Semiempirical Surface Research Articles

Snow depth retrieval method for PolSAR data using multi-parameters snow backscattering model

Snow depth (SD) is a crucial property of snow, its spatial and temporal variation is important for global change, snowmelt runoff simulation, disaster prediction, and freshwater storage estimation. Polarimetric Synthetic Aperture Radar (PolSAR) can precisely describe the backscattering of the target and emerge as an effective tool for SD retrieval. The backscattering component of dry snow is mainly composed of volume scattering from the snowpack and surface scattering from the snow-ground interface. However, the existing method for retrieving SD using PolSAR data has the problems of over-reliance on in-situ data and ignoring surface scattering from the snow-ground interface. We proposed a novel SD retrieval method for PolSAR data by fully considering the primary backscattering components of snow and through multi-parameter estimation to solve the snow backscattering model. Firstly, a snow backscattering model was formed by combining the small permittivity volume scattering model and the Michigan semi-empirical surface scattering model to simulate the different scattering components of snow, and the corresponding backscattering coefficients were extracted using the Yamaguchi decomposition. Then, the snow permittivity was calculated through generalized volume parameters and the extinction coefficient was further estimated through modeling. Finally, the snow backscattering model was solved by these parameters to retrieve SD. The proposed method was validated by Ku-band UAV SAR data acquired in Altay, Xinjiang, and the accuracy was evaluated by in-situ data. The correlation coefficient, root mean square error, and mean absolute error are 0.80, 4.49 cm, and 3.95 cm, respectively. Meanwhile, the uncertainties generated by different SD, model parameters estimation, solution method, and underlying surface are analyzed to enhance the generality of the proposed method.

Modelling Analysis of MEA Dynamic Operation Under Real-World Automotive Driving Cycle

In last years, Proton Exchange Membrane Fuel Cells (PEMFC) have received an increasing interest as a candidate for light and heavy-duty vehicle application1. To guarantee an efficient and durable operation of PEMFC, the understanding of local conditions, experienced by MEA, during dynamic operation is highly important. Effective numerical modelling can help to gain insights on the local heterogeneity linked to water and thermal management of the cell.A 1 + 1D multiphase dynamic non isothermal PEMFC model, implemented into MATLAB Simulink environment, was used to investigate experimental data obtained on a segmented cell hardware 2. Particular attention was paid to properly model the mass transport and kinetic related phenomena in catalyst layer(CL), as the local oxygen transport resistance through the ionomer thin film3 and the platinum oxide formation/reduction reaction, and their mutual interaction. Exploiting experimental data, obtained from tests on the segmented cell hardware, it was possible to give a robust validation to the 1+1D model, both at global and local level, thanks to the spatially-resolved information available. By simulating real-world driving cycle (DLC) protocol, developed within the ID-FAST project4, drying-up periods of ionomer contained in catalyst layers and in membrane, as well as flooding periods of porous layers and channels, were identified and analyzed during low-load and high-load operation.The performance model was coupled with a catalyst durability model, such to investigate the effect of ageing on dynamic performance of PEMFC. An innovative semi-empirical electrochemical surface area (ECSA) loss model was developed, based on the platinum dissolution mechanism, and validated initially on accelerated stress tests on different operating conditions5. The simulated load profile and local operating conditions in CLs were processed by a proper algorithm, to convert a driving cycle into a combination of elementary steps and to associate a loss factor for the ECSA to each of the identified elements, thus to estimate the retained local and global ECSA. Exploiting experimental database made up of 1000 operating hours performing ID-FAST DLC, the effect of local operating conditions on PEMFC degradation and performance was furtherly comprehended, focusing mainly on the effect of reactant humidification. The developed model framework succeeded in estimating performance loss during driving cycle operation, as visible in Figure 1(a-b). Simulated and experimental ECSA decay over time is shown in Figure 1(c), demonstrating good model prediction capability. The obtained results showed the potentiality of the model to capture complex two-phase and non-isothermal transport phenomena both in the along channel and through-MEA direction, consistently with a large set of experimental data.Experimental data on single cell were collected under ID-FAST project (Grant Agreement No 779565, Joint Undertaking, EU Horizon 2020).References D. A. Cullen et al., Nat. Energy, 6, 462–474 (2021) https://www.nature.com/articles/s41560-021-00775-z.E. Colombo, A. Baricci, A. Bisello, L. Guetaz, and A. Casalegno, J. Power Sources, 553, 232246 (2023) https://doi.org/10.1016/j.jpowsour.2022.232246.T. Suzuki, H. Yamada, K. Tsusaka, and Y. Morimoto, J. Electrochem. Soc., 165, F166–F172 (2018) https://iopscience.iop.org/article/10.1149/2.0471803jes.F. Wilhelm et al., ID-FAST -D4.3 – Analysis of coupling between mechanisms and definition of combined ASTs, p. 49, (2021) https://www.id-fast.eu/uploads/media/ID-FAST_D4-3_Analysis_of_coupling_between_mechanisms_and_definition_of_combined_ASTs_OK.pdf.A. Kneer and N. Wagner, J. Electrochem. Soc., 166, F120–F127 (2019) https://iopscience.iop.org/article/10.1149/2.0641902jes. Figure 1

Asteroseismic modelling strategies in the PLATO era

Context. Asteroseismology experienced a breakthrough in the last two decades thanks to the so-called photometry revolution with space-based missions such as CoRoT, Kepler, and TESS. Because asteroseismic modelling will be part of the pipeline of the future PLATO mission, it is relevant to compare some of the current modelling strategies and discuss the limitations and remaining challenges for PLATO. In this first paper, we focused on modelling techniques treating directly the seismic information. Aims. We compared two modelling strategies by directly fitting the individual frequencies or by coupling a mean density inversion with a fit of the frequency separation ratios. Methods. We applied these two modelling approaches to six synthetic targets with a patched atmosphere, for which the observed frequencies were obtained with a non-adiabatic oscillation code. We then studied ten actual targets from the Kepler LEGACY sample. Results. As is well known, the fit of the individual frequencies is very sensitive to the surface effects and to the choice of the underlying prescription for semi-empirical surface effects. This significantly limits the accuracy and precision that can be achieved for the stellar parameters. The mass and radius tend to be overestimated, and the age therefore tends to be underestimated. In contrast, the second strategy, which is based on mean density inversions and on the ratios, efficiently damps the surface effects and allows us to obtain precise and accurate stellar parameters. The average statistical precision of our selection of targets from the LEGACY sample with this second strategy is 1.9% for the mass, 0.7% for the radius, and 4.1% for the age. This is well within the PLATO mission requirements. The addition of the inverted mean density to the constraints significantly improves the precision of the stellar parameters by 20%, 33%, and 16% on average for the stellar mass, radius, and age, respectively. Conclusions. The modelling strategy based on mean density inversions and frequencies separation ratios showed promising results for PLATO because it achieved a precision and accuracy on the stellar parameters that meet the PLATO mission requirements with ten Kepler LEGACY targets. The strategy also left some margin for other unaccounted systematics, such as the choice of the physical ingredients of the stellar models or the stellar activity.

Machine Learning Approaches for Road Condition Monitoring Using Synthetic Aperture Radar

Airborne Synthetic Aperture Radar (SAR) has the potential to monitor remotely the road traffic infrastructure on a large scale. Of particular interest is the road surface roughness, which is an important road safety parameter. For this task, novel algorithms need to be developed. Machine learning approaches, such as Artificial Neural Networks (ANN) and Random Forest Regression, which can perform non-linear regression, can achieve this goal. This work considers fully polarimetric airborne radar datasets captured with DLR's airborne F-SAR radar system. Several machine learning-based approaches were tested on the datasets to estimate road surface roughness. The resulting models were then compared with ground truth surface roughness values and also with the semi-empirical surface roughness model studied in previous work.

Surface Roughness Analysis of H13 Steel during Electrical Discharge Machining Process Using Cu-TiC Sintered Electrode.

In electrical discharge machining (EDM), the machined surface quality can be affected by the excessive temperature generation during the machining process. To achieve a longer life of the finished part, the machined surface quality plays a key role in maintaining its overall integrity. Surface roughness is an important quality evaluation of a material’s surface that has considerable influence on mechanical performance of the material. Herein, a sintered cermet tooltip with 75% copper and 25% titanium carbide was used as tool electrode for processing H13 steel. The experiments have been performed to investigate the effects of EDM parameters on the machined surface roughness. The findings show that, as the pulse current, pulse length, and pulse interval are increased, the surface roughness tends to rise. The most significant determinant for surface roughness was found to be pulse current. A semi-empirical surface roughness model was created using the characteristics of the EDM technique. Buckingham’s theorem was used to develop a semi-empirical surface roughness prediction model. The semi-empirical model’s predictions were in good agreement with the experimental studies, and the built empirical model based on physical features of the cermet tooltip was tested using dimensional analysis.

Intercalibration of FY-3D MWTS Against S-NPP ATMS Based on Microwave Radiative Transfer Model

Accurate and stable in-orbit radiometric calibration of a satellite instrument is fundamental to Earth geophysical parameter estimation. This article addresses the intercalibration of the microwave temperature sounder (MWTS) on the Chinese second-generation polar-orbiting meteorological satellite, Fengyun 3D (FY-3D), against the advanced technology microwave sounder (ATMS) aboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite. First, ocean and land microwave radiative transfer models (RTM) are constructed by combining the sea and land surface emissivity models and atmospheric absorption model, as well as the intercalibration equations. Then, the MWTS and ATMS observations are resampled into a 1° × 1° regular grid space, and the matching brightness temperatures (TBs) under clear-sky/near clear-sky conditions are collected. Next, the TBs at top-of-atmosphere are simulated using the RTM and the fifth generation of European Centre for Medium-Range Weather Forecast atmospheric reanalysis (ERA5) data. After that, the double differences between FY-3D MWTS and S-NPP ATMS and the theoretical observations in FY-3D MWTS channels are calculated. Finally, the radiometric calibration coefficients of FY-3D MWTS are successfully derived from the observations of S-NPP ATMS by linear fits on the matching TBs. In contrast to the ATMS measurements, FY-3D MWTS observations are generally overestimated, and the in-orbit radiometric calibration errors (mean ± standard deviation at the mean) are 1.83 ± 1.45, 0.45 ± 0.94, 1.87 ± 0.60, −0.20 ± 0.36, −0.02 ± 0.37, 0.19 ± 0.24, 1.69 ± 0.28, 2.25 ± 0.29, 1.97 ± 0.33, 1.74 ± 0.42, 2.84 ± 0.42, 0.07 ± 0.65, and 0.32 ± 1.18 K in FY-3D MWTS channels 1–13, respectively. The results with Hewison's semi-empirical land surface emissivity (LSE) model and the results with LSEs derived from the coincident ATMS observations at 50.3 GHz are consistent. Moreover, the intercalibration results obtained by the RTM in this work also agree well with the results obtained by the community radiative transfer model.

Nonadiabatic dynamics with quantum nuclei: simulating charge transfer with ring polymer surface hopping.

Investigation of many electronic processes in molecules and materials, such as charge and exciton transport, requires a computational framework that incorporates both non-adiabatic electronic effects and nuclear quantum effects, in particular at low temperatures. We have recently developed an efficient semi-empirical fewest switches surface hopping method, denoted fragment orbital-based surface hopping (FOB-SH), that was tailored towards highly efficient simulation of charge transport in molecular materials, yet with nuclei treated classically. In this work, we extend FOB-SH and include nuclear quantum effects by combining it with ring-polymer molecular dynamics (RPMD) in three different flavours: (i) RPSH with bead approximation (RPSH-BA) as suggested in Shushkov et al., J. Chem. Phys., 2012, 137, 22A549, (ii) a modification of (i) denoted RPSH with weighted bead approximation (RPSH-wBA) and (iii) the isomorphic Hamiltonian method of Tao et al., J. Chem. Phys., 2018, 148, 10237 (SH-RP-iso). We present here applications to hole transfer in a molecular dimer model and analyze detailed balance and internal consistency of all three methods and investigate the temperature and driving force dependence of the hole transfer rate. We find that RPSH-BA strongly underestimates and RPSH-wBA overestimates the exact excited state population, while SH-RP-iso gives satisfactory results. We also find that the latter predicts a flattening of the rate vs. driving force dependence in the Marcus inverted regime at low temperature, as often observed experimentally. Overall, our results suggest that FOB-SH combined with SH-RP-iso is a promising method for including zero point motion and tunneling in charge transport simulations in molecular materials and biological systems.

High Spatial Resolution Soil Moisture Mapping Using a Lobe Differencing Correlation Radiometer on a Small Unmanned Aerial System

A persistent challenge in the measurement of soil moisture from satellites stems from the inherently low spatial resolution using active or passive system. The lobe differencing correlation radiometer (LDCR) on a small unmanned aerial system (sUAS) is shown to provide a capability to measure soil moisture at high spatial resolution for a range of scientific and operational purposes. Flight tests of LDCR on a fixed wing sUAS were performed at the Canton, Oklahoma Soilscape site in September 2015, and Irrigation Research Foundation (IRF) in Yuma, Colorado, in June 2016. The LDCR design and performance are discussed, and the calibration using both preflight lab test data and in-flight data over a calm pond was performed to calibrate the radiometer. Radio frequency interference (RFI) from the sUAS platform was observed and mitigated. The LDCR sampling processes are detailed and an implementation of the $\tau -\omega $ vegetation correction model along with a semiempirical surface roughness correction model incorporating a full-domain soil moisture mapping algorithm is presented. The algorithm uses a weakly nonlinear observation operator suitable for irregular sUAS flight trajectories that maps volumetric soil moisture (VSM) on a user-defined product grid from the sUAS sampling grid. Using LDCR radiometric and thermal measurements, along with Landsat-based vegetation water content (VWC) and soil texture information, soil moisture was mapped at decameter spatial resolution. The retrieved VSM data are favorably compared with in situ VSM measurements and irrigation records. A method for determining LDCR VSM estimation errors is developed to quantify the mapping algorithm accuracy and assess the impact of error sources.

The application of molar surface Gibbs energy to predicting ILs' properties

1-alky-3-methylimidazolium taurate ionic liquid, [Cnmim]Tau (n = 3,4) were synthesized by neutralization method and were confirmed by 1H NMR and 13C NMR. Standard addition method was applied to determine the densities, surface tensions and refractive indexes for [Cnmim]Tau (n = 3,4). By using Tong's vaporization enthalpy model, the vaporization enthalpies of [Cnmim]Tau (n = 3,4) and some other ILs were estimated. The association enthalpies, ΔAHm0 of them in gas phase were estimated. Combined gs with Lorentz–Lorenz equation, a semi-empirical surface tension predicting equation can be obtained. The surface tensions of [Cnmim][Tau](n = 3,4), some other ILs and some molecular liquids were predicted. The predicted values are highly similar with the corresponding experimental ones. In current work, we put forward a new method for predicting ΔlgCp,m0,which was used for [Cnmim]Tau (n = 3,4) and other ILs. Our results are in the same order magnitude with Verevkin's, but ours are relatively higher.

Molar surface Gibbs energy of the aqueous solution of ionic liquid [C4mim][OAc

The values of density and surface tension for aqueous solution of ionic liquid(IL) 1-butyl-3-methylimidazolium acetate([C4mim][OAc]) with various molalities were measured in the range of 288.15—318.15 K at intervals of 5 K. On the basis of thermodynamics, a semi-empirical model-molar surface Gibbs energy model of the ionic liquid solution that could be used to predict the surface tension or molar volume of solutions was put forward. The predicted values of the surface tension for aqueous [C4mim][OAc] and the corresponding experimental ones were highly correlated and extremely similar. In terms of the concept of the molar Gibbs energy, a new Eotvos equation was obtained and each parameter of the new equation has a clear physical meaning.

Versatile Large-Area Custom-Feature van der Waals Epitaxy of Topological Insulators.

As the focus of applied research in topological insulators (TI) evolves, the need to synthesize large-area TI films for practical device applications takes center stage. However, constructing scalable and adaptable processes for high-quality TI compounds remains a challenge. To this end, a versatile van der Waals epitaxy (vdWE) process for custom-feature bismuth telluro-sulfide TI growth and fabrication is presented, achieved through selective-area fluorination and modification of surface free-energy on mica. The TI features grow epitaxially in large single-crystal trigonal domains, exhibiting armchair or zigzag crystalline edges highly oriented with the underlying mica lattice and only two preferred domain orientations mirrored at 180°. As-grown feature thickness dependence on lateral dimensions and denuded zones at boundaries are observed, as explained by a semiempirical two-species surface migration model with robust estimates of growth parameters and elucidating the role of selective-area surface modification. Topological surface states contribute up to 60% of device conductance at room temperature, indicating excellent electronic quality. High-yield microfabrication and the adaptable vdWE growth mechanism with readily alterable precursor and substrate combinations lend the process versatility to realize crystalline TI synthesis in arbitrary shapes and arrays suitable for facile integration with processes ranging from rapid prototyping to scalable manufacturing.

Photochemical Reactions of Cyclohexanone: Mechanisms and Dynamics.

Photochemistry of carbonyl compounds is of major importance in atmospheric and organic chemistry. The photochemistry of cyclohexanone is studied here using on-the-fly molecular dynamics simulations on a semiempirical multireference configuration interaction potential-energy surface to predict the distribution of photoproducts and time scales for their formation. Rich photochemistry is predicted to occur on a picosecond time scale following the photoexcitation of cyclohexanone to the first singlet excited state. The main findings include: (1) Reaction channels found experimentally are confirmed by the theoretical simulations, and a new reaction channel is predicted. (2) The majority (87%) of the reactive trajectories start with a ring opening via C-Cα bond cleavage, supporting observations of previous studies. (3) Mechanistic details, time scales, and yields are predicted for all reaction channels. These benchmark results shed light on the photochemistry of isolated carbonyl compounds in the atmosphere and can be extended in the future to photochemistry of more complex atmospherically relevant carbonyl compounds in both gaseous and condensed-phase environments.

Study on thermophysical properties for aqueous solutions of ionic liquid N-ethyl-pyridine dicyanamide [C2py][DCA

The IL (ionic liquid) [C2py][DCA] (N-ethyl-pyridine dicyanamide) was prepared and confirmed by 1H NMR (1H nuclear magnetic resonance) spectroscopy, 13C NMR (13C nuclear magnetic resonance) spectroscopy. The values of density and surface tension of the IL [C2py][DCA] and its aqueous solutions with various molalities were determined at T = (288.15 to 318.15)K. The values of the molar volume for aqueous solutions of [C2py][DCA], V, were calculated and increase with the increase of temperature and molarity. On the basis of thermodynamics, a semi-empirical model-molar surface Gibbs free energy model of the ionic liquid solution was put forward. It can be used to predict the surface tension and the predicted values of the surface tension for aqueous [C2py][DCA] are all most the same with the corresponding experimental ones. In terms of the concept of the molar surface Gibbs free energy, a new Eötvös equation is obtained. Each parameter of the new equation has a clear physical meaning, energy dimension and is more convenient to use.

Modeling of photodegradation process to remove the higher concentration of environmental pollution

Environmental organic pollutants are mineralized to harmless final-products such H2O and CO2 by photocatalytic advanced oxidation processes (AOPs). In photocatalytic-AOPs, an appropriate concentration of p-Cresol was mixed with certain amount of ZnO in 500 mL deionized water according to an experimental-design. Then the mixture was irradiated by UV-A lamp at different pH for 6 h. At specific time intervals, the sampling was carried out to calculate the efficiency of the photodegradation. Therefore, the photodegradation as a system consists of four input variables such irradiation time, pH, amount of ZnO and p-Cresol’s concentration while the only output was the efficiency. In this work, the system was modeled and optimized by semi-empirical response surface methodology. To obtain the empirical responses, the design was performed in laboratory. Then observed responses were fitted with several well-known models by regression process to suggest a provisional model. The suggested model which was validated by several statistical evidence, predicted the desirable condition with higher efficiency. The predicted condition consisted of irradiation time (280 min), pH (7.9), photocatalyst (1.5g L−1), p-Cresol (95 mg L−1) and efficiency (95%) which confirmed by further experiments. The closed confirmation results has presented the removal (efficiency = 94.7%) of higher p-Cresol concentration (95 mg L−1) at shorter irradiation time in comparison with the normal photodegradation efficiency (97%) which included irradiation time (300 min), pH (7.5), photocatalyst amount (1.5g L−1) and p-Cresol (75 mg L−1). As a conclusion, the modeling which is able to industrial scale up succeeded to remove higher concentration of environmental organic pollutants with ignorable reduction of efficiency.

Modeling the surface tension of complex, reactive organic–inorganic mixtures

Abstract. Atmospheric aerosols can contain thousands of organic compounds which impact aerosol surface tension, affecting aerosol properties such as heterogeneous reactivity, ice nucleation, and cloud droplet formation. We present new experimental data for the surface tension of complex, reactive organic–inorganic aqueous mixtures mimicking tropospheric aerosols. Each solution contained 2–6 organic compounds, including methylglyoxal, glyoxal, formaldehyde, acetaldehyde, oxalic acid, succinic acid, leucine, alanine, glycine, and serine, with and without ammonium sulfate. We test two semi-empirical surface tension models and find that most reactive, complex, aqueous organic mixtures which do not contain salt are well described by a weighted Szyszkowski–Langmuir (S-L) model which was first presented by Henning et al. (2005). Two approaches for modeling the effects of salt were tested: (1) the Tuckermann approach (an extension of the Henning model with an additional explicit salt term), and (2) a new implicit method proposed here which employs experimental surface tension data obtained for each organic species in the presence of salt used with the Henning model. We recommend the use of method (2) for surface tension modeling of aerosol systems because the Henning model (using data obtained from organic–inorganic systems) and Tuckermann approach provide similar modeling results and goodness-of-fit (χ2) values, yet the Henning model is a simpler and more physical approach to modeling the effects of salt, requiring less empirically determined parameters.

Quantifying the combined effects of climatic, crop and soil factors on surface resistance in a maize field

• The response of surface resistance to climatic, crop and soil factors was investigated. • A surface resistance model was construed based on the response pattern. • The surface resistance model was compared to the widely used Jarvis and KP models. • The surface resistance model combined with PM estimated sparse vegetation ET accurately. Land surface evapotranspiration (ET) is the central process in hydrological cycle. The accuracy in simulating ET is affected by the calculation of underlying surface resistance. However, the surface resistance is difficult to be measured and greatly affected by climatic, crop and soil factors. How to quantify the combined effects of these factors on surface resistance is still a challenge for hydrologists. Our study attempted to construct and validate a semi-empirical surface resistance model based on the analysis of the response pattern of surface resistance to climatic resistance, leaf area index (LAI) and soil moisture. The surface resistance was derived by the re-arranged Penman–Monteith (PM) equation and the measured maize ET using eddy covariance in 2007. Results indicate that the ratio of surface resistance to climatic resistance showed a logarithmic relationship with LAI, and an exponential function as soil moisture when LAI was below 2. But the ratio was nearly constant and not sensitive to variation in LAI and soil moisture when LAI exceeded 2. Based on the relationships, a surface resistance model was further constructed and compared to the widely used Katerji–Perrier and Jarvis models over the sparse maize and grape canopy. Our resistance model combined with the PM equation improved the accuracy in estimating daily maize ET by 11% in 2007 and 4% in 2008, and vineyard ET by 7% against the Katerji–Perrier model combined with PM method, while by 32% in 2007 and 104% in 2008, and vineyard ET by 5% against the Jarvis model combined with PM method. Thus our model significantly improved the performance in simulating sparse vegetation ET and can be used to estimate daily surface resistance under the partial canopy condition.

Global spectroscopy of the water monomer

Given the large energy required for its electronic excitation, the most important properties of the water molecule are governed by its ground potential energy surface (PES). Novel experiments are now able to probe this surface over a very extended energy range, requiring new theoretical procedures for their interpretation. As part of this study, a new, accurate, global spectroscopic-quality PES and a new, accurate, global dipole moment surface are developed. They are used for the computation of the high-resolution spectrum of water up to the first dissociation limit and beyond as well as for the determination of Stark coefficients for high-lying states. The water PES has been determined by combined ab initio and semi-empirical studies. As a first step, a very accurate, global, ab initio PES was determined using the all-electron, internally contracted multi-reference configuration interaction technique together with a large Gaussian basis set. Scalar relativistic energy corrections are also determined in order to move the energy determinations close to the relativistic complete basis set full configuration interaction limit. The electronic energies were computed for a set of about 2500 geometries, covering carefully selected configurations from equilibrium up to dissociation. Nuclear motion computations using this PES reproduce the observed energy levels up to 39 000 cm(-1) with an accuracy of better than 10 cm(-1). Line positions and widths of resonant states above dissociation show an agreement with experiment of about 50 cm(-1). An improved semi-empirical PES is produced by fitting the ab initio PES to accurate experimental data, resulting in greatly improved accuracy, with a maximum deviation of about 1 cm(-1) for all vibrational band origins. Theoretical results based on this semi-empirical surface are compared with experimental data for energies starting at 27 000 cm(-1), going all the way up to dissociation at about 41 000 cm(-1) and a few hundred wavenumbers beyond it.

Critical energy for direct initiation of spherical detonations in H 2/N 2O/Ar mixtures

Although the detonation phenomenon in hydrogen–nitrous oxide mixtures is a significant issue for nuclear waste storage facilities and development of propulsion materials, very limited amount of critical energy data for direct initiation – which provides a direct measure of detonability or sensitivity of an explosive mixture − is available in literature. In this study, the critical energies for direct blast initiation of spherical detonations in hydrogen–nitrous oxide–Ar mixtures obtained from laboratory experiments and theoretical predictions at different initial conditions (i.e., different initial pressure, equivalence ratio and amount of argon dilution) are reported. In the experiments, direct initiation is achieved via a spark discharge from a high voltage and low inductance capacitor and the initiation energy is estimated accordingly from the current output. Characteristic detonation cell sizes of hydrogen–nitrous oxide–Ar mixtures are estimated from chemical kinetics using a recently updated reaction mechanism. A correlation expression is developed as a function of initial pressure, argon dilution and equivalence ratio, which is fitted to provide good estimation of the experimental measured data. The direct link between cell size and critical energy for direct blast initiation is then analyzed. Good agreement is found between experimental results and theoretical predictions, which make use of the cell size estimation correlation and the semi-empirical surface energy model. The effects of the initial pressure, equivalence ratio and the amount of Ar dilution on the critical initiation energy H 2–N 2O–Ar mixtures are investigated. By comparing the critical energies with those of H 2–O 2–Ar mixtures, it is shown that H 2–N 2O mixtures are more detonation sensitive with smaller initiation energies than H 2–O 2 mixtures at the same initial pressure, equivalence ratio and amount of argon dilution, except for higher diluted condition with amount of argon in the mixture above 20%. Attempt is made to explain the critical energy variation and comparison between the two H 2–N 2O–Ar and H 2–O 2–Ar mixtures from the induction length analysis and detonation instability consideration.

Ultrafast photochemistry of methyl hydroperoxide on ice particles.

Simulations show that photodissociation of methyl hydroperoxide, CH(3)OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH(3)OOH is found to prefer the exterior of the cluster, with the CH(3)O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH(3)O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH(3)O to form CH(3)OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH(2)O, H(2)O, H(2), CO, CH(3)OH, and H(2)O(2).

Modeling Cd(II) adsorption to heterogeneous subsurface soils in the presence of citric acid using a semi-empirical surface complexation approach

Laboratory batch sorption experiments were conducted to understand the effect of citrate on cadmium sorption to heterogeneous subsurface soils. Our results indicate that citrate may have a pronounced effect on Cd(II) sorption depending on system conditions (e.g. pH, ligand concentration). While the presence of citrate had no effect on Cd(II) sorption at pH < 4, it led to a decrease in Cd sorption under slightly acidic to alkaline pH range depending on the concentration of citrate used. Maximum effect of citrate on Cd(II) sorption was observed at pHs between 5 and 7. This coincides with the observed range of maximum citrate adsorption and the formation of Cd–citrate complexes. A two-site non-electrostatic surface chemical model (SCM) based on the Generalized Composite (GC) approach was able to describe the experimental data well over a wide range of conditions, with only six different surface reactions including two ternary (Cd/citrate/soil) surface complexes. Although the semi-empirical surface model used in the simulations does not accurately represent the actual mechanisms at the molecular level, it is relatively simple, and can be effectively used in transport calculations as an alternative to the K d approach.