Absorption Linewidth Research Articles

Sequential Co-Passivation in InAs Colloidal Quantum Dot Solids Enables Efficient Near-Infrared Photodetectors.

III-V colloidal quantum dots (CQDs) are promising materials for optoelectronic applications, for they avoid heavy metals while achieving absorption spanning the visible to the infrared (IR). However, the covalent nature of III-V CQDs requires the development of new passivation strategies to fabricate conductive CQD solids for optoelectronics: this work shows herein that ligand exchanges, previously developed in II-VI and IV-VI quantum dots and employing a single ligand, do not fully passivate CQDs, and that this curtails device efficiency. Guided by density functional theory (DFT) simulations, this work develops a co-passivation strategy to fabricate indium arsenide CQD photodetectors, an approach that employs the combination of X-type methyl ammonium acetate (MaAc) and Z-type ligands InBr3 . This approach maintains charge carrier mobility and improves passivation, seen in a 25% decrease in Stokes shift, a fourfold reduction in the rate of first-exciton absorption linewidth broadening over time-under-stress, and leads to a doubling in photoluminescence (PL) lifetime. The resulting devices show 37% external quantum efficiency (EQE) at 950nm, the highest value reported for InAs CQD photodetectors.

High-quality factor mid-infrared absorber based on all-dielectric metasurfaces.

The absorption spectrum of metasurface absorbers can be manipulated by changing structures. However, narrowband performance absorbers with high quality factors (Q-factor) are hard to achieve, mainly for the ohmic loss of metal resonators. Here, we propose an all-dielectric metasurface absorber with narrow absorption linewidth in the mid-infrared range. Magnetic quadrupole resonance is excited in the stacked Ge-Si3N4 nanoarrays with an absorption of 89.6% and a Q-factor of 6120 at 6.612 µm. The separate lossless Ge resonator and lossy Si3N4 layer realize high electromagnetic field gain and absorption, respectively. And the proposed method successfully reduced the intrinsic loss of the absorber, which reduced the absorption beyond the resonant wavelength and improved the absorption efficiency of Si3N4 in the low loss range. Furthermore, the absorption intensity and wavelength can be modulated by adjusting the geometric parameters of the structure. We believe this research has good application prospects in mid-infrared lasers, thermal emitters, gas feature sensing, and spectral detection.

Simulation of a continuum-source atomic absorption spectrometer

Continuum source atomic absorption spectrometry becomes a method for simultaneous multi-element analysis leading to a renewed interest in this method. The linear range of the calibration curve and the detection limits of continuum source atomic absorption spectrometers depend on the parameters of the instrumental function of the spectral instrument and the absorption line, the intensity of the radiation source, the level of radiation scattered in the instrument, and other factors. Calculating these dependences was the aim of this work. A theoretical model of the process of the absorption signal formation was constructed, and a computer program for its simulation was developed. Adequacy of the simulation was verified by comparing the absorption signals and calibration curves with the experimental ones obtained on a Grand-AAS spectrometer for several elements: Cd 228.8022 nm, Be 234.8610 nm, Mn 279.4817 nm, and Pb 283.3053 nm, as well as for the Ag 338.289 nm line recorded by two polychromators with different spectral resolution and scattered radiation levels. The relative deviation of the measured analytical signal from the simulated one did not exceed 15% in the concentration range of five orders of magnitude. The simulation results showed that nonlinearity of the calibration curve was primarily determined by the ratio of the width of the instrumental function (resolution) of the spectral instrument to the absorption line. The contribution of scattering in the spectrometer to the nonlinearity was secondary. The influence of the aperture characteristics of the photodetector arrays on the analytical signal was not significant. It was shown that decreasing resolution of the spectral instrument resulted in expanding the linear part of the calibration graphs from above to the level determined by scattering in the spectrometer. The detection limits limiting the graphs from below decrease with decreasing resolution until it approaches twice the width of the element's absorption line. After that, they start to increase.

Ar-Kr resonance energy transfer in He/Ar/Kr optically pumped rare gas laser medium

High metastable density is one of the research hotspots of optically pumped rare gas laser (OPRGL). Considering that the Ar metastable state energy level is only 20 cm<sup>–1</sup> different from the Kr excited state 5p[3/2]<sub>2</sub>, argon gas is added to the He/Kr discharge system. Owing to the long lifetime of the Ar metastable state atoms, through the collision resonance energy transfer process of Ar(4s[3/2]<sub>2</sub>)→Kr(5p[3/2]<sub>2</sub>), the purpose of supplementing and increasing the metastable density of Kr (Kr<sup>*</sup>) can be realized. In the case of obtaining the same metastable density, the pressure of the discharge power source is reduced, and a new idea is provided for further obtaining a high metastable density in a large discharge volume. In this work, the experimental analysis is carried out from the perspectives of spectral diagnosis and measurement of metastable density by laser absorption spectroscopy. The results show that the peak of radiative transition line of Kr high energy level atoms participating in the collision to the metastable state energy level is significantly enhanced after adding argon, and the tail signal of the transition line is extended within one discharge cycle. Under the gas conditions of 100 mbar, 1% Kr and 12.5% Ar, the peak value of the spectral line can be enhanced by about 10 times, and the tail signal of the transition line can be extended from 0.6 μs to 14.25 μs. At the same time, the density of Kr metastable energy level atoms is measured under different Ar content. Under the gas conditions of 100 mbar, 15% Ar and 1% Kr, the density of Kr<sup>*</sup> increases from 4.94×10<sup>11</sup> cm<sup>–3</sup> to 6.96×10<sup>12</sup> cm<sup>–3</sup>. At low pressure, the absorption linewidth of Kr metastable atoms narrows with the increase of Ar content. Under the gas condition of 600 mbar and 1% Kr, when the content of Ar is increased to 5%, the peak density of Kr<sup>*</sup> increases from 4.69×10<sup>13</sup> cm<sup>–3</sup> to 5.79×10<sup>13</sup> cm<sup>–3</sup>, i.e. the increment is 20%. Although the enhancement of metastable-atom-generation at high pressure is not so significant as those at low pressure, an increasing trend can still be observed. The results verify that the Kr metastable atoms generated in each discharge period can be supplemented by Ar-Kr resonance energy transfer.

Comparison of Low-Redshift Lyman-α Forest Observations to Hydrodynamical Simulations with Dark Photon Dark Matter.

Recent work has suggested that an additional ≲6.9 eV per baryon of heating in the intergalactic medium is needed to reconcile hydrodynamical simulations with Lyman-α forest absorption line widths at redshift z≃0.1. Resonant conversion of dark photon dark matter into low frequency photons is a viable source of such heating. We perform the first hydrodynamical simulations including dark photon heating and show that dark photons with mass m_{A^{'}}∼8×10^{-14} eV c^{-2} and kinetic mixing ε∼5×10^{-15} can alleviate the heating excess. A prediction of this model is a nonstandard thermal history for underdense gas at z≳3.

The CARMENES search for exoplanets around M dwarfs

Context. The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Échelle Spectrographs (CARMENES) instrument is searching for periodic radial-velocity (RV) variations of M dwarfs, which are induced by orbiting planets. However, there are other potential sources of such variations, including rotational modulation caused by stellar activity. Aims. We aim to investigate four M dwarfs (Ross 318, YZ CMi, TYC 3529-1437-1, and EV Lac) with different activity levels and spectral sub-types. Our goal is to compare the periodicities seen in 22 activity indicators and the stellar RVs, and to examine their stability over time. Methods. For each star, we calculated generalised Lomb-Scargle periodograms of pseudo-equivalent widths of chromospheric lines, indices of photospheric bands, the differential line width as a measure of the width of the average photospheric absorption line, the RV, the chromatic index that describes the wavelength dependence of the RV, and parameters of the cross-correlation function. We also calculated periodograms for subsets of the data and compared our results to TESS photometry. Results. We find the rotation periods of all four stars to manifest themselves in the RV and photospheric indicators, particularly the TiO 7050 index, whereas the chromospheric lines show clear signals only at lower activity levels. For EV Lac and TYC 3529-1437-1, we find episodes during which indicators vary with the rotation period, and episodes during which they vary with half the rotation period, similarly to photometric light curves. Conclusions. The changing periodicities reflect the evolution of stellar activity features on the stellar surface. We therefore conclude that our results not only emphasise the importance of carefully analysing indicators complementary to the RV in RV surveys, but they also suggest that it is also useful to search for signals in activity indicators in subsets of the dataset, because an activity signal that is present in the RV may not be visible in the activity indicators all the time, in particular for the most active stars.

The iron and oxygen content of LMC Classical Cepheids and its implications for the extragalactic distance scale and Hubble constant

Classical Cepheids are primary distance indicators and a crucial stepping stone to determining the present-day Hubble constant Ho to the precision and accuracy required to constrain apparent deviations from the LCDM Concordance Cosmological Model. We have measured the iron and oxygen abundances of of 89 Cepheids in the LMC, one of the anchors of the local Distance Scale, quadrupling the prior sample and including 68 of the 70 Cepheids used to constrain Ho by the SH0ES program. The goal is to constrain the extent to which the Cepheid luminosity is influenced by their chemical composition, an important contributor to the uncertainty on the determination of the Ho itself and a critical factor in the internal consistency of the distance ladder. We have derived stellar parameters and abundances from a self-consistent spectroscopic analysis based on Equivalent Width of absorption lines. The [Fe/H] distribution of LMC Cepheids is a single Gaussian with a mean of -0.4079+-0.003 dex (0.1 dex systematic uncertainty) and sigma 0.076+-0.003 dex. The latter is fully compatible with the measurement error and supports the low dispersion of 0.069 mag seen in the NIR HST LMC period-luminosity relation. The uniformity of the abundance has the important consequence that the LMC Cepheids alone cannot provide any meaningful constraint on the dependence of the Cepheid Period-Luminosity relation on chemical composition at any wavelength. This revises a prior claim based on a small sample of 22 LMC Cepheids that there was little dependence (or uncertainty) between composition and NIR luminosity, a conclusion which would produce a conflict between anchors of the distance ladder with different mean abundance. The chemical homogeneity of the LMC Cepheid population makes it an ideal environment to calibrate the metallicity dependence between the more metal poor SMC and metal rich Milky Way and NGC4258.

Vibrational spectroscopy in materials

An absorption line at 900 nm has been observed in Perspex. Samples of 1 and 2 cm thickness were used and the integrated absorption (line width times peak absorption) of the line was proportionate to the thickness. Facilities for lowering the sample temperature were not available and therefore both measurements were carried out at room temperature. A Red Tide spectrophotometer was used to measure this absorption. According to the Optics Group at National Institute of Standards and Technology, USA, the line can be assigned to the 3rd overtone CH stretch of the methyl and methylene group. The present result was compared with historic data for boron impurities in indium phosphide to demonstrate the versatility of this form of spectroscopy

Cyclotron resonance linewidth due to electron scattering by five different phonon modes in semiparabolic quantum wells

A theory of the cyclotron resonance absorption linewidth (CRALW) in the presence of electron scattering by five different phonon modes in a semiparabolic quantum well has been developed. The cyclotron resonance absorption power and the CRALW are calculated by use of the projection-operator and profile methods, respectively. The main results are as follows: The CRALW increases with increase of the confinement frequency, temperature, and magnetic field, but decreases with increase of the Landau level number for the interface mode, the bulk mode, and three types of confined modes. Besides, the CRALW is largest for the Huang-Zhu phonon model but is smallest for the bulk mode. The contributions of the interface modes to the CRAWL are considerable and should be taken into account in the cyclotron resonance effect. Moreover, as the Landau level number increases, both the CRALW and the difference in the CRALW between two Landau levels decrease. In addition, the agreement between the guided modes and the Huang-Zhu phonon model is better than that between the slab modes and the Huang-Zhu phonon model. Our theoretical calculations accord well with previous experimental data.

Combined influences of temperature, pressure, and alloy-composition on magneto-optical properties of the semi-parabolic well under all four different phonon models

In this study, combined influences of the pressure, alloy composition, and temperature on the magneto-optical properties of the semiparabolic well under all four different phonon models, including three confining phonon models are considered in detail through the magneto-phonon resonance oscillation (MPRO). The magneto-optical transport absorption power (MOTAP) and MO-conductivity tensor (MOCT) expressions are obtained utilizing the method of operator projection. Simultaneously the profile method also is used in order to measure the inter-subband absorption linewidth. Our calculated result shows that (i) the aluminum composition y has significant effect on the position of the resonant peak; (ii) the absorption line-width as a function of the Al-composition, temperature of electron, and hydrostatic pressure for all four models of phonons; (iii) the influence of the Al-composition, temperature of electron, and pressure on the intersubband absorption line-width is different for each of phonon model; (iv) our present result is fairly consistent with experimental measurement; (v) combined effects of the aluminum composition, the temperature of electron, and the hydrostatic pressure on the intersubband absorption line-width of the semiparabolic QWs are important when the phonon confinement is taken into consideration.

Quantum plasmonics of few electrons in strongly confined doped semiconducting oxide: A DFT + U study of ZnGaO

It has been reported in photodoping experiments that localized surface plasmonic resonances can be sustained with electrons as few as 3. We performed first principles calculations of density functional theory, with the Hubbard U correction, to see if localized surface plasmonic resonances can also be sustained by doping a wide bandgap ZnO with few shallow donors of Ga. We distributed 3–6 dopants approximately uniformly, due to quasi-spherical geometry of the quantum dot, in the dilute doping limit. The uniform distribution of dopants in quantum dots has been reported experimentally. Although the dopant configurations are limited due to computational cost, our findings shed light on absorption trends. Results for quantum dots of 1.4 nm, passivated with pseudo-hydrogens, show that localized surface plasmonic resonances can be generated in the near infrared range. The absorption linewidths for such small-sized quantum dots are broad. We find that the resonance linewidth depends on the orientation of surfaces and the number of secondary peaks on the concentration of dopants. The absorption coefficients, as functions of the principal values of the dielectric tensor, indicate that an electric field with orientation parallel to that of the most symmetric surface will produce localized surface plasmonic resonances with high quality factors.

Strong Coupling of Carbon Quantum Dots in Liquid Crystals.

Carbon quantum dots (CDs) have recently received a tremendous amount of interest owing to their attractive optical properties. However, CDs have broad absorption and emission spectra limiting their application ranges. We herein, for the first time, show synthesis of water-soluble red emissive CDs with a very narrow line width (∼75 meV) spectral absorbance and hence demonstrate strong coupling of CDs and plasmon polaritons in liquid crystalline mesophases. The excited state dynamics of CDs has been studied by ultrafast transient absorption spectroscopy, and CDs display very stable and strong photoluminescence emission with a quantum yield of 35.4% and a lifetime of ∼2 ns. More importantly, we compare J-aggregate dyes with CDs in terms of their absorption line width, photostability, and ability to do strong coupling, and we conclude that highly fluorescent CDs have a bright future in the mixed light–matter states for emerging applications in future quantum technologies.

Intersubband absorption linewidth in a semi-parabolic quantum well: Comparison among three different phonon models

In this article, effects of different confined phonon models on intersubband absorption linewidth in semiparabolic quantum well (SPQW) is studied in detail through considering the optically detected magneto-confined phonon resonance (ODMCPR) effect. The projection operator and profile methods are utilized in order to calculate the linear conductivity tensor as well as magneto-phonon resonance absorption power (MPRAP) and measure the absorption linewidth or the full width at half maximum (FWHM) at the ODMCPR peak, respectively. We find that (i) the FWHM (absorption linewidth) as functions of the temperature, Landau level, and confining potential frequency; (ii) there is a different dependence in slab and guided phonon modes, as well as Huang–Zhu (HZ) phonon mode model on the FWHM; (iii) the confined phonon modes have great influence on the intersubband absorption linewidth in semiparabolic quantum well; (iv) the present calculations accord well with previous experimental studies.

Rotational Modulation of Spectroscopic Zeeman Signatures in Low-mass Stars

Abstract Accurate tracers of the stellar magnetic field and rotation are cornerstones for the study of M dwarfs and for reliable detection and characterization of their exoplanetary companions. Such measurements are particularly challenging for old, slowly rotating, fully convective M dwarfs. To explore the use of new activity and rotation tracers, we examined multiyear near-infrared (NIR) spectroscopic monitoring of two such stars—GJ 699 (Barnard’s Star) and Teegarden’s Star—carried out with the Habitable-zone Planet Finder spectrograph. We detected periodic variations in absorption line widths across the stellar spectrum, with higher amplitudes toward longer wavelengths. We also detected similar variations in the strength and width of the 12435.67 Å neutral potassium (K i) line, a known tracer of the photospheric magnetic field. Attributing these variations to rotational modulation, we confirm the known 145 ± 15 day rotation period of GJ 699, and measure the rotation period of Teegarden’s Star to be 99.6 ± 1.4 days. Based on simulations of the K i line and the wavelength dependence of the line-width signal, we argue that the observed signals are consistent with varying photospheric magnetic fields and the associated Zeeman effect. These results highlight the value of detailed line profile measurements in the NIR for diagnosing stellar magnetic field variability. Such measurements may be pivotal for disentangling activity and exoplanet-related signals in spectroscopic monitoring of old, low-mass stars.

A Spectroscopic Angle on Central Engine Size Scales in Accreting Neutron Stars

Abstract Analyses of absorption from disk winds and atmospheres in accreting compact objects typically treat the central emitting regions in these systems as point sources relative to the absorber. This assumption breaks down if the absorbing gas is located within a few × 1000 GM/c 2, in which case a small component of the absorber’s Keplerian motion contributes to the velocity width of absorption lines. Here, we demonstrate how this velocity-broadening effect can be used to constrain the sizes of central engines in accreting compact objects via a simple geometric relationship, and develop a method for modeling this effect. We apply this method to the Chandra/HETG spectra of three ultracompact and short-period neutron star X-ray binaries in which evidence of gravitationally redshifted absorption, owing to an inner-disk atmosphere, has recently been reported. The significance of the redshift is above 5σ for XTE J1710−281 (this work) and 4U 1916−053, and is inconsistent with various estimates of the relative radial velocity of each binary. For our most sensitive spectrum (XTE J1710−281), we obtain a 1σ upper bound of 310 km s−1 on the magnitude of this geometric effect and a central engine of size R CE < 60 GM/c 2 (or < 90 GM/c 2 at the 3σ level). These initial constraints compare favorably to those obtained via microlensing in quasars and approach the sensitivity of constraints via relativistic reflection in neutron stars. This sensitivity will increase with further exposures, as well as the launch of future microcalorimeter and grating missions.

The iron and oxygen content of LMC Classical Cepheids and its implications for the extragalactic distance scale and Hubble constant

Context. Classical Cepheids are primary distance indicators and a crucial stepping stone in determining the present-day value of the Hubble constant H0 to the precision and accuracy required to constrain apparent deviations from the ΛCDM Concordance Cosmological Model. Aims. We measured the iron and oxygen abundances of a statistically significant sample of 89 Cepheids in the Large Magellanic Cloud (LMC), one of the anchors of the local distance scale, quadrupling the prior sample and including 68 of the 70 Cepheids used to constrain H0 by the SH0ES program. The goal is to constrain the extent to which the luminosity of Cepheids is influenced by their chemical composition, which is an important contributor to the uncertainty on the determination of the Hubble constant itself and a critical factor in the internal consistency of the distance ladder. Methods. We derived stellar parameters and chemical abundances from a self-consistent spectroscopic analysis based on equivalent width of absorption lines. Results. The iron distribution of Cepheids in the LMC can be very accurately described by a single Gaussian with a mean [Fe/H] = −0.409 ± 0.003 dex and σ = 0.076 ± 0.003 dex. We estimate a systematic uncertainty on the absolute mean values of 0.1 dex. The width of the distribution is fully compatible with the measurement error and supports the low dispersion of 0.069 mag seen in the near-infrared Hubble Space Telescope LMC period–luminosity relation. The uniformity of the abundance has the important consequence that the LMC Cepheids alone cannot provide any meaningful constraint on the dependence of the Cepheid period–luminosity relation on chemical composition at any wavelength. This revises a prior claim based on a small sample of 22 LMC Cepheids that there was little dependence (or uncertainty) between composition and near-infrared luminosity, a conclusion which would produce an apparent conflict between anchors of the distance ladder with different mean abundance. The chemical homogeneity of the LMC Cepheid population makes it an ideal environment in which to calibrate the metallicity dependence between the more metal-poor Small Magellanic Cloud and metal-rich Milky Way and NGC 4258.

Strong Coupling of Multimolecular Species to Soft Microcavities.

Can we couple multiple molecular species to soft cavities? The answer to this question has relevance in designing open cavities for polaritonic chemistry applications. Because of the differences in adhesiveness, it is difficult to couple multiple molecular species to open cavities in a controlled and precise manner. In this Letter, we discuss the procedure to coat multiple dyes, TDBC and S2275, onto a dielectric microsphere using a layer-by-layer deposition technique so as to facilitate the multimolecule coupling. We observed the formation of a middle polariton branch due to the intermolecular mixing facilitated by the whispering gallery modes. The coupling strength, 2g, of the TDBC molecules was found to be 98 meV, while that of the S2275 molecules was 78 meV. The coupling strength was found to be greater than the cavity line width and the molecular absorption line width, showing that the system is in the strong coupling regime.

Narrowing the absorption linewidth and its limitation in a four-level inverted-Y atomic system

Depending on a four-level inverted-Y atomic system, we demonstrate the limitation of linewidth-narrowing for the probe absorption spectrum in the electromagnetic induced absorption platform. Thanks to the use of an auxiliary control field which couples one hyperfine ground state and one middle-excited state we show that the linewidth limitation can be constrained by a coherence decay rate between two hyperfine ground states, rather than by the decay rate between the ground and the excited states as in previous Ladder schemes. That fact makes the theoretically-predicted absorption linewidth at least two orders of magnitude narrower. By using a suitable adjustment for the control-field amplitude and the detuning we numerically show that an extremely-narrowed probe absorption spectrum accompanied by a higher spectra contrast can be obtained, which confirms well with our theoretical predictions. We study the transient time response to the absorption spectrum and show that a relatively longer response time arises due to the small coherence decay rate between two hyperfine ground states. Furthermore, we reduce the influence on linewidth-narrowing from the Doppler effect via an optimized design of lasers, and reveal that no Doppler-free effect exists due to the lack of three-photon process. Our results may pave a route to the development of high-resolution spectroscopy in current experiments.

A new automated tool for the spectral classification of OB stars

Context. As an increasing number of spectroscopic surveys become available, an automated approach to spectral classification becomes necessary. Due to the significance of the massive stars, it is of great importance to identify the phenomenological parameters of these stars (e.g., the spectral type), which can be used as proxies to their physical parameters (e.g., mass and temperature). Aims. In this work, we aim to use the random forest (RF) algorithm to develop a tool for the automated spectral classification of OB-type stars according to their sub-types. Methods. We used the regular RF algorithm, the probabilistic RF, which is an extension of RF that incorporates uncertainties, and we introduced the KDE – RF method which is a combination of the kernel-density estimation and the RF algorithm. We trained the algorithms on the equivalent width (EW) of characteristic absorption lines measured in high-quality spectra (signal-to-noise (S/N)≳50) from large Galactic (LAMOST, GOSSS) and extragalactic surveys (2dF, VFTS) with available spectral types and luminosity classes. By following an adaptive binning approach, we grouped the labels of these data in 11 spectral classes within the O2-B9 range. We examined which of the characteristic spectral lines (features) are more important for the classification based on a number of feature selection methods, and we searched for the optimal hyperparameters of the classifiers to achieve the best performance. Results. From the feature-screening process, we find that the full set of 17 spectral lines is needed to reach the maximum performance per spectral class. We find that the overall accuracy score is ∼70%, with similar results across all approaches. We apply our model in other observational data sets providing examples of the potential application of our classifier to real science cases. We find that it performs well for both single massive stars and for the companion massive stars in Be X-ray binaries, especially for data of similar quality to the training sample. In addition, we propose a reduced ten-features scheme that can be applied to large data sets with lower S/N ∼ 20 − 50. Conclusions. The similarity in the performances of our models indicates the robustness and the reliability of the RF algorithm when it is used for the spectral classification of early-type stars. The score of ∼70% is high if we consider (a) the complexity of such multiclass classification problems (i.e., 11 classes), (b) the intrinsic scatter of the EW distributions within the examined spectral classes, and (c) the diversity of the training set since we use data obtained from different surveys with different observing strategies. In addition, the approach presented in this work is applicable to products from different surveys in terms of quality (e.g., different resolution) and different formats (e.g., absolute or normalized flux), while our classifier is agnostic to the luminosity class of a star, and, as much as possible, it is metallicity independent.

Absorption spectra measurements at ~1 cm<sup>–1</sup> spectral resolution of <sup>32</sup>S, <sup>33</sup>S, <sup>34</sup>S, and <sup>36</sup>S sulfur dioxide for the 206–220 nm region and applications to modeling of the isotopic self-shielding

The sulfur isotope fractionation that occurs during SO2 photolysis is key to explaining the isotope signatures stored in ancient sedimentary rocks and understanding the atmospheric compositions of the early Earth and early Mars. Here, we report the photoabsorption cross-sections of 32SO2, 33SO2, 34SO2, and 36SO2 measured from 206 to 220 nm at 296 K. The wavelength resolution was set to 1 cm–1, 25 times higher than that of previous SO2 isotopologue absorption spectra measurements. The precision of ~10% is in agreement with previously reported SO2 absorption spectra. In comparison with previously reported high-resolution spectra of natural abundance, SO2 measurements demonstrate smaller cross-sectional magnitudes at absorption peaks and an offset wavelength by ~0.016 nm. Using the newly recorded isotopologue spectra, we calculated the sulfur isotope fractionation for self-shielding during SO2 photolysis. The calculated 34S fractionation (34ε) roughly reproduces the observed relationship between 34ε and the SO2 column density in previous photolysis experiments. Thus, the cross-section is useful for predicting 34S/32S isotope fractionation in an optically thick SO2 atmosphere. In contrast, for mass-independent fractionation (MIF-S, i.e., non-zero Δ33S), the measured spectra predicted a weakly negative Δ33S/δ34S slope of about –0.1. The small Δ33S/δ34S slope is consistent with the slopes of SO2 photolysis experiments under high-pressure atmospheres (i.e., the pressure broadened absorption line width will be comparable to the spectral resolution). Therefore, MIF-S during photolysis experiments was linked to spectroscopic measurements for the first time. We conclude that reasonable precision and high-resolution spectroscopic measurements are key to explaining the origin of MIF-S at column densities below 1018 cm–2. However, MIF-S production in chamber experiments or atmospheric conditions may require understanding pressure or temperature effects, such as linewidth broadening on the UV-absorption spectra, and how these effects manifest themselves on isotopologues.