A clear understanding of sublimation kinetics is critical for developing detection techniques. Sublimation affects the shape and size of small particles as well as the environment. The particle characteristics are essential for various types of measurements including optical response. Molecular dynamic simulations were used to better understand the kinetics of both the condensation of molecules onto and the sublimation of molecules from the surfaces of explosives materials such as 2,4-dinitrotoluene and 2,4,6-trinitrotoluene. These studies were undertaken to better understand the persistence of trace quantities of particles on surfaces to aid efforts to optical detect strategies of explosives as well as physical harvesting approaches such as collection with swabs. Potential-energy-function parameters for the molecular dynamic (MD) simulations are designed and values for the probability of recondensation onto the surface (i.e., sticking coefficient) and the velocity distribution of molecules escaping the particle surface calculated. These values were compared with other experimental and simulation efforts for the studied materials.
Hydrogen-bonding plays an important role in interactions of molecular structures, and is associated with distinct features in vibrational spectra of molecular systems. Interpretation of these features is essential for monitoring and control of structural changes and kinetic processes in large ensembles of molecular structures. Computational experiments based on molecular dynamics provide interpretation of vibrational-spectrum features. This report extends previously reported simulations concerning nerve-agent-sorbent binding for examination of spectral features correlated with annealing
Molecular dynamics (MD) modeling of hydrogen-bond oscillations is described. Hydrogen-bonding plays an important role in interactions of molecular structures, and is associated with distinct features in vibrational spectra of molecular systems. Interpretation of these features is essential for monitoring and control of structural changes and kinetic processes in large ensembles of molecular structures. Computational experiments based on MD provide interpretation of vibrational-spectrum features. In the case of molecular binding spectroscopy, hydrogen-bond vibrational modes, which are associated with sorbent-sorbate interactions, can be correlated with characteristic spectral features at finite temperature.
Molecular dynamics (MD) simulations of nerve-agent-sorbent binding provide, in principle, interpretation of measured infrared (IR) spectra obtained using molecular binding spectroscopy. Comparison of IR absorption spectra for nerve-agent-sorbent binding obtained using MD and those measured experimentally, however, indicate inconsistencies with respect to interpretation of underlying molecular interactions. Accordingly, there is a need to examine physical assumptions underlying these MD simulations and potential functions representing the molecular dynamics. This study examines aspects of MD simulated nerve-agent-sorbent binding for more quantitative interpretation of ATR spectra associated with nerve-agent detection.
This study describes parametric modeling of diffuse IR reflectance for sparsely surface-distributed particles of the explosive RDX. Diffuse IR spectra are modeled using a formulation that considers spectral features due to target-material reflectance, i.e., RDX, substrate reflectance and resonance scattering resulting from finite sizes of surface-distributed particles. The results of this study demonstrate an approach for parametric modeling of diffuse IR reflectance for sparsely surface-distributed particles. The mathematical formulation of this approach is that of a phenomenological scattering-matrix representation.
Threat chemicals such as explosives may persist on surfaces, enabling them to be detected by non-contact or standoff optical methods such as diffuse IR reflectance. However, due to particle size effects and optical coupling to the substrate, their IR spectral signatures will differ from laboratory reference measurements of bulk materials. This study presents an inverse analysis of diffuse IR reflectance from sparsely surface-distributed particles of the explosive PETN. A methodology using spectrum templets is applied for inverse analysis of measured spectra. The methodology is based on a generalization of extended multiplicative signal correction (EMSC). The results of this study demonstrate application of the inverse analysis methodology for extraction of spectral features for surface-distributed particles of specified dielectric response.
This study examines using parametric models for inverse analysis of diffuse IR reflectance from particulate materials that are sparsely distributed upon a surface. Parametric models are applied for inverse analysis of simulated spectra, which are calculated using ensembles of reflectance spectra for non-interacting material particles on surfaces, which have specified dielectric response properties and particle-size distributions Simulated reflectance spectra for individual particles upon surfaces, used for prototype inverse analysis, are calculated numerically using a model based on Mie scattering theory, which assumes spherical particles on surfaces. Parametric models of diffuse reflectance spectra provide encoding of dielectric response features for physical interpretation and convenien representation.
Molecular dynamics (MD) simulations of nerve agent-sorbent binding are presented for interpretation of infrared molecular binding spectroscopy, which is for nerve agent detection. Potential-energy-function parameters for the MD simulations are adjusted according to attenuated total reflection (ATR) measurements of surface interaction between a custom sorbent material and nerve agent simulant. The physical assumptions underlying these MD simulations and sensitivity of potential-function parameters are examined with respect to viability for quantitative prediction and interpretation of ATR spectra associated with nerve agent detection.
Prototype simulations of attenuated total re ection (ATR) applied for infrared molecular binding spectroscopy, which is for nerve-agent detection, are presented. The simulations use: calculated estimates of permitivity functions (for the custom sorbent SiFA4H, nerve agent simulant DMMP and molecular structure SiFA4H-DMMP); and a model of re ection from multicomponent-multilayer systems, which is based on the scattering-matrix representation of electromagnetic-wave propagation. The physical assumptions and approximations underlying these simulations, and model-parameter sensitivity are examined with respect to quantitative prediction of ATR spectra associated with nerve-agent detection. Experimentally measured ATR spectra are utilized for qualitative comparison and quantitative adjustment of model parameters.
This study examines using simulated spectra for analysis of diffuse IR reflectance from explosive materials that are sparsely distributed upon a surface. The simulated spectra are calculated using ensembles of reflectance spectra for non-interacting explosive particles on surfaces, which have specified dielectric response properties and particle-size distributions. Reflectance spectra for individual particles upon a surface are calculated numerically using a model based on Mie scattering theory, which assumes a spherical particle on a surface. This validation study considers a prototype system comprising a sparse distribution of RDX particles upon a soda-lime glass surface compared with experimental results.
An inverse analysis of experimentally measured infrared absorption spectra for the custom sorbent SiFA4H, nerve agent precursor and simulant DMMP, and intermolecularly bonded structure SiFA4H+DMMP is presented. These structures and their associated infrared spectra provide general understanding of the process whereby an analyte chemical may be detected using infrared spectral analysis. The inverse analysis presented provides estimates of permittivity functions, which when combined with the Clausius-Mossotti relation, can predict molecular polarizabilities associated with SiFA4H-SiFA4H and SiFA4H-DMMP interactions. Molecular polarizabilities deduced from measured absorption coefficients are modeled using molecular dynamics simulations.
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