James A. Dodd

Baseline subtraction using robust local regression estimation

Abstract A technique entitled robust baseline estimation is introduced, which uses techniques of robust local regression to estimate baselines in spectra that consist of sharp features superimposed upon a continuous, slowly varying baseline. The technique is applied to synthetic spectra, to evaluate its capabilities, and to laser-induced fluorescence spectra of OH (produced from the reaction of ozone with hydrogen atoms). The latter example is a particularly challenging case for baseline estimation because the experimental noise varies as a function of frequency.

Vibrational relaxation of NO(υ=1) by oxygen atoms

The rate constant kO(υ=1) for NO(υ=1) vibrational relaxation by O has been measured at room temperature using a laser photolysis-laser probe technique. Vibrationally excited NO and relaxer O atoms were formed using 355 nm laser photolysis of a dilute mixture of NO2 in argon bath gas. The time evolution of both the NO(υ=1) and the O atoms was monitored using laser-induced fluorescence (LIF). The required absolute O-atom densities were obtained through a comparison of O-atom LIF signals from the photolysis source and from a titrated cw microwave source. At early times the O atoms constitute the most important loss mechanism for the nascently produced NO(υ=1). Possible effects from NO(υ=1) vibrational ladder-climbing and from thermal expansion have been shown to be minimal. The rate constant kO(υ=1)=(2.4±0.5)×10−11 cm3 s−1 determined herein is a factor of 2 to 3 lower than the generally accepted value of kO(υ=1) used in thermospheric modeling. The present value for kO(υ=1) is the same, within the error bars,...

Identification of vapor-phase chemical warfare agent simulants and rocket fuels using laser-induced breakdown spectroscopy

Application of laser-induced breakdown spectroscopy (LIBS) to the identification of security threats is a growing area of research. This work presents LIBS spectra of vapor-phase chemical warfare agent simulants and typical rocket fuels. A large dataset of spectra was acquired using a variety of gas mixtures and background pressures and processed using partial least squares analysis. The five compounds studied were identified with a 99% success rate by the best method. The temporal behavior of the emission lines as a function of chamber pressure and gas mixture was also investigated, revealing some interesting trends that merit further study.

New insights for mesospheric OH: multi-quantum vibrational relaxation as a driver for non-local thermodynamic equilibrium

The question of whether mesospheric OH(υ) rotational population distributions are in equilibrium with the local kinetic temperature has been debated over several decades. Despite several indications for the existence of non-equilibrium effects, the general consensus has been that emissions originating from low rotational levels are thermalized. Sky spectra simultaneously observing several vibrational levels demonstrated reproducible trends in the extracted OH(υ) rotational temperatures as a function of vibrational excitation. Laboratory experiments provided information on rotational energy transfer and direct evidence for fast multi-quantum OH(high-υ) vibrational relaxation by O atoms. We examine the relationship of the new relaxation pathways with the behavior exhibited by OH(υ) rotational population distributions. Rapid OH(high-υ) + O multi-quantum vibrational relaxation connects high and low vibrational levels and enhances the hot tail of the OH(low-υ) rotational distributions. The effective rotational temperatures of mesospheric OH(υ) are found to deviate from local thermodynamic equilibrium for all observed vibrational levels.

Laboratory studies of CO2(ν2)-O vibrational energy transfer

For altitudes above about 80 km, oxygen molecules are increasingly dissociated by solar vacuum ultraviolet absorption, and O atoms, together with N2, become a principal constituent of the atmosphere. Through collisions with the ambient O atoms, the ground vibrational state of CO2 is efficiently excited to its lowest excited vibrational state, with one quantum of energy in the ν2 bending mode. In the near-space environment, a sizable fraction of this population relaxes via 15-μm spontaneous infrared emission, which effectively converts ambient kinetic energy into radiative energy that passes into space. This process is the principal upper atmospheric cooling mechanism in the 75-120 km altitude range. Despite the importance of this mechanism, current estimates of the CO2(ν2)-O vibrational relaxation rate constant kO(ν2) vary over a factor of six, with the laboratory measurements clustering in the 1-1.5 × 10-12 cm3s-1 range, and the aeronomical estimates in the 3-6 × 10-12 cm3s-1 range. We are currently pursuing vibrational relaxation measurements on the CO2(ν2)-O system in the laboratory, using the temperature jump method together with transient diode laser absorption spectroscopy detection of the CO2 vibrational level populations. We will present the current state of progress of the experimental effort, as well as possible future directions.

Hyperthermal atomic oxygen source for near-space simulation experiments

A hyperthermal atomic oxygen (AO) beam facility has been developed to investigate the collisions of high-velocity AO atoms with vapor-phase counterflow. Application of 4.5 kW, 2.4 GHz microwave power in the source chamber creates a continuous discharge in flowing O(2) gas. The O(2) feedstock is introduced into the source chamber in a vortex flow to constrain the plasma to the center region, with the chamber geometry promoting resonant excitation of the TM(011) mode to localize the energy deposition in the vicinity of the aluminum nitride (AlN) expansion nozzle. The approximately 3500 K environment serves to dissociate the O(2), resulting in an effluent consisting of 40% AO by number density. Downstream of the nozzle, a silicon carbide (SiC) skimmer selects the center portion of the discharge effluent, prior to the expansion reaching the first shock front and rethermalizing, creating a beam with a derived 2.5 km s(-1) velocity. Differential pumping of the skimmer chamber, an optional intermediate chamber and reaction chamber maintains a reaction chamber pressure in the mid-10(-6) to mid-10(-5) Torr range. The beam has been characterized with regard to total AO beam flux, O(2) dissociation fraction, and AO spatial profile using time-of-flight mass spectrometric and Kapton-H erosion measurements. A series of reactions AO+C(n)H(2n) (n=2-4) has been studied under single-collision conditions using mass spectrometric product detection, and at higher background pressure detecting dispersed IR emissions from primary and secondary products using a step-scan Michelson interferometer. In a more recent AO crossed-beam experiment, number densities and predicted IR emission intensities have been modeled using the direct simulation Monte Carlo technique. The results have been used to guide the experimental conditions. IR emission intensity predictions are compared to detected signal levels to estimate absolute reaction cross sections.

Room temperature measurements of CO 2 (v 2 )-O vibrational energy transfer

In the Earths upper atmosphere, collisions between ground state CO2 molecules and translationally excited O atoms effectively populate the bending (v2) vibrational modes of CO2. Subsequent relaxation of the v2 modes occurs through spontaneous or stimulated emission of 15-μm radiation. Much of this radiation escapes into space, thereby removing ambient kinetic energy from the atmosphere. This cooling mechanism is especially important at altitudes between 75 and 120 km where the O atom density is relatively high and the conditions are optically thin. We have performed laboratory measurements to better characterize the vibrational energy transfer efficiency for this system. Several improvements to the experiment have been made since our preliminary manuscript on this topic. The temperature-jump method is used to form vibrationally excited CO2, and transient diode laser absorption spectroscopy is used to monitor the vibrational level populations following collisions with atomic oxygen. Using this approach, the room-temperature vibrational relaxation rate coefficient, kO(v2), has been measured to be (2.0±0.3)x10-12 cm3s-1. This value is slightly higher than previous laboratory measurements, which have clustered in the (1-1.5)x10-12 cm3s-1 range, and on the low end of aeronomical estimates of (2-6)x10-12 cm3s-1.