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Register a new userBaseline-free Quantitative Absorption Spectroscopy Based on Cepstral Analysis
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The accuracy of quantitative absorption spectroscopy depends on correctly distinguishing molecular absorption signatures in a measured transmission spectrum from the varying intensity or baseline of the light source. Baseline correction becomes particularly difficult when the measurement involves complex, broadly absorbing molecules or non-ideal transmission effects such as etalons. We demonstrate a technique that eliminates the need to account for the laser intensity in absorption spectroscopy by converting the measured transmission spectrum of a gas sample to a modified form of the time-domain molecular free induction decay (m-FID) using a cepstral analysis technique developed for audio signal processing. Much of the m-FID signal is temporally separated from and independent of the source intensity, and this portion can be fit directly with a model to determine sample gas properties without correcting for the light source intensity. We validate the new approach in several complex absorption spectroscopy scenarios and discuss its limitations. The technique is applicable to spectra obtained with any absorption spectrometer and provides a fast and accurate approach for analyzing complex spectra.
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We present here an experimental set-up to perform simultaneously measurements of surface plasmon resonance (SPR) and X-ray absorption spectroscopy (XAS) in a synchrotron beamline. The system allows measuring in situ and in real time the effect of X-ray irradiation on the SPR curves to explore the interaction of X-rays with matter. It is also possible to record XAS spectra while exciting SPR in order to detect the changes in the electronic configuration of thin films induced by the excitation of surface plasmons. Combined experiments recording simultaneously SPR and XAS curves while scanning different parameters can be carried out. The relative variations in the SPR and XAS spectra that can be detected with this set-up ranges from 10-3 to 10-5, depending on the particular experiment.
Dual-comb spectroscopy is a rapidly developing technique that enables moving parts-free, simultaneously broadband and high-resolution measurements with microseconds of acquisition time. However, for high sensitivity measurements and extended duration of operation, a coherent averaging procedure is essential. To date, most coherent averaging schemes require additional electro-optical components, which increase system complexity and cost. Instead, we propose an all-computational solution that is compatible with real-time architectures and allows for coherent averaging of spectra generated by free-running systems. The efficacy of the computational correction algorithm is demonstrated using spectra acquired with a THz quantum cascade laser-based dual-comb spectrometer.
X-ray Absorption Spectroscopy (XAS) is a widely used X-ray diagnostic method. While synchrotrons have large communities of XAS users, its use on X-Ray Free Electron Lasers (XFEL) facilities has been rather limited. At a first glance, the relatively narrow bandwidth and the highly fluctuating spectral structure of XFEL sources seem to prevent high-quality XAS measurements without accumulating over many shots. Here, we demonstrate for the first time the collection of single-shot XAS spectra on an XFEL, with error bars of only a few percent, over tens of eV. We show how this technique can be extended over wider spectral ranges towards Extended X-ray Absorption Fine Structure (EXAFS) measurements, by concatenating a few tens of single-shot measurements. Such results open indisputable perspectives for future femtosecond time resolved XAS studies, especially for transient processes that can be initiated at low repetition rate.
The ability to map and estimate the activity of radiological source distributions in unknown three-dimensional environments has applications in the prevention and response to radiological accidents or threats as well as the enforcement and verification of international nuclear non-proliferation agreements. Such a capability requires well-characterized detector response functions, accurate time-dependent detector position and orientation data, an algorithmic understanding of the surrounding 3D environment, and appropriate image reconstruction and uncertainty quantification methods. We have previously demonstrated 3D mapping of gamma-ray emitters with free-moving detector systems on a relative intensity scale using a technique called Scene Data Fusion (SDF). Here we characterize the detector response of a multi-element gamma-ray imaging system using experimentally benchmarked Monte Carlo simulations and perform 3D mapping on an absolute intensity scale. We present experimental reconstruction results from hand-carried and airborne measurements with point-like and distributed sources in known configurations, demonstrating quantitative SDF in complex 3D environments.
Dual-comb spectroscopy has emerged as an indispensable analytical technique in applications that require high resolution and broadband coverage within short acquisition times. Its experimental realization, however, remains hampered by intricate experimental setups with large power consumption. Here, we demonstrate an ultra-simple free-running dual-comb spectrometer realized in a single all-fiber cavity suitable for the most demanding Doppler-limited measurements. Our dual-comb laser utilizes just a few basic fiber components, allows to tailor the repetition rate difference, and requires only 350 mW of electrical power for sustained operation over a dozen of hours. As a demonstration, we measure low-pressure hydrogen cyanide within 1.7 THz bandwidth, and obtain better than 1% precision over a terahertz in 200 ms enabled by a drastically simplified all-computational phase correction algorithm. The combination of the unprecedented setup simplicity, comb tooth resolution and high spectroscopic precision paves the way for proliferation of frequency comb spectroscopy even outside the laboratory.
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