Abstract:The report of an IUPAC Task Group, formed in 2011 on "Intensities and line shapes in high-resolution spectra of water isotopologues from experiment and theory" (Project No. 2011-022-2-100), on line profiles of isolated high-resolution rotational-vibrational transitions perturbed by neutral gas-phase molecules is presented. The well-documented inadequacies of the Voigt profile (VP), used almost universally by databases and radiative-transfer codes, to represent pressure effects and Doppler broadening in isolated vibrationalrotational and pure rotational transitions of the water molecule have resulted in the development of a variety of alternative line-profile models. These models capture more of the physics of the influence of pressure on line shapes but, in general, at the price of greater complexity. The Task Group recommends that the partially Correlated quadratic-Speed-Dependent Hard-Collision profile (pCqSD-HCP) should be adopted as the appropriate model for high-resolution spectroscopy. For simplicity this should be called the Hartmann-Tran profile (HTP). The HTP is sophisticated enough to capture the various collisional contributions to the isolated line shape, can be computed in a straightforward and rapid manner, and reduces to simpler profiles, including the Voigt profile, under certain simplifying assumptions.

A spectroscopic method of molecular detection based on dispersion measurements using a frequency-chirped laser source is presented. An infrared quantum cascade laser emitting around 1912 cm(-1) is used as a tunable spectroscopic source to measure dispersion that occurs in the vicinity of molecular ro-vibrational transitions. The sample under study is a mixture of nitric oxide in dry nitrogen. Two experimental configurations based on a coherent detection scheme are investigated and discussed. The theoretical models, which describe the observed spectral signals, are developed and verified experimentally. The method is particularly relevant to optical sensing based on mid-infrared quantum cascade lasers as the high chirp rates available with those sources can significantly enhance the magnitude of the measured dispersion signals. The method relies on heterodyne beatnote frequency measurements and shows high immunity to variations in the optical power received by the photodetector.

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The advent of quantum cascade lasers has provided matured continuously tunable solid state laser sources emitting from mid-infrared to terahertz wavelengths. Such sources, used as local oscillators, offer the practical prospect of aircraft, high altitude platform, and satellite deployment of compact and shot noise limited heterodyne radiometers for Earth observation and astronomy. A ground-based prototype of a quantum cascade laser heterodyne radiometer operating in the mid-infrared has been developed and is presented. The instrument design and concepts are described, together with evaluation of the instrument in the laboratory and during field measurements of atmospheric ozone. In this study the best performance achieved by the prototype quantum cascade laser heterodyne radiometer was a signal-to-noise ratio of three times the theoretical shot-noise limit. The prototype has allowed the main sources of excess noise to be identified as residual optical feedback in the local oscillator optical path and a lack of mechanical and thermal stability in the local oscillator collimation system. Instrument improvements are currently being implemented and enhanced performance is expected in the near future.

Simultaneous exhaled carbonyl sulfide (OCS) and carbon dioxide concentration measurements in human breath are demonstrated with a compact pulsed quantum-cascade laser-based gas sensor. We achieved a noise-equivalent sensitivity (1sigma) of 1.2 parts per billion by measuring a well-isolated OCS P(11) absorption line in the v3 band at 2057.6 cm(-1) using an astigmatic Herriott cell of 36-m optical path length and 0.4-s acquisition time.

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A recently developed distributed feedback quantum cascade laser (QCL) capable of thermoelectric-cooled (TEC) continuous-wave (cw) operation and emitting at ∼ 9 µm is used to perform laser chemical sensing by tunable infrared spectroscopy. A quasi-continuous-wave mode of operation relying on long current pulses (∼ 5 Hz, ∼ 50% duty cycle) is utilized rather than pure cw operation in order to extend the continuous frequency tuning range of the quantum cascade laser. Sulfur dioxide and ammonia were selected as convenient target molecules to evaluate the performance of the cw TEC QCL based sensor. Direct absorption spectroscopy and wavelengthmodulation spectroscopy were performed to demonstrate chemical sensing applications with this novel type of quantum cascade laser. For ammonia detection, a 18-ppm noise-equivalent sensitivity (1 σ) was achieved for a 1-m absorption path length and a 25-ms data-acquisition time using direct absorption spectroscopy. The use of second-harmonic-detection wavelengthmodulation spectroscopy instead of direct absorption increased the sensitivity by a factor of three, achieving a normalized noiseequivalent sensitivity of 82 ppb Hz −1/2 for a 1-m absorption path length, which corresponds to 2 × 10 −7 cm −1 Hz −1/2 . PACS 42.55.Px; 42.62.Fi; 07.88.+y IntroductionTunable laser spectroscopy has proved to be a technique well suited for achieving gas-phase concentration measurements from the ppm level to the low ppb range. Compact laser sources that emit in the mid infrared, where most molecules exhibit fundamental and therefore strong absorption ro-vibrational bands, are particularly useful. Among the available tunable laser sources, distributed feedback (DFB) quantum cascade lasers (QCLs) offer several unique advantages for the design of compact field-deployable optical sensors, such as high output power, narrow laser line width, compactness, robustness, single-mode operation, and . Until now, due to the relatively short upper-state lifetime of the involved intersubband transitions occurring in QCLs, their high operating voltage of nearly 10 V, and the associated large heat dissipation within the active zone, single-frequency operation has been achieved only in a pulsed mode at room temperature. Pulsed QCL operation suffers from three drawbacks: (1) due to thermal chirping, even a ∼ 10-ns pulse generates typical QCL line widths of ∼ 200-300 MHz, which is substantially larger than the Fourier-transform limit; (2) the main sensitivity limitation of a pulsed DFB QCL based spectrometer arises from the pulse to pulse intensity fluctuations that require the use of an additional reference beam for normalization [3,4]; and (3) the generation of nanosecond current pulses requires high-speed driving electronics and detectors as well as fast data acquisition. To overcome these drawbacks, considerable effort has been made towards achieving a DFB QCL that is able to operate in a continuouswave (cw) mode at room temperature.A first device, employing high-reflection-coated facets and operating up t...