Absorption Cross-section Measurements Research Articles

An exploratory study on online quantification of isoprene in human breath using cavity ringdown spectroscopy in the ultraviolet

Breath analysis offers a promising method of noninvasive analyses of volatile metabolites and xenobiotics present in human body. Isoprene is one of the highest abundant volatile organic compounds (VOCs) present in human exhaled breath. Breath isoprene (50–200 part per billion by volume (ppbv) or higher) can be analyzed by using mass spectroscopy-based methods, yet laser absorption spectral detection of breath isoprene has not been much reported, partially due to its ultraviolet (UV) absorption wavelength and the spectral overlap with other breath VOCs such as acetone in the same wavelength region. These facts make it challenging to develop a spectroscopy-based breath isoprene analyzer for a potential portable instrument. Here we report on the development of a cavity ringdown spectroscopy (CRDS) system for detection of breath isoprene in the UV region near 226 nm. First, we investigated spectral absorption interferences near 226 nm and selected an optimal detection wavelength at 226.56 nm with minimum to no spectral interference. We then measured absorption cross-sections of isoprene at 225.5–227.4 nm under controlled cavity pressures, and the measured absorption cross-section 1.93 × 10−17 cm2/molecule at 226.56 nm was used to quantify isoprene in different cases including human breath gas samples. Finally, we validated the CRDS system by measuring breath gas samples from 19 human subjects using proton transfer reaction mass spectrometry (PTR-MS). The CRDS system shows good linear response (R2 = 0.999), high stability (0.2%), and high accuracy (R2 = 0.906 with PTR-MS). The limit of detection of the system was 0.47 ppbv, with average over 100 ringdown events (equivalent to 5 s). This work represents the first exploratory study of the detection of breath isoprene using CRDS. The results demonstrate the potential of developing a CRDS-based breath analyzer for online, near-real time, sensitive analysis of breath isoprene for further research that would help to elucidate its physiological and clinical significance.

Pressure scaling of measured absorption cross-sections by modifying the molecular free induction decay signal

The accuracy of absorption-based measurement is directly linked to the fidelity of the pressure- and temperature-dependent reference absorption cross-sections used for fitting and data interpretation. For large, complex molecules or species for which line-by-line absorption databases do not exist, cross-sections are often measured in the laboratory at a particular set of known temperature and pressure conditions and published for use as reference spectra. For subsequent simulations or measurements at conditions between or outside of the reference conditions, no exact reference cross-sections exist. We propose a technique which enables the scaling of reference absorption cross-sections to different pressures. Pressure influences the shape of the cross-section through collisional broadening. We modify the collisional broadening of the cross-section by performing an inverse Fourier transform on the dataset to view the free induction decay of the molecule, and then computationally adjusting the decay rate of the FID. This effectively pressure scales the broadening parameters of the multitude of contributing molecular resonances. The technique is validated here against reference absorption cross-sections, and can yield residuals between modified and measured reference spectra below one percent. The approach is simple to apply, and allows users to generate reference cross-sections at any pressure above the pressure condition of the reference dataset. It therefore also significantly reduces the need to measure reference absorption cross-sections at many pressures.

Ultrafast photophysical and nonlinear optical properties of novel free base and axially substituted phosphorus (V) corroles

The progression in synthetic procedures over the last two decades gave admittance to a wide variety of corroles for suitable potential applications such as photovoltaics, photonics, and bio-imaging. In this communication, we present results from our investigations of ultrafast photophysical processes and third-order nonlinear optical properties of newly synthesized donor-acceptor based free-base [(C6F5)3] and phosphorus [P-(OH)2(C6F5)3] corroles. The global analysis of the femtosecond transient absorption data based on the compartmental model revealed the corresponding time constants of several photophysical processes such as (a) internal conversion (τIC) in the 260–280 fs range (b) vibrational relaxation (τVR) in the 2.5-5 ps range and (c) nonradiative relaxation times (τnr) in the 4.15–7.6 ns range and finally (d) triplet lifetimes in the range of 25–50 μs. The two-photon absorption (TPA) cross-section measurements were performed using the femtosecond, kHz pulse Z-Scan technique at 600 nm and 800 nm and the retrieved TPA cross-section values were in the range of ~102 GM. Degenerate four-wave mixing measurements illustrated a large third-order nonlinear optical susceptibility χ(3) with a magnitude of 6.9 × 10−14 esu and instantaneous (sub-picosecond) response, suggesting a pure electronic contribution to the nonlinearity of these corroles. The discoveries from this study may help further to extend the capability of corroles as NLO materials for photonic applications.

Quantitative measurements of broad-band mid-infrared absorption spectra of formaldehyde, acetaldehyde, and acetone at combustion-relevant temperatures near 5.7 µm

Temperature-dependent absorption cross sections of formaldehyde, acetaldehyde, and acetone are presented for their CO stretching bands near 5.7 µm. The measurement methodology employs a broad-scan, rapid-tuning EC-QCL in conjunction with shock tube facilities and can provide quantitative spectroscopic information with an average spectral resolution of 0.6 cm−1 at a tuning rate in excess of 30 cm−1/ms. The absorption spectrum of formaldehyde in the ν2 vibrational band is reported at 998 K and 3.8 atm, showing reasonable agreement with spectral simulations based on HITRAN 2016. The residuals are ascribed to the emergence of hotbands and high-J rovibrational transitions at elevated temperatures. The first high-temperature absorption cross section measurements of acetaldehyde and acetone are reported near atmospheric pressure at combustion-related temperatures up to 1000 K, with strong temperature broadening observed in the measured spectra. The results presented are archived in the Stanford ShockGas-IR database and are expected to be useful in future spectroscopic studies.

Ultraviolet absorption cross-section measurements of shock-heated O2 from 2,000–8,400 K using a tunable laser

Accurate spectroscopic modeling is critical when measuring time-resolved, state-specific chemical kinetics of diatomic molecules. Here, a spectroscopic model (Stanford model) was developed to accurately simulate oxygen absorption cross-sections in the Schumann-Runge system for non-equilibrium conditions. Cross-sections of shock-heated oxygen (O2) have been measured using a picosecond pulsed ultraviolet (UV) laser, and the viability of two spectroscopic models has been demonstrated. Measurements were taken behind reflected shocks in 2% and 5% O2 in argon (Ar) mixtures around 211.2 nm and 236.9 nm up to initial post-reflected shock temperatures of 10,700 K. Cross-sections were plotted against vibrational temperature and compared to calculated cross-sections from the Stanford model and the Adjusted Spectrum model. Vibrational temperatures for cross-section measurements were calculated for plateaus and peaks in experimental absorbances using a Bethe-Teller relaxation model up to 6,000 K and a steady-state approach above 6,000 K. Vibrational temperatures calculated using the steady-state approach were 3–5% higher than coupled vibration-dissociation (CVD) calculations. The experimental cross-sections agree to within 15% of the Stanford model for both wavelength regimes.

Measurement of the absorption cross sections of unstable CS based on the dynamic equilibrium of CS2 and CS in the photolysis of CS2

The absorption cross sections, as important spectroscopic data, can be used for the identification and quantitative measurement of components. However, in view that carbon monosulfide (CS) is unstable and highly reactive, it is difficult to measure the absorption cross sections of CS. Therefore, a method for measuring absorption cross sections of CS based on the dynamic equilibrium between carbon disulfide (CS2) and CS in the photolysis CS2 is proposed in the paper, and the absorption cross sections of CS are obtained for the first time. Firstly, the method for evaluating CS2 concentrations is introduced, and the calculation formula of CS2 concentrations is given. Secondly, CS2 is converted to CS by increasing the intensity of incident light. Factors affecting CS2 conversion efficiency, such as the flow rate of CS2 and the intensity of incident light, are studied. Next, according to the conversion relationship between CS2 and CS, the CS concentration has been obtained by measuring the change of CS2 concentration. Finally, the CS absorption cross sections of (0.29 ± 0.01) × 10−16 cm2/molecule and (2.15 ± 0.04) × 10−16 cm2/molecule are provided at wavelengths of 250.88 nm and 257.60 nm, respectively.

Two-Photon Absorption and Cell Imaging of Fluorene-Functionalized Epicocconone Analogues.

Epicocconone 1 is a natural chromophore isolated from the fungus Epicoccum nigrum that has shown applications in proteomics and fluorescent microscopy thanks to its unique pro-fluorescence properties. The modification of the skeleton of the natural product by replacing the triene side chain by a fluorenyl scaffold can noticeably increase the fluorophore's absorption coefficient. The synthesis of the analogues of the natural product has been made possible by the use of a palladium-catalyzed carbonylation reaction, allowing the construction of the β-keto-dioxinone key intermediate. Two-photon absorption cross-section measurements of the fluorenyl epicocconone analogues show a structure dependency with values ranging from 60 to 280 GM and live cell imaging show intense staining of intracellular vesicle-like structures around the nucleus.

Temperature dependence of Alexandrite effective emission cross section and small signal gain over the 25-450 °C range

We present detailed measurements of effective emission cross section spectra of the Alexandrite gain medium in the 25-450 °C temperature range and provide analytic formulas that can be used to match the measured spectra. The measurement results have been used to investigate the wavelength and temperature dependence of small signal gain, as well as gain bandwidth relevant for ultrafast pulse generation/amplification. We show that the estimated laser performance based on the measured spectroscopic data provides a good fit to the results in the literature. We further discuss the need for a detailed measurement of excited-state absorption cross section in future studies.

Four-photon absorption cross-section measurements in UV fused silica at 473 nm.

With the use of I- and z-scan techniques, the four-photon absorption cross section in a UV fused silica sample has been measured to be σ4=(1.0±0.5)×10-115 cm8 s3 at 473nm. This value is more than an order of magnitude lower than that known for other optical materials (KBr, KI, NaCl, TeO2, and GeS2).

Morphological Study on Human Body Absorption Cross Section in a Reverberation Chamber From 1 GHz to 16 GHz

The human body absorption cross section (ACS) is important in nonionizing radiation dosimetry, but it is always hard to accurately evaluate the ACS of an individual from his or her morphological parameters, such as height and weight. To obtain an empirical formula that can evaluate the ACS from morphological parameters, 48 subjects with different morphological parameters were measured from 1 to 16 GHz in a reverberation chamber. The ACS was extracted from the power delay profile. This has the advantage of not requiring antenna radiation efficiency, and it has not been previously used in a wideband group study like this. The accuracy of the ACS measurement is demonstrated by comparison with the ACS of a spherical model with known structure and material, and the mean absolute percentage error of the sphere measurement is just 3.4%. Statistical analysis shows that the body surface area (BSA) has the strongest correlation with the ACS among all the morphological parameters; therefore, we present a new empirical ACS formula as a function of BSA and frequency. This will be of interest to those considering dosimetry, and in computing the effect of human body absorption on radio propagation in vehicles and other structures.

Radiative lifetime of boron-bound excitons in diamond

We report the ultraviolet absorption of boron-bound excitons at low temperature in a single crystal of diamond grown by chemical vapor deposition. The no-phonon (NP) and phonon-assisted lines are identified by comparison with cathodoluminescence. The oscillator strength of the NP lines was found to be 3.0 × 10−5 based on the measured absorption cross-section. This value is discussed in terms of the scaling law known for doped silicon, where the oscillator strength varies proportionally to Eloc2.5, with Eloc being the localization energy of excitons on acceptors. More importantly, we also could assess the oscillator strength of the dominant transverse optical phonon-assisted transition, which is found to be equal to 1.2×10−3. The associated radiative lifetime for the boron-bound exciton is 1.8 μs, which is much longer than the non-radiative Auger lifetime that governs its decay.

Measurement of the relative non-degenerate two-photon absorption cross-section for fluorescence microscopy.

In non-degenerate two-photon microscopy (ND-TPM), the required energy for fluorescence excitation occurs via absorption of two photons of different energies derived from two synchronized pulsed laser beams. ND-TPM is a promising imaging technology offering flexibility in the choice of the photon energy for each beam. However, a formalism to quantify the efficiency of two-photon absorption (TPA) under non-degenerate excitation, relative to the resonant degenerate excitation, is missing. Here, we derive this formalism and experimentally validate our prediction for a common fluorophore, fluorescein. An accurate quantification of non-degenerate TPA is important to optimize the choice of photon energies for each fluorophore.

Aerosol light absorption from optical measurements of PTFE membrane filter samples: sensitivity analysis of optical depth measures

Abstract. Mass absorption cross section (MAC) measurements of atmospherically relevant aerosols are required to quantify their effect on Earth's radiative budget. Estimating aerosol light absorption from transmittance and/or reflectance measurements through filter deposits is an attractive option because of their ease of deployment in field settings, low cost, and the ability to revisit previously analyzed samples. These measurements suffer from artifacts that depend on a given filter measurement system and aerosol optics. Empirical correction algorithms are available for commercial instruments equipped with optically thick fiber filters, but optically thin filter media have not been characterized in detail. Here, we present empirical relationships between particle light absorption optical depth – measured using multiwavelength integrated photoacoustic spectrometers – and filter optical depth measurements for polytetrafluoroethylene (PTFE) membrane filter samples of carbonaceous aerosols generated from combustion of diverse biomass fuels and kerosene (surrogate for fossil fuel combustion). Through radiative transfer modeling, we assessed the suitability of three measures of filter-based optical depth for robustly describing particulate-phase light absorption over a range of single scattering albedo (SSA) values: (1) ODs – a measure of transmission of the fraction of incident radiation that is not backscattered by the filter system – which utilizes transmittance and reflectance of the sample side of the filter; (2) the commonly used ODc, which uses transmittance and reflectance of the clean side of the filter; and (3) ATN or the Beer–Lambert attenuation. Modeling results were also validated experimentally, with ODs showing the least variability around the mean in this multidimensional parameter space. We establish a simple, wavelength-independent formulation for calculating aerosol MAC and absorption coefficients from measurements of ODs. We find the ratio between in situ particulate absorption optical depth and ODs to be inversely proportional to aerosol SSA. Our findings underscore that ODs is a better optical depth measure than ODc for applying appropriate correction factors when estimating particle-phase light absorption from filter-based techniques.

A two-wavelength ethylene-absorption temperature diagnostic

Recent high-temperature absorption cross-section measurements of ethylene have revealed spectral features with strong sensitivity to temperature. Motivated by these observations, this work presents a novel two-wavelength laser absorption diagnostic for inferring temperature from the ethylene spectrum. Two CO2 gas laser wavelengths, 10.532 µm and 10.591 µm, were systematically chosen for their cross-section strengths and the temperature sensitivity of their absorbance ratio. Cross-sections at each wavelength and absorbance ratio measurements were collected over conditions 418–1671 K, 0.3–4.3 atm in a shock tube. The absorbance ratio was found to be insensitive to pressure between 1.1 atm and 4.3 atm, and a correlation was developed to describe the ratio as a function of temperature. At lower pressures, the correlation incorporates an additional pressure factor. The diagnostic was used to study the shock tube pyrolysis of ethane at 1330 K and 2.2 atm, and further applications are suggested.

Practical papers, articles and application notes

I would like to introduce three wonderful papers to all our readers. Q. Xu and Y. Zhao from the Nanjing University of Aeronautics and Astronautics, China; X. Shen and K. Chen from the Nanjing Rongce Testing Technology Ltd, China; and Y. Huang from the University of Liverpool, U. K., authored the first paper "Absorption Cross Section Measurement of a Vehicle in Reverberation Chamber for Quick Estimation of Field Strength". For electromagnetic immunity testing of a vehicle, a reverberation chamber (RC) loaded with the vehicle-under-test has to be calibrated so that the necessary input power to generate the required field strength can be determined. In this paper, the average absorption cross section (ACS) of a vehicle in the frequency range of 80 MHz — 24 GHz is measured so that the required electric field strength in the RC loaded with a car can be estimated before the RC is constructed. Once the typical ACS of a car is known, one only needs to measure the Q factor of an unloaded RC and the Q factor of the RC loaded with a car can be predicted. This is very useful before the RC is constructed, as once the RC is built the unloaded Q factor cannot be increased.

Absorption cross section measurement of a vehicle in reverberation chamber for quick estimation of field strength

The average absorption cross section (ACS) of a car in the frequency range of 80 MHz–24 GHz is measured in a reverberation chamber (RC) based on the time domain method. The measured ACS of a car is very useful: it can be used to predict the electric field strength in a given RC loaded with a car, which can accelerate the calibration process of the RC greatly. The measurement results are provided with uncertainties. Summary on potential applications is also given.

A multi-wavelength speciation framework for high-temperature hydrocarbon pyrolysis

A framework for the study of high-temperature hydrocarbon pyrolysis is presented. The proposed framework details a multi-wavelength speciation method using convex optimization, a review of complementary fixed-wavelength laser diagnostics, and a database of high-temperature absorption cross-sections to enable use of the framework for 11 different species. The proposed speciation method involves a vectorized Beer-Lambert system solved under solution constraints using convex optimization. In support of the proposed method, a review of pertinent laser diagnostics and a database of high-temperature absorption cross-sections are also provided. The database includes measured absorption cross-sections of methane (CH4), ethylene (C2H4), propene (C3H6), isobutene (i-C4H8), 1-butene (1-C4H8), benzene (C6H6), toluene (C7H8), and four jet fuels (samples of Jet A, JP5, JP8, and Gevo ATJ) at the wavelengths of 3.1758 µm, 3.17595 µm, 3.283 µm, 3.392 µm, 3.410 µm, 10.532 µm, 10.675 µm, 10.89 µm, 10.958 µm, 10.962 µm, and 11.345 µm within the range of 300–1600 K and 0.25–4 atm. The database is comprised of over 1000 high–temperature absorption cross-section measurements from new shock tube experiments, new Fourier transform infrared spectrometer (FTIR) experiments, and an aggregated dataset from the literature. The convex speciation method and cross-section database provided will facilitate laser absorption studies of hydrocarbon pyrolysis used in the development of chemical kinetic mechanisms.

Absolute optical absorption cross-section measurement of Rb atoms injected into superfluid helium using energetic ion beams

We report on a new in situ laser spectroscopy technique using superfluid helium and a highly energetic ion beam. In taking advantage of superfluid helium as a host matrix for laser spectroscopy, we were able to develop an extremely low background detection system. The first demonstration using 85Rb31+ ion beam (66 MeV/nucleon) accelerated at the RIKEN accelerator facility shows improvement of detection limit by two orders of magnitude. A quantitative evaluation of Rb atoms in the observation region enables an analysis of their optical absorption cross-section in superfluid helium from the fluorescence intensity dependence on ion-beam intensity and applied laser power.

Measurement of the mid-infrared absorption spectra of ethylene (C2H4) and other molecules at high temperatures and pressures

A methodology for the measurement of mid-infrared absorption cross sections of gaseous molecules at high temperatures and pressures is presented. A rapid-tuning, broad-scan external cavity quantum cascade laser with a tuning rate in excess of 30000 cm−1s−1 has been employed in the measurement of full vibrational bands ( > 100 cm−1) in shock-heated test gases. The approach is demonstrated with measurements of the absorption cross section of ethylene (C2H4) in the 8.5 µm to 11.7 µm region for temperatures from 800 K to 1600 K and pressures from 1 to 5 atm. Decreasing peak strength with temperature is observed as well as pressure-insensitivity. The measurements are compared with existing experimental, empirical, and ab initio databases. Additionally, illustrative absorption cross section measurements of propene (C3H6), 1-butene (1-C4H8), 2-butene (2-C4H8), 1,3-butadiene (1,3-C4H6), and methanol (CH3OH) are presented near 1000 K and 2.3 atm.

Development of a two-wavelength IR laser absorption diagnostic for propene and ethylene

A two-wavelength infrared laser absorption diagnostic for non-intrusive, simultaneous quantitative measurement of propene and ethylene was developed. To this end, measurements of absorption cross sections of propene and potential interfering species at 10.958 µm were acquired at high-temperatures. When used in conjunction with existing absorption cross-section measurements of ethylene and other species at 10.532 µm, a two-wavelength diagnostic was developed to simultaneously measure propene and ethylene, the two small alkenes found to generally dominate the final decomposition products of many fuel hydrocarbon pyrolysis systems. Measurements of these two species is demonstrated using this two-wavelength diagnostic scheme for propene decomposition between 1360 and 1710 K.