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Weapons and Materials Research Directorate Research Areas

Combustion, Kinetics, and Spectroscopy

Flame Diagnostics Employing Molecular Beam/Mass Spectrometry and Laser Spectroscopy

Advisor:  RC Sausa

Key words: flames, mass spectra, laser spectra, fire suppressants, propellant combustion, thermal conditions and measurements,

This research centers on the detailed structure studies of premixed flames doped with fire suppressants or flames important in solid nitramine propellant combustion. It is important to understand the detailed chemistry of these flames in order to predict and model flame extinguishment, and to successfully model combustion, which will lead to optimal propellant formulation and propulsion performance.

We stabilize the flames on a burner at subatmospheric pressures to increase the spatial resolution of the reaction zone, and we measure the temperature and species concentrations as a function of height above the burner surface. Temperature profiles are obtained with coated thermocouples and cross-checked against those obtained by the sodium line reversal technique and/or by laser-induced fluorescence (LIF) using Boltzmann rotational spectral analysis of OH or other suitable species.

Species concentration profiles are obtained using molecular beam sampling with triple quadrupole mass spectrometric detection and by various laser spectroscopies techniques, including LIF and resonance-enhanced multiphoton ionization (REMPI). We then compare the experimental flame profiles of the major and radical species to calculated profiles generated using the flame code PREMIX.

The mechanisms used to generate these model profiles are derived from critical literature reviews. Rate and sensitivity analyses are also employed to investigate the intricacies of the mechanism, and the reactions important in modeling the experimental results.

Research equipment consists of a low-pressure burner apparatus equipped with molecular beam sampling capabilities and triple quadrupole mass spectrometric detection for isobaric species discrimination, an excimer pumped/dye laser with second harmonic generation, a computer acquisition and analysis system, and assorted electro-optics.

This research complements Dr WR Anderson’s opportunity, “Detailed Chemical Modeling of Combustion.”

Computational Chemistry for Rocket Propellant Design and Analysis

Advisor:  MJ McQuaid

Key words: reaction kinetics and dynamics, molecular modeling, quantum chemistry, energetic materials

A variety of computational chemistry techniques are employed to characterize properties and processes relevant to the development of rocket propellants. Computational quantum mechanics is employed for the determination of molecular structure, thermochemical properties, and the calculation of potential energy barriers to reactions thought to be important in ignition. Molecular dynamics simulations are employed to predict densities, enthalpies of phase change, and to address miscibility issues. Because of the relatively unique electronic properties of rocket propellant ingredients, force field development can be required to accomplish goals. Proposals to model the properties of gels would be welcome.

Associates have access to considerable computational resources throughout the ARL Major Shared Resource Center. Available quantum chemistry codes include GAUSSIAN 03, ACES II, DMOL3, and CASTEP. Available classical molecular modeling software includes DL_Poly and a suite of polymer modeling codes from Accelrys. Publication of research results in the open literature is strongly encouraged.

References
McQuaid MJ, et al: Journal of Molecular Structure (Thermochemistry) (Theochem) 587: 199, 2002.

McQuaid MJ, Sun H, Rigby D: Journal of Computational Chemistry 25: 61, 2004.

Flame Combustion Diagnostics and Modeling

Advisor:  KL McNesby

Key words: computer simulation, flames, mass spectra, pressure effects and measurements, propellant combustion, solid propellants, infrared spectra, Fourier-transformation spectra

This research opportunity involves using Fourier-transform infrared (FTIR) spectroscopy, infrared diode laser (IRDL) spectroscopy, and mass spectrometry (MS) to probe the combustion chemistry of premixed and counterflow diffusion burner flames that are relevant to the probable flame-zone chemistry of solid propellants. The burner flames are studied at subatmospheric pressure (e.g., 20-100 torr) in order to expand the reaction zone permitting spatial resolution of the combustion chemistry. Species profiles, as a function of height above the burner surface, are then measured using FTIR, IRDL, or MS. In the case of the FTIR and IRDL diagnostics, temperature profiles can also be determined from the intensity distributions in the vibrational-rotational lines for the smaller molecules.

Flame modeling focuses on using a one-dimensional, laminar flame code. Using literature or estimated elementary reaction rate constants, time- and height-dependent species concentration (and temperature) profiles are computed for direct comparison with those determined experimentally. Another research opportunity involves flame combustion modeling of experimental data published by other investigators.

Flame Suppression Mechanisms

Advisor: KL McNesby

Key words: fire suppressants, flame spectra

The goal of this research is to determine how chemicals that are as/more efficient than halon 1301 quench the various flames of military interest. Research will focus on investigations of inhibited methane/air and propane/air opposed flow diffusion flames at reduced pressure. Optical diagnostics (laser-induced fluorescence, mid- and near-infrared tunable diode laser spectroscopy) are used to determine temperature, and permanent (CO, H2O) and radical (H, O, OH) species concentration profiles within the opposed flow flame. We compare these experimental results with results from flame modeling calculations to determine the reactions most important to flame suppression chemistry.

Tunable Diode Laser Spectroscopic Sensing of Fuels, Oxidizers, and Toxic Gases

Advisor: KL McNesby

Key words: diode laser spectroscopy, high resolution, remote sensing, toxic gases

Tunable diode laser spectroscopy is used to measure and quantify fuels, oxidizers, and combustion products during fire and fire suppressant testing of laboratory and real scale flames and fires. The research focuses on the use of room temperature semiconductor diode lasers and detectors operating in the near- and mid-infrared spectral region. Research takes advantage of state-of-the-art laser characterization and gas measurement facilities in our laboratory. Current efforts include high sensitivity measurements of toxic gases using novel modulation techniques and sensing of broad band absorbers using narrow band laser sources.

Multiphoton Ignition and Flow Diagnostics Studies

Advisor:  AW Miziolek

Key words: gas phase and gases, ignition, multiphoton processes and effects, photochemistry, propellant combustion, ultraviolet lasers

Research on the detection of nascent combustion atoms, such as O and H, using two-photon resonance fluorescence has demonstrated that multiphoton photochemistry may occur under certain experimental conditions and thus affect the “nonintrusive” nature of the laser probe. Subsequent experiments have led to the discovery of a new type of laser-ignition source, which is based on multiphoton photochemistry using ultraviolet lasers. Research opportunities are available in the areas of laser ignition and flow characterization (temperature, velocity, and number density) of subsonic and supersonic flows of reactive gases.

Major equipment utilized in this research includes fully tunable Nd:YAG-based picosecond and nanosecond laser systems, excimer lasers, a two-dimensional intensified photodiode array camera, high-speed detection electronics, a streak camera, and a computer-based data acquisition and analysis system.

Combustion, Plasma, and Photochemistry Studies for Environmental Applications

Advisor:  AW Miziolek

Key words: computer simulation, propellant combustion, thermal conditions and measurements, flames

The Army has recently increased its emphasis on environmental research in areas such as pollution prevention, cleanup, and compliance. Research opportunities exist in the following areas: (1) identification of mechanisms responsible for flame extinguishment to support a major DOD R&D effort in order to identify successful Halon alternative compounds; (2) study of plasmas, particularly nonthermal ones, as a method for toxic and hazardous compound destruction; and (3) application of photochemistry for waste destruction and cleanup of contaminated sites. Our laboratory is well-equipped with appropriate instrumentation including low-pressure burners with molecular beam/mass spectrometric and tunable diode laser diagnostics. For most of these projects, we have developed close collaborations with research groups at the National Institute of Standards and Technology. Opportunities exist in both the experimental and modeling aspects of this work.

Detailed Modeling of the Chemistry and Physics of Combustion

Advisor:  WR Anderson

Key words: computer simulation, propellant combustion, thermal conditions and measurements, flames

The objective of this research is to understand and model reaction mechanisms and physical effects, which occur in combustion of propellants and gas phase flames. Emphasis is placed on understanding the generation of power and pollutants during combustion. Elementary reaction rate constants, obtained primarily from experiments in the literature, are used to predict the reaction network that is responsible for converting from fuel/oxidizer mixture to products. If elementary rate data are not available and the reaction rates cannot be measured, the rate constants may be calculated by theoretical chemists, with whom we are in close collaboration. The models are compared with results from shock tube, stirred reactor, premixed laminar flame, or propellant strand combustion experiments. Mechanisms of primary interest range from simple reactions of small organic molecules with nitrogen oxides to the complex chemistry of propellant combustion. Reactions of a plasma containing C+, H+, and O+ ions with air and propellant combustion gases are also of interest. Available computing facilities are excellent, ranging from PCs to supercomputers.

Studies in Laser Chemical Kinetics and Reaction Dynamics

Advisor: RC Sausa

Key words: reaction kinetics and dynamics, atmospheric kinetics, air pollution, atmospheric ozone, laser-induced fluorescence, greenhouse effect

The objective of this research is to measure elementary rate constants and branching ratios of important reactions involved in atmospheric pollution, ozone depletion, global warming, and energetic material decomposition and combustion; as well as to understand the reaction dynamics of the species involved. An understanding of the chemical kinetics and reaction dynamics is a prerequisite for successfully modeling such phenomena. These experiments will be performed using the well-established, time-resolved laser pump/probe technique. A pump laser is employed to generate the transient species from selected precursors; then, a probe detects and monitors the disappearance of the reactant species or appearance of the products by laser-induced fluorescence (LIF) or resonance-enhanced multiphoton ionization (REMPI). Proposals are also welcome that center on the theoretical characterization of the potential energy surfaces.

Available equipment includes a flow chamber equipped for LIF and REMPI-time-of-flight studies, an excimer or CO2 pump laser, a probe dye laser with second harmonic generator, various PC-AT computers with data acquisition and analysis software, fast oscilloscopes, boxcars, monochromator/PMT, and assorted electro-optics. Main frame and supercomputer capabilities are available for theoretical studies.

Environmental Fate Research

Advisor:  AW Miziolek

Key words: air pollution, atmospheric ozone, volatile organic compounds, fire suppressants

The DOD releases many volatile compounds during normal operations. Some of these compounds are of concern because of their ultimate atmospheric fate, and their possible deleterious effect on the troposphere and/or stratosphere. For example, we need to develop new fire extinguishing agents as a result of the stratospheric ozone depletion caused by the currently used Halons. The atmospheric fate of possible Halon replacement compounds must be determined in order to ensure their environmental acceptability. Research opportunities exist in laboratory measurements of spectroscopy, photochemistry, and kinetics for such compounds. This work is carried out in collaboration with atmospheric chemistry researchers at the National Institute of Standards and Technology, as well as atmospheric modelers at the University of Illinois.

Laser Diagnostics and Collisional Dynamics

Advisor:  AW Miziolek

Key words: flame spectra, lasers and laser processes, multiphoton processes and effects, collisions (physics)

This opportunity focuses on developing new laser-based diagnostics for various flame species, particularly light atoms and small radicals. New approaches are being pursued, which involve multiphoton excitation leading to emission, ionization, or photochemistry. Related research centers on detailed studies of excited-state collisional dynamics, which impact the quantitative nature of the measurements. Research tools include nanosecond and picosecond tunable lasers, a tunable diode laser, excimer lasers, various high-speed photodetectors, signal-averaging electronics, and computers.

Laser Spectroscopy and Novel Laser Diagnostics

Advisor: RC Sausa

Key words: laser spectra, photolysis, photodissociation, atmospheric chemistry

This research focuses on using laser-based techniques to detect and characterize stable and transient species important in atmospheric pollution, global warming, and/or combustion. The transient species are formed from the photolysis of various precursor molecules in a collision-free or collisional environment using a CO2 or an excimer laser, and detected by various laser-based techniques, such as laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and photoacoustic spectroscopy. We use these techniques to obtain fundamental spectroscopic constants, nascent state distributions, quantum yields, and limits of detection; and to determine the photodissociative pathways leading to their formation.

Proposals are welcome that center on the development and application of novel laser-based diagnostic techniques to monitor highly reactive species, which utilize nonlinear spectroscopies and/or new pumping schemes.

Available equipment includes an ultrahigh vacuum chamber coupled to a monochromator/PMT for LIF studies, an optogalvanic probe and a time-of-flight mass spectrometer for REMPI studies, and an excimer pumped dye laser with second harmonic generator to probe the transient species. Various data acquisition and analysis systems, fast digital oscilloscopes, and boxcar integrators are also available.

Advanced Optical Sensors

Advisor: RC Sausa

Key words: atmospheric trace constituents, laser remote sensing, mass spectra, multiphoton ionization, optical sensors, laser-induced fluorescence, molecular beams

This research opportunity centers on developing analytical sensors for the rapid detection and real-time monitoring of trace atmospheric pollutants or vapors of energetic materials. These sensors are important for potential applications in pollution prevention and compliance, antiterrorist aviation security, and demilitarization safety.

Laser radiation is employed to softly ionize the target molecules by a multiphoton process for mass spectrometric detection or photodissociation into characteristic fragments. A pulsed discharge source is also used to fragment the target molecules. Fragments typically include atoms and di- and tri-atoms, which generally have structured, readily identifiable transitions in the ultraviolet-visible spectral region. Thus, they may be detected by their prompt emission if electronically excited during the photolysis pulse; by amplified stimulated emission; by laser-induced fluorescence; or by resonance-enhanced multiphoton ionization. The last two techniques are more effective when coupled with pulsed molecular beam sampling and time-of-flight mass spectrometric detection.

Molecular beam sampling greatly improves the sensitivity and selectivity over ambient conditions as a result of supersonic expansion. This expansion produces the molecules in few distinct ro-vibrational states; as a result, their absorption spectrum is considerably less congested.

Available equipment includes various lasers (e.g., a pulsed tunable CO2, excimer, and nano- and picosecond lasers), pulsed discharge sources for microplasma generation, a TOF spectrometer equipped with molecular beam sampling, a differentially pumped vacuum chamber, a computer acquisition and analysis system, and assorted electro-optics.

Reference
Cabalo J, Sausa RC: Applied Optics 57(9): 1196, 2003

Experimental and Theoretical Studies of van der Waals Complexes

Advisor:  RC Sausa

Key words: mass spectra, multiphoton ionization, gas phase and gases, van der Waals forces, molecular collisions

Many radical species are important intermediates in flame and combustion processes, and play prominent roles in interstellar and atmospheric chemistry. The modeling of these environments depends on the measurement and/or theoretical calculation of numerous bimolecular rates involving many different collisional partners.

A prerequisite for this calculation is the determination of the intermolecular potential surface on which the reaction rate takes place. However, the potential function needed to describe the intermolecular surface is difficult to generate, particularly for reactive collisions. We can construct such a surface by studying the collisions between a radical and nonreactive species. The characterization of these surfaces has become feasible by the spectroscopic determination of the short range intermolecular potentials of van der Waals complexes.

This research centers on experimental and theoretical studies of gas phase, van der Waals complexes formed by an open-shell species and a stable gas. The complexes are detected and characterized in free jet expansions by laser-induced fluorescence (LIF) and/or resonance-multiphoton ionization (REMPI) with time-of-flight (TOF) mass spectrometric analysis. The experimental results yield spectroscopic constants, binding energy, and geometry of the ground and excited states.

Proposals are also welcome that center on perturbation theory for experimental rotational spectral simulations and ab initio calculations for determining electronic potential energy surfaces with ro-vibrational eigenvalues and functions.

Equipment available for this research includes a differentially pumped molecular beam apparatus equipped for LIF and REMPI-TOF studies, an excimer and CO2 laser for photolysis, an excimer/pumped dye laser with second harmonic generator for complex excitation, various PC-AT computers with data acquisition and analysis software, fast oscilloscopes, boxcars, monochromator/PMT, and assorted electro-optics. Main frame and supercomputer capabilities are available for theoretical studies.

Basic and Applied Studies in Atomic Collisions & Spectroscopy

Advisor:  GM Thomson

Key words: atomic collisions, wweapon signatures, ion beams and bombardment, X rays,

Research is centered on experimental studies of fundamental atomic interactions and/or their application to problems of military interest. Processes related to high current switching devices, x-ray production, or optical/IR signature generation in military scenarios are of special concern.

For collision studies the laboratory can furnish ions in beams ranging in energy from suprathermal to a few MeV. Equipment for detailed charged-particle analysis and for photon spectroscopy spanning the range from the infrared to gamma rays is also available. Associates will have ready access to an extensive array of dedicated and major shared data-processing facilities.

Currently the Laboratory pursues efforts in the development of devices to collect and unravel battlefield signatures for weapon location/characterization; in the creation of pulsed power devices for military applications and in the basic physics of the ionization, excitation, and charge-transfer processes that are behind them. Other ongoing collisions-related work delves into problems in radiography, materials characterization, and trace-element analysis.

Proposals to expand these projects in new directions or to start whole new atomic physics-based efforts are both welcome.

Theoretical Chemistry

Advisor: SW Bunte and BM Rice

Key words: surface chemistry, surface energy, energetic materials

We use theoretical chemistry to study a wide range of chemical problems. Current areas of application include the calculation of potential energy surfaces of a wide variety of chemical reactions, including degradation pathways of chemical warfare agents and the combustion of energetic materials. In addition, we use theoretical methods to characterize the properties of new high-energy compounds, to investigate intermolecular interactions with applications to condensed phase chemistry and the chemistry occurring on metallic surfaces, to study interfacial phenomena, and to predict the interaction of radiation with matter. Other interest areas include the modeling of polymeric systems and the development of new theoretical chemistry methods.

Methods currently available range from ab initio and semi-empirical quantum chemistry (e.g., GAUSSIAN 98, ACES II, GAMESS, SAPT, AMSOL, CADPAC, CRYSTAL95, WIEN97, and DOD PLANEWAVE) to classical molecular modeling techniques (MSI suite of polymer modeling software; we are members of the MSI Polymer Consortium). Our laboratory is well equipped with computational resources through our close relationship with the ARL Major Shared Resource Center.

Computational Chemistry: Molecular Dynamics and Monte Carlo Simulations and Molecular Packing

Advisor:  BM Rice

Key words: molecular modeling, Monte Carlo method, condensed phase and condensation, explosives and explosions, computational chemistry, ab initio crystal prediction, molecular packing, quantitative structure activity/property relationships (QSAR/QSPR)

Opportunities exist for method and model development, and their application in simulations of systems of chemical interest. Classical simulations by molecular dynamics, Monte Carlo, or molecular packing methods are invaluable in providing detailed molecular level information of properties and processes that might be difficult to detect experimentally. We are interested in determining rates and mechanisms of complex chemical reactions occurring in the conversion of energetic materials to products, which can be accomplished through molecular dynamics and variational transition state theory.

We also utilize molecular simulation methods to calculate equilibrium properties of condensed phase materials at different temperatures and pressures, and apply molecular dynamics methods to model shock propagation in non-energetic and energetic materials. We are also interested in modeling detonation in energetic solids.  Finally, we are interested in developing computational tools that are based on quantum mechanical calculations to predict properties associated with performance or sensitivity of energetic materials.

Our main research area involves energetic materials. Work in this area focuses on modeling chemical and physical processes that occur in the gas, liquid, and solid phases.  We encourage the development and utilization of scientific visualization tools to analyze these results and the development of highly scalable molecular simulation codes to be used on DOD parallel computational platforms. Available resources include visualization facilities, several workstations, and the wide variety of supercomputers located at the DOD Major Shared Resource Centers. We strongly recommend publication in the open literature for research in both the environmental and energetic materials areas.