The spectroscopic and transition properties of the disulfur anion (S2–) were investigated employing the internally contracted multireference configuration interaction method, with spin-orbit coupling effects incorporated via the Breit-Pauli Hamiltonian. Potential energy curves for 24 low-lying Λ-S and 54 Ω states were constructed to determine the spectroscopic parameters of bound and quasi-bound states, which were validated against available experimental data. The X2Πg (υ" = 0‒21) state of S2– is found to be stable at a calculated adiabatic electron affinity of 12738.29 cm‒1 for S2. Furthermore, transition dipole moments between Λ-S states were calculated to predict transition probabilities, including Einstein A coefficients, Franck-Condon factors, and band origins of vibronic emissions. The vibrational radiative lifetimes vary significantly across different electronic states: the A2Πu state exhibits the shortest lifetime (∼0.1 μs), followed by the double-well 12Σu– state (0.1∼1 μs). The 12Δg and 12Σg– states show lifetimes of ∼1 μs, whereas the 12Δu and 12Σu+ states range from 1 to 10 μs. Broader ranges are observed for the 14Πu and 12Σg+ states (1∼100 μs for the former and first well of the latter, 100 μs for the second well of 12Σg+), with the 24Πu state reaching 1000 μs. The radiative lifetimes for the a4Σ–→X2Πg and A2Πu→X2Πg transitions encompassing all relevant Ω components (1/2 and 3/2) were also evaluated.
Ground-based solar absorption Fourier transform infrared (FTIR) spectroscopy is widely used for satellite trace-gas validation, but adapting a conventional laboratory FTIR spectrometer for direct-sun observations introduces nonideal angular effects. The finite solar disk, wide acceptance field of view, and residual pointing errors from a custom solar tracker can produce solar self-apodization, off-axis beam shifts, and mismatched detector footprints between the fixed-arm and moving-arm beams. If these effects are not represented in the forward model, they may distort the measured spectrum and trade off with the retrieved column-averaged dry-air CO2 mole fraction (XCO2). This study develops a solar-line-constrained joint retrieval strategy for the weak CO2 band at 6180–6280 cm−1. The method retains source-fixed Fraunhofer-line structure in the solar irradiance while jointly fitting the gas column and key instrumental parameters. Ray-traced synthetic spectra, generated by simulating the internal FTIR optical paths and beam interference, show that the proposed retrieval recovers the target gas column from two different initial values with negligible bias, whereas the retrieval without the solar-line constraint retains a bias of approximately 3 ppm. The method is further applied to 1870 screened spectra collected from July 2025 to May 2026 using a Nicolet iS50 spectrometer at Shanghai Lingang. The daily median XCO2 shows a seasonal evolution broadly consistent with the GGG prior, with a mean daily median–prior offset of ＋1.08 ppm. These results support background-relative internal consistency and demonstrate the feasibility of laboratory FTIR direct-sun XCO2 retrieval using solar-line constraints.
As a first-order approximation, spheroidal shapes can be used as a surrogate to compute the single-scattering properties of complex particles. The Separation of Variables Method (SVM) provides a rigorous analytical framework for solving light scattering by homogeneous and two-layer spheroids with large size parameters and extreme aspect ratios. However, the computational cost of SVM increases rapidly with increasing size parameter. To improve computational efficiency while maintaining numerical accuracy, this study combines the T-matrix method with SVM, forming a hybrid approach referred to as the separation of variables and T-matrix method (SV-TM). This approach significantly reduces computational costs by circumventing repetitive calculations associated with multiple particle orientations. In practice, SV-TM accelerates computations by approximately two orders of magnitude compared with the conventional SVM while retaining validity for size parameters as large as 2000 and being applicable to highly elongated or flattened spheroids. Moreover, the T-matrix formulation ensures numerical accuracy for randomly oriented particles because of its analytical average of the optical properties over particle orientations. The enhanced efficiency and robustness of SV-TM enable a wide range of practical applications that involve light scattering by nonspherical and inhomogeneous particles.
In this work, a high-resolution absorption spectrum of the main isotopologue of sulfur dioxide in the infrared range of 4700–5200 cm–1, which is currently not represented in the HITRAN database, is reanalyzed. The experimental spectrum was recorded on a Bruker IFS-120 HR Fourier spectrometer and was studied for the first time in (Ulenikov et al., 2009), where 949 experimental energy levels for the (301) and (103) vibrational states were reported. In this study, the results of the previous work were sufficiently extended. Using the Watson effective Hamiltonian method, 1427 and 1337 vibrational-rotational transitions were identified for the (301)-(000) and (103)-(000) bands, respectively. The interpolyad Fermi resonance between the (103) state and the "dark" (231) state was detected and taken into account for the first time. As a result of the analysis, highly accurate energy levels were determined: 631 for the (301) state and 605 for the (103) state, of which 153 and 134 levels, respectively, were determined for the first time.The main result of this work is the creation of a new empirical list of lines, including 5239 vibrational-rotational transitions. To ensure high data reliability, the final list combines experimental wavenumbers and high-precision line positions calculated using the effective Hamiltonian method with intensities from variational calculations, allowing for a more complete description of the selective absorption of sulfur dioxide in the studied range. The obtained results can be integrated into the HITRAN spectroscopic database for remote sensing of the Earth's and planetary atmospheres.
Stark broadening is used as a plasma diagnostic technique to infer electron density. Modulated pump–probe spectroscopy was performed on the 42D5/2→122F∘,152F∘,202F° transitions in a partially ionized rubidium vapor. The rubidium vapor was optically pumped along the D2 transition using a 3 W CW laser at helium pressures of 0.001 Torr, 3.0 Torr, and 30.0 Torr for alkali densities from 2.0 × 1012cm-3 to 6.0 × 1013cm-3. Electron density was estimated from Stark broadening of the 42D5/2→122F∘,152F∘,202F° spectral lines. Griem theory was used to calculate the broadening and shift parameters, giving values of 2.8±0.5Hz cm3, 7.8±1.5Hz cm3, and 28.0±5.6Hz cm3 for the 42D5/2→122F∘,152F∘,202F° transitions respectively. The subsequently inferred electron densities for all three energy levels (122F, 152F, and 202F) show a consistency despite a reasonable overlap of nearby perturbing levels. Stark widths increased linearly with alkali density, with slopes from 0.017(2)ne/nRb to 0.9(1)ne/nRb and intercepts at alkali densities of 1.60(2) × 1013cm-3 to 0.33(7) × 1013cm-3, the linear relationship between alkali density and electron density indicating the presence of another electron production process in addition to energy pooling.
Passive daytime radiative cooling is a promising zero-energy approach for thermal management, yet achieving broadband thermal emission within micron-scale multilayer structures remains a significant challenge. Inorganic multilayers, limited by the narrow bandwidth of materials, typically require complex architectures to broaden emission, whereas polymer coatings, constrained by their limited intrinsic absorption, often demand several hundred micrometers of thickness for satisfactory performance.Herein, we propose a physically interpretable forward-design strategy for microscale two- or three-layer radiative coolers, which utilizes the refractive index and extinction coefficient of materials to govern the internal optical-field distribution and intrinsic absorption to maximize the emissivity. By coordinating impedance matching, complementary absorption, and interference enhancement, common easily fabricated materials are made to work synergistically within two- or three-layer structures of only a few micrometers. Guided by this strategy, bilayer and trilayer configurations generally reach normal-incidence atmospheric-window weighted emissivities of 0.91–0.94 and 0.93–0.97, respectively, while maintaining solar reflectivity above 0.96. The representative configuration approaches an ideal blackbody at 0.981 within a thickness of ∼3.6 μm. This study establishes a robust design framework for microscale multilayer radiative coolers, offering theoretical guidance for high-performance photothermal device optimization.
The scattering phase function is a fundamental determinant of radiative transfer in particulate media, yet the predictive accuracy of widely used empirical phase function models remains poorly quantified, directly limiting the reliability of optical simulations and quantitative remote sensing inversion. Here, we systematically compare four models (HG, CS, RM, and TTRM) across non‑absorbing, weakly absorbing, and strongly absorbing particle systems, employing rigorous Lorenz–Mie theory as the benchmark and Monte Carlo simulations to provide, for the first time, a comprehensive quantification of their effects on both directional‑hemispherical reflectance and angularly resolved BRDF. The influences of size parameter, particle size distribution, and optical thickness are evaluated. Using exact Mie calculations as the reference, errors in directional-hemispherical reflectance and angularly resolved BRDF are systematically quantified. The Monte Carlo results show that the TTRM model reproduces the angularly resolved BRDF with the highest fidelity in the vast majority of cases. For the polydisperse systems considered in this work, the relative errors of its BRDF·cosθr are typically below 5%, while the absolute errors range from several times to two orders of magnitude smaller than those of the HG, CS, and RM models. For the hemispherical reflectance, the TTRM maintains relative errors below 3% under polydisperse conditions, while the HG, CS, and RM models produce maximum errors of 168.37%, 70.59%, and 129.93%, respectively. These findings confirm that, within the range of conditions investigated, the TTRM model exhibits superior performance and high reliability for radiative transfer simulations and BRDF calculations compared to the other empirical models tested. This work provides a quantitative basis for the selection of scattering phase function models and for improving the accuracy of remote sensing inversions.
H2- and He-broadening coefficients of CO2 and their temperature dependence were measured for the R(18) and R(20) transitions of the ν1+ν3 band at 3728.4101 and 3729.7122 cm−1, respectively. Measurements were performed using a tunable distributed-feedback laser near 2.7 µm in a high-temperature optical cell. Absorbance spectra were recorded for mixtures of 1–4% CO2 in H2 or He at ten pressures between 75 and 600 Torr, covering temperatures of 295–805 K for H2 and 295–1005 K for He. The upper temperature limit for the H2 mixtures was imposed by the onset of the reverse water-gas shift reaction. The measured line shapes were fitted with Voigt profiles, with residuals typically less than 1%. Broadening coefficients were obtained from the linear pressure dependence of the fitted collisional widths, and temperature exponents were obtained from power-law fits. The room-temperature H2-broadening coefficients are 0.1124 and 0.1118 cm−1 atm−1 for the R(18) and R(20) transitions, with temperature exponents of 0.676 and 0.655, respectively. The corresponding He-broadening coefficients are 0.0591 and 0.0583 cm−1 atm−1, with exponents of 0.569 and 0.543. The measured H2 exponents are 13–16% larger than the constant value adopted in HITRAN 2024 but agree within 6% with recent requantized classical molecular dynamics predictions. These results provide the first H2- and He-broadening data for the ν1+ν3 band and the first experimental validation of He-broadening predictions above 760 K.
Tholins are complex organic aerosols formed through photochemical reactions involving methane and nitrogen. They are inherent components of Titan's atmosphere and have also been identified on several other bodies in the outer solar system. Their unique optical properties significantly influence the radiative transfer processes, making them critical for understanding planetary climate and atmospheres containing these particles. In this work, theoretical simulations using the discrete dipole approximation (DDA) were performed to examine how tholins scatter light. Four different irregular particle shapes were considered to mimic realistic tholin morphologies. Simulations were carried out at representative wavelengths of 500, 642.9, and 900 nm across a size range of 0.5 to 5 µm, using refractive indices based on previously published laboratory measurements. The phase function and degree of linear polarization were calculated and analysed as functions of phase angle. The simulated results were further compared qualitatively with available laboratory and spacecraft observations. The calculations show that irregular tholin particles exhibit distinct scattering patterns that differ from mineral dusts, displaying predominantly forward scattering behaviour, pronounced wavelength dependence, and characteristic negative polarization branches at small phase angles. The results also demonstrate the influence of particle size, shape, and morphology on the scattering response. These results provide a computational framework for investigating the optical behaviour of tholins and contributes to the interpretation of remote-sensing observations of Titan and other planetary environments.
We report the first theoretical predictions of the density dependence of self-broadened CO2 line intensities retrieved from fits of absorption spectra using an impact line-shape model. This effect, previously shown to arise from the breakdown of the impact approximation, results in the redistribution of a fraction of the line intensity from a narrow spectrum to a broad pedestal whose width is inversely proportional to the collision duration. Using requantized classical molecular dynamics simulations (rCMDSs), we predicted the spectral density of a CO2 parallel band at three temperatures (212 K, 250 K, and 296 K) and various densities. For each temperature, the simulated spectra were fitted with the speed-dependent Nelkin–Ghatak profile including first-order line mixing. The results show a clear linear decrease in the retrieved line intensities with increasing density. The theoretical intensity depletion coefficients are in good agreement with previously measured values obtained from Fourier-transform spectra. At 296 K, the predicted intensity depletion ranges from 0.15 to 1.3 % amagat−1, while the measured values range from 0.6 to 1.3 % amagat−1. The effect increases at lower temperatures, reaching 1.5 to 3 % amagat−1 at 212 K. The results obtained at the three temperatures suggest a 296T3.4 temperature dependence for the intensity depletion coefficient. We also determined the continuum absorption - a broad pedestal resulting from the redistribution of intensity from the narrow line spectrum. The predicted continua are in excellent agreement with the measured ones at both 212 K and 296 K. Analysis of the time evolution of molecular populations obtained from rCMDSs clearly shows that the finite duration of collisions contributes to the observed intensity depletion and the associated continuum absorption.
Radiative heat transfer in gas turbine combustors is essential due to high-temperature sooting flames under elevated pressures. The Full-Spectrum Correlated-K distribution (FSCK) method is widely adopted in practical industrial applications owing to its high efficiency, but its accuracy is dependent on the quadrature node distribution and the reference thermodynamic state, especially for gas–soot mixtures across a wide pressure range. To evaluate this dependence, four one-dimensional flames with different soot profiles are constructed in this work, and the calculations for each flame are performed under different pressures and length scales. The results indicate that the quadrature scheme has a strong impact on FSCK accuracy. The Gauss-Chebyshev odd-rank scheme yields larger errors than Gauss-Chebyshev even-rank and Gauss-Legendre schemes. Increasing the number of quadrature nodes reduces the errors for all three quadrature schemes. As the transformation factor increases, the error of Gauss-Chebyshev odd-rank quadrature scheme rises, while those of the other two quadrature schemes change slightly, particularly when the number of quadrature nodes exceeds 8. The reference temperature significantly affects FSCK accuracy, and its influence depends on the Correlated-K scheme. In contrast, the reference pressure has a minor effect on the accuracy of FSCK. Furthermore, FSCK accuracy decreases as the reference soot volume fraction rises, and the best accuracy is achieved when the reference soot volume fraction is 0 ppm.
Electron-impact ionization cross sections are investigated for the ground and metastable states of the Ar3+, Ar4+, and Ar5+ ions. The direct and indirect ionization processes are included in the study. The direct ionization is analyzed from the 3s and 3p subshells of the studied configurations. The 3d subshell is investigated for the metastable states of the excited configurations of the Ar3+ and Ar4+ ions. The ionization from the 2p subshell is not included in the analysis since this process produces autoionizing configurations that decay further to the next ionization stage, contradicting previous studies which included the direct ionization from the 2p subshell to explain experimental data. Influence of excitations from the 2p, 3s, 3p, and 3d subshells (if applicable) with subsequent Auger decays to the indirect process is analyzed for the studied configurations. Important contribution to the indirect ionization process is determined for the excitations from the 2p subshell.
