J. William Rich

Vibrational energy transfer rates using a forced harmonic oscillator model

This paper addresses the analysis, validation, and application of analytic, nonperturbative, semiclassical vibration-translation (V-T) and vibration-vibration-translation (V-V-T) rate models for atom-diatom and diatom-diatom vibrational molecular energy transfer collisions. These forced harmonic oscillator (FHO) rate models are corrected and validated by comparison with recent experiments, and with three-dimensional semiclassical trajectory calculations for N 2 -N 2 , O 2 -O 2 , and N 2 -O 2 , which are considered to be the most reliable theoretical data available. A remarkably good overall agreement is shown for both the temperature and quantum number dependence of single-quantum and double-quantum V-V-T transitions in the temperature range 200 < T < 8000 K and for vibrational quantum numbers 0 < ν < 40. It is demonstrated that the multiquantum vibrational energy transfer processes occur via a sequential FHO mechanism, as a series of virtual single-quantum steps during one collision. An important exception, asymmetric multiquantum V-V exchange at low temperatures, that occurs via a direct first-order mechanism, is discussed. Analytic thermally averaged FHO V-T and V-V rates are suggested. The FHO model gives new insight into vibrational kinetics and may be easily incorporated into kinetic modeline calculations under conditions when first-order theories are not applicable.

Singlet oxygen generation in a high pressure non-self-sustained electric discharge

This paper presents results of singlet oxygen generation experiments in a high-pressure, non-self-sustained crossed discharge. The discharge consists of a high-voltage, short pulse duration, high repetition rate pulsed discharge, which produces ionization in the flow, and a low-voltage dc discharge which sustains current in a decaying plasma between the pulses. The sustainer voltage can be independently varied to maximize the energy input into electron impact excitation of singlet delta oxygen (SDO). The results demonstrate operation of a stable and diffuse crossed discharge in O2–He mixtures at static pressures of at least up to P0 = 380 Torr and sustainer discharge powers of at least up to 1200 W, achieved at P0 = 120 Torr. The reduced electric field in the positive column of the sustainer discharge varies from E/N = 0.3 × 10 −16 to 0.65 × 10 −16 Vc m 2 , which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O2(a 1 �) excitation. Measurements of visible emission spectra O2(b 1 � → X 3 �) in the discharge afterglow show the O2(b 1 �) concentration to increase with the sustainer discharge power and to decrease as the O2 fraction in the flow is increased. Rotational temperatures inferred from these spectra in 10% O2–90% He flows at P0 = 120 Torr and mass flow rates of ˙ m = 0.73–2.2 g s −1 are 365–465 K. SDO yield at these conditions, 1.7% to 4.4%, was inferred from the integrated intensity of the (0,0) band of the O2(a 1 � → X 3 �) infrared emission spectra calibrated using a blackbody source. The yield remains nearly constant in the discharge afterglow, up to at least 15 cm distance from the discharge. Kinetic modelling calculations using a quasi-one-dimensional nonequilibrium pulser–sustainer discharge model coupled with the Boltzmann equation for plasma electrons predict gas temperature rise in the discharge in satisfactory agreement with the experimental measurements. However, the model overpredicts the O2(a 1 �) yield by a factor of 2–2.5, which suggests that the model’s description of nonequilibrium O2–He plasma kinetics at high pressures is not quite adequate. (Some figures in this article are in colour only in the electronic version)

Low-temperature supersonic boundary layer control using repetitively pulsed magnetohydrodynamic forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short-pulse duration, high-voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization results show that as the Reynolds number increases, the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using laser differential interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0∼200–250Torr. A crossed discharge (pulser+dc sustainer) in M=3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0A in nitrogen and up to 0.8A in air. The electrical conductivity and the Hall parameter in these f...

Nonequilibrium dissociation rates behind strong shock waves : classical model

Abstract A model is suggested for the analytical calculation of dissociation rates behind shock waves where the vibrational temperature T v is less than the gas temperature T . The model is based on an analysis of the threshold translational energy for collision-induced dissociation as a function of initial vibrational and rotational energies. The threshold function method combined with a classical impulsive model for energy exchange yields explicit formulae for the rate coefficient k ( T v , T ) and the mean vibrational energy removed in dissociation. The mechanism of nonequilibrium dissociation is predicted to change during vibrational relaxation: dissociation from low vibrational levels dominates at low T v / T , while dissociation from all levels contributes almost equally as T v / T approaches unity. The formulae obtained exhibit an explicit dependence on the mass ratio of the dissociating molecule and its collision partner, the lighter mass of the partner making dissociation from high levels more favorable. Dissociation in a molecular gas at T > T v is demonstrated to occur predominantly via noncollinear collisions with simultaneous transfer of rotational and translational energy to the vibrational mode of the dissociating molecule.

Repetitively Pulsed Nonequilibrium Plasmas for Magnetohydrodynamic Flow Control and Plasma-Assisted Combustion

This paper demonstrates significant potential of the use of high-voltage, nanosecond pulse duration, high pulse repetition rate discharges for aerospace applications. The present results demonstrate key advantages of these discharges: 1) stability at high pressures, high flow Mach numbers, and high-energy loadings by the sustainer discharge, 2) high-energy fractions going to ionization and molecular dissociation, and 3) targeted energy addition capability provided by independent control of the reduced electric field of the direct current sustainer discharge. These unique capabilities make possible the generation of stable, volume-filling, low-temperature plasmas and their use for high-speed flow control, nonthermal flow ignition, and gasdynamic lasers. In particular, the crossed pulsersustainerdischargewasusedformagnetohydrodynamic flowcontrolincoldM � 3 flows,providing firstevidenceof cold supersonic flow deceleration by Lorentz force. The pulsed discharge (without sustainer) was used to produce plasma chemical fuel oxidation, ignition, and flameholding in premixed hydrocarbon–air flows, in a wide range of equivalence ratios and flow velocities and at low plasma temperatures, 150–300 � C. Finally, the pulser-sustainer discharge was used to generate singlet oxygen in an electric discharge excited oxygen–iodine laser. Laser gain and output power are measured in the M � 3 supersonic cavity.

Time-resolved Fourier transform infrared spectroscopy of optically pumped carbon monoxide

Abstract The paper discusses measurements of vibration-to-vibration (V–V) energy transfer rates for CO–CO using time-resolved step-scan Fourier transform infrared spectroscopy of optically pumped carbon monoxide. In the experiments, time evolution of all vibrational states of carbon monoxide excited by a CO laser and populated by V–V processes (up to v∼40) is monitored simultaneously. The V–V rates are inferred from these data using a kinetic model that incorporates spatial power distribution of the focused laser beam, transport processes, and multi-quantum V–V processes. Although the model predictions agree well with the time-dependent step-scan relaxation data, there is variance between the model predictions and the up-pumping data, however. Comparison of calculations using two different sets of V–V rates with experimental spectra showed that the use of the semi-empirical V–V rates of DeLeon and Rich provides better agreement with experiment. It is also shown that the multi-quantum V–V rates among high vibrational quantum numbers, calculated by Cacciatore and Billing, are substantially overpredicted. The results provide some new insight into nonequilibrium vibrational kinetics, and also demonstrate the capabilities of the step-scan Fourier transform spectroscopy for time-resolved studies of molecular energy transfer processes and validation of theoretical rate models.

Three-dimensional nonperturbative analytic model of vibrational energy transfer in atom-molecule collisions

A three-dimensional semiclassical analytic model of vibrational energy transfer in collisions between a rotating diatomic molecule and an atom has been developed. The model is based on analysis of classical trajectories of a free-rotating (FR) molecule acted upon by a superposition of repulsive exponential atom-to-atom potentials. The energy transfer probabilities have been evaluated using the nonperturbative forced harmonic oscillator (FHO) model. The model predicts the probabilities for vibrational energy transfer as functions of the total collision energy, orientation of a molecule during a collision, its rotational energy, and impact parameter. The model predictions have been compared with the results of three-dimensional close-coupled semiclassical trajectory calculations using the same potential-energy surface. The comparison demonstrates not only remarkably good agreement between the analytic and numerical probabilities across a wide range of collision energies, but also shows that the analytic FHO...

Phenomenological Analysis of Shock-Wave Propagation in Weakly Ionized Plasmas

Shock propagation into weakly ionized gases shows several features differing markedly from conventional, nonionized-gas shock structure. Phenomenological analysis of general macroscopic features of the previously observed plasma shock effects allows only two possible interpretations: existence of an energy (momentum) flux toward the wave precursor or volumetric energy release (exothermic phase transition) in the upstream portion of the wave (precursor) followed by reverse transition in the downstream portion of the wave. It is shown that known microscopic mechanisms are not capable of producing such a flux or energy release: Typical processes involving electrons, ions, and excited species do not couple strongly to neutral atoms and molecules, and there is not enough energy stored in these species because of the low ionization fraction. The theoretical basis for phase transitions in low-density, weakly ionized plasmas also is unknown. Analysis of the steady two-wave system created by either of the two effects raises a question as to whether the observed plasma shocks are stable objects. Another question is whether there exists phase transition within the plasma shock. It also remains unclear to what extent twodimensional thermal inhomogeneity effects contribute to the observed phenomena. Answering these fundamental questions requires additional experimental studies of the problem. p D d E E/N F h j k

Low-temperature M=3 flow deceleration by Lorentz force

This paper presents results of cold magnetohydrodynamic (MHD) flow deceleration experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows in the presence of transverse direct current electric field and transverse magnetic field. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17%–20%, while accelerating force of the same magnitude results in static pressure increase of up to 5%–7%. The measured static pressure changes are compared with modeling calculations using quasi-one-dimensional MHD flow equations. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. The effect is produced for two possible combinations of the magne...

Vibrational energy storage in high pressure mixtures of diatomic molecules

Abstract CO/N 2 , CO/Ar/O 2 , and CO/N 2 /O 2 gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T V =1900–2300 K for N 2 , T V =2600–3800 K for CO, and T V =2200–2800 K for O 2 . The translational–rotational temperature of the gases does not exceed T =700 K. Line-of-sight averaged CO vibrational level populations up to v =40 are inferred from infrared emission spectra. Vibrational level populations of CO ( v =0–8), N 2 ( v =0–4), and O 2 ( v =0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.