infrared volume gratings in the bulk of chalcogenide glass IG2 (Ge33As12Se55). These gratings are spectrally characterized using a Fourier transform infrared spectrometer, by measuring the transmission spectrum over the entire transmission band of the IG2 substrate. The gratings exhibit first order diffraction across the entire 2–5 μm spectral band, with the specific transmission band tuned by changing the angle of incidence. Higher diffraction orders are also observed. Highresolution spectral data is presented alongside detailed modelling and analysis. This work provides the basis for future design of mid-infrared transmission gratings; ULI provides a low-cost, robust alternative to mid-infrared reflection gratings, with the added capability to engineer the grating response to a specific application. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. 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Ultrafast laser inscription is a versatile manufacturing technique which can be used to modify the refractive index of various glasses on a microscopic scale. This enables the production of a number of photonic devices such as waveguides, beam-splitters, photonic lanterns, and diffraction gratings. In this paper, we report on the use of ultrafast laser inscription to fabricate volume phase transmission gratings in mid-infrared transmitting chalcogenide glass.We describe the optimisation of the laser inscription process parameters enhancing grating performances via the combination of spectrally resolved grating transmission measurements and theoretical analysis models. The first order diffraction efficiency of the gratings was measured at mid-infrared wavelengths (3-5 mu m), and found to exceed 60% at the Littrow blaze wavelength, compared to a substrate external transmittance of 67%. This impressive result implies the diffraction efficiency should exceed 90% for a grating substrate treated with an anti-reflection coating. There is excellent agreement between the modelled grating efficiency and the measured data, and from a least squares fit to the measured data the refractive index modulation achieved during the inscription process is inferred. These encouraging initial results demonstrate that ultrafast laser inscription of chalcogenide glass may provide a potential new and alternative technology for the manufacture of astronomical diffraction gratings for use at near-infrared and mid-infrared wavelengths.

Expired breath δ(13)CO2 measured in real time serves as a useful biomarker of altered macronutrient metabolism in response to changes in energy balance. Altered breath δ(13)CO2 is believed to be a result of changes in macronutrient oxidation and the kinetic isotope effect where enzymatic processes discriminate against metabolites naturally enriched with (13)C. Use of breath δ(13)CO2 as a rapid biofeedback of energy balance status will enhance an individual's ability to modify behavior during weight loss efforts. Herein we describe a novel approach for immediate biofeedback for energy deficit using a moderate exercise challenge. Our new mid-infrared isotope ratio-meter for δ(13)CO2 is a step toward miniaturization of a personal device for instant biofeedback for people attempting to lose weight.

Accurate and sensitive methods of monitoring greenhouse gas (GHG) emission over large areas has become a pressing need to deliver improved estimates of both human-made and natural GHG budgets. These needs relate to a variety of sectors including environmental monitoring, energy, oil and gas industry, waste management, biogenic emission characterization, and leak detection. To address the needs, long-distance open-path laser spectroscopy methods offer significant advantages in terms of temporal resolution, sensitivity, compactness and cost effectiveness. Path-integrated mixing ratio measurements stemming from long open-path laser spectrometers can provide emission mapping when combined with meteorological data and/or through tomographic approaches.

Atmospheric methane concentration levels were detected using a custom built laser dispersion spectrometer in a long open-path beam configuration. The instrument is driven by a chirped distributed feedback mid-infrared quantum cascade laser centered at ~1283.46 cm-1 and covers intense rotational-vibrational transitions from the fundamental ν4 band of methane. A full forward model simulating molecular absorption and dispersion profiles, as well as instrumental noise, is demonstrated. The instrument's analytical model is validated and used for quantitative instrumental optimization. The temporal evolution of atmospheric methane mixing ratios is retrieved using a fitting algorithm based on the model. Full error propagation analysis on precision gives a normalized sensitivity of ~3 ppm.m.Hz-0.5 for atmospheric methane.

A molecular detection method based on laser dispersion spectroscopy has been implemented to detect methane concentrations with an open-path configuration and using transitions from the ν4 band. Preliminary results on transient seeps are presented.

A grating-tuned extended-cavity quantum cascade laser (EC-QCL) operating around 7.6 µm was assembled to provide a tuning range of ~80 cm⁻¹ with output power of up to 30 mW. The EC-QCL output power was shown to be sensitive to the presence of a broadband absorbing gas mixture contained in a 2-cm cell introduced inside the extended laser cavity. In this arrangement, enhanced absorption relative to single path linear absorption was observed. To describe observations, in the QCL rate-equation model was included the effect of intracavity absorption. The model qualitatively reproduced the absorption behavior observed. In addition, it allowed quantitative measurements of mixing ratio of dimethyl carbonate, which was used as a test broadband absorber. A number of alternative data acquisition and reduction methods were identified. As the intracavity absorber modifies the laser threshold current, phase-sensitive detection of the laser threshold current was found to be the most attractive way to determine the mixing ratio of the absorber. The dimethyl carbonate detection limit was estimated to be 1.4 ppmv for 10 second integration. Limitations and possible ways of improvements were also identified.