Lawrence G. Piper

Energy transfer studies on N2(X 1Σ+g,v) and N2(B 3Πg)

We have developed a novel discharge–flow technique for studying the quenching of N2(Xu20091Σ+g,v‘≥5) and N2(Bu20093Πg) by a variety of molecules. The technique involves adding small number densities of N2(Au20093Σ+u) to a flow of N2(X,v) producing, thereby, N2(B). By comparing N2(B) fluorescence intensities generated when a quencher is added to the N2(X,v) flow 2–3 ms before N2(A) addition with intensities observed after the N2(X,v) and quencher have been mixed for times of 10–30 ms, we can separate the effects of N2(B) fluorescence quenching from effects of N2(X,v) quenching. Our results indicate that CH4, CO2, CO, O2, and N2O quench N2(B) at rates approaching gas kinetic while H2, N2, SF6, and CF4 are about ten times slower. Rate coefficients for N2(X,v‘≥5) quenching by H2 and N2 are on the order of 10−15 cm3u2009molecule−1u2009s−1, those for CO2, CH4, and CF4 roughly an order of magnitude faster, and CO and N2O yet another order of magnitude faster.

Reevaluation of the transition‐moment function and Einstein coefficients for the N2(A 3Σ+u–X 1Σ+g) transition

We have measured the relative intensities of the nitrogen Vegard–Kaplan bands N2(Au20093Σu+–Xu20091Σg+) for transitions covering a range in r centroids between 1.22 and 1.48 A. With this data we constructed a relative electronic transition moment function that diverges significantly from previously reported functions. We place our data on an absolute basis by normalizing our relative function by the experimentally determined Einstein coefficient for the v’=0 to v‘=6 transition. Combining our normalized data from 1.22 to 1.48 A with absolute transition moment data measured by Shemansky between 1.08 and 1.14 A results in a function covering the range between 1.08 and 1.48 A. The radiative lifetimes calculated from this function are longer than those currently accepted by amounts varying between 25% for v’=0%–50% for v’=4–6.

Electronic transition moment variation and Einstein coefficients for the NO(B 2Π–X 2Π) system

This paper details an investigation of the variation in the electronic transition moment with internuclear separation for the NO(B2Π–X2Π) transition. Measurements of the relative intensities of a number of NO B–X vibronic transitions having a common upper level were used to construct a relative transition‐moment function between 1.27 and 1.60 A. After normalizing this relative function by experimentally determined radiative lifetimes, the transition‐moment function was extended down to 1.23 A by incorporating data from oscillator strength measurements. In contrast to empirical transition‐moment functions that have been proposed previously, the function in this paper decreases with increasing internuclear separation. Unlike these other functions, however, this one is consistent with theoretical predictions, with most available oscillator strength data, and with the observed trend in B‐state radiative lifetimes as a function of vibrational level.

Rapid field screening of soils for heavy metals with spark-induced breakdown spectroscopy

Spark-induced breakdown spectroscopy (SIBS) is a recently developed atomic-fluorescence-based analytical technique that is analogous to laser-induced breakdown spectroscopy. SIBS, however, uses an electrical plasma generation method on nonconductive samples instead of a focused laser beam. Here we describe the basic characteristics of SIBS and its application to the field-screening analysis of soil, using a standard addition analytical approach. Detection limits of ∼25 mg/kg have been seen for lead, chromium, barium, mercury, and cadmium. A variety of soils have been tested, some cocontaminated with organic material and uranium (238U).

Further observations on the nitrogen orange afterglow

We have extended earlier observations on the nitrogen orange afterglow that results from the excitation of N2(Bu20093Πg,v’=1–12) in the energy transfer reaction between N2(Au20093Σ+u) and N2(X,v≥4). Spectral observations out to 1550 nm show that N2(B,v’=0) accounts for about 38% of the total N2(B) excitation. This makes the rate coefficient for N2(B) excitation in the energy‐transfer reaction between N2(A) and N2(X,v≥4) equal to (4±2)×10−11 cm3u2009molecule−1u2009s−1. Experiments involving 14N2(A) and isotopically labeled 15N2(X,v) show 15N2(B) is the principal product. This demonstrates that the mechanism involves electronic energy transfer from the N2(A) to the N2(X,v). The vibrational distributions of N2(B,v≥4) are qualitatively similar whether 15N2(v) or 14N2(v) is excited although the magnitude of 15N2(B,v≥4) excitation is about 20% larger. These distributions can be characterized roughly as a 5200 K Boltzmann distribution. In contrast, the vibronic levels of 14N2(B,v=0–2) are substantially more excited than are tho...

Advanced Diagnostics and Kinetics of Oxygen-Iodine Laser Systems

*† ‡ § ** This paper describes a comprehensive, multispecies diagnostic suite for the characterization of chemical and electrical oxygen-iodine laser kinetics. Oxygen-iodine lasers involve reactions and energy transfer among several key species, including electronically excited O2(a) and O2(b), the reagent I2, ground-state I, electronically excited I * , and, in the case of electric-discharge driven systems, atomic oxygen. We have implemented highly sensitive and accurate methods for the measurement of all of these species in a chemically reacting flow system. We have used quantitative near-infrared emission spectroscopy to detect O2(a), O2(b), and I * , ultra-high precision absorption spectroscopy to detect I2, ultra-high precision tunable diode laser absorption spectroscopy to detect I:I * small-signal gain, and a chemiluminescent titration method to detect O. All of these methods provide well-resolved species concentrations, and the spectral emission and laser absorption measurements also provide spectroscopic determinations of gas temperature. Multispecies measurements of this type constrain reacting flow models so that gaps in our understanding of these systems can be identified and resolved. Through accurately calibrated spectral emission measurements, we have shown that high yields of O2(a) (>20%) can be attained via microwave discharge excitation of flowing dilute mixtures of O2 and Ar. For these conditions, we have observed positive small-signal gain on the I * - I transition, however the multispecies concentration data clearly show the existence of previously unknown kinetics limitations related to the presence of O. We discuss the calibration and accuracy of the diagnostic methods, and the implications of the results for the kinetics of discharge-driven oxygen-iodine laser systems.

The reactions of N(2P) with O2 and O

We have studied the kinetics of metastable N(2P) with atomic and molecular oxygen. The measurements were made in a discharge flow apparatus in which N(2P) was generated from the energy transfer reaction between N2(A) and ground state N(4S) and was monitored either by vacuum ultraviolet resonance fluorescence at 174 nm or else by observing the forbidden N(2P–4S) emission at 347 nm. The rate coefficient for N(2P) quenching by O2 is (2.2±0.4)×10−12 cm3u2009molecule−1u2009s−1 and that for quenching by O is (1.7±0.4)×10−11 cm3u2009molecule−1u2009s−1. One channel of the reaction between N(2P) and atomic oxygen appears to produce NO+ via a chemi‐ionization mechanism.

Spectroscopic studies of a prototype electrically pumped COIL system

This paper discusses methods, using non-intrusive diagnostic techniques, to characterize the detailed dynamics of I* gain and O2(a1Δ) yield on a laboratory microwave-discharge flow reactor, for conditions relevant to the electrically driven COIL concept. The key diagnostics include tunable diode laser absorption measurements of I* small-signal gain and temperature, high-precision absorption measurements of reactor I2 concentrations, absolute and relative spectral emission measurements of O2(a1Δ) and I* concentrations, and air-afterglgow determinations of O concentrations. We have characterized variations in O and O2(a) yields with discharge power and oxygen mole fraction. We observe O2(a) yields to increase dramatically with decreasing oxygen mole fraction. We have also demonstrated a spectral fitting analysis technique capable of quantifying the presence of vibrationally excited O2(a,v). This combined suite of diagnostics offers a comprehensive approach to performance characterization for electrically driven COIL concepts.

The Electric Oxygen-Iodine Laser: Chemical Kinetics of O2(a1 delta) Production and I(2P1/2) Excitation in Microwave Discharge Systems

Generation of singlet oxygen metastables, O2(a1Δ), in an electric discharge plasma offers the potential for development of compact electric oxygen-iodine laser (EOIL) systems using a recyclable, all-gas-phase medium. The primary technical challenge for this concept is to develop a high-power, scalable electric discharge configuration that can produce high yields and flow rates of O2(a) to support I(2P1/2->2P3/2) lasing at high output power. This paper discusses the chemical kinetics of the generation of O2(a) and the excitation of I(2P1/2) in discharge-flow reactors using microwave discharges at low power, 40-120 W, and moderate power, 1-2 kW. The relatively high E/N of the microwave discharge, coupled with the dilution of O2 with Ar and/or He, leads to increased O2(a) production rates, resulting in O2(a) yields in the range 20-40%. At elevated power, the optimum O2(a) yield occurs at higher total flow rates, resulting in O2(a) flow rates as large as 1 mmole/s (~100 W of O2(a) in the flow) for 1 kW discharge power. We perform the reacting flow measurements using a comprehensive suite of optical emission and absorption diagnostics to monitor the absolute concentrations of O2(a), O2(b), O(3P), I2, I(2P3/2), I(2P1/2), small-signal gain, and temperature. These measurements constrain the kinetics model of the system, and reveal the existence of new chemical loss mechanisms related to atomic oxygen. The results for O2(a) production at 1 kW have intriguing implications for the scaling of EOIL systems to high power.

Effects Of Excitation Mechanism On Linewidth Parameters Of Conventional Vacuum Ultraviolet (VUV) Discharge Line Sources

Conventional vacuum ultraviolet line sources often consist of electrodeless rf or microwave discharges of flowing or sealed inert gases at 1-10 torr. The inert buffer, typically helium or argon, is seeded with traces of parent species of the desired transitions (e.g., 02 or N2 for OI or NI transitions, respectively). The sensitivity of resonance sources as absorption or fluorescence diagnostics depends critically upon the effective line width of the source resonance radiation. This property is determined primarily by source self-absorption, and by Doppler broadening of the source radiation, which is itself a function of the translational-energy distribution of the radiating species. Self-absorption is easily minimized or characterized experimentally. However, Doppler broadening is a complex function of the lamp excitation processes and should be characterized for each type of resonance lamp. The major competing excitation mechanisms for transitions such as 01 (130 nm) or NI (120 nm, 149 nm, 175 nm) in such line sources are electron impact processes, where the excess kinetic energy of the collision is retained with the electrons, and energy transfer or dissociative excitation by rare gas metastables (Ar(3P2,0) at 11.5 eV or He(23,1S) at , 20 eV), where a significant fraction of the energy defect may appear as excess translational energy in the radiating species. The kinetics of these processes as they relate to various VUV atomic line sources are reviewed. In addition, preliminary experimental data from absorption measurements on atomic nitrogen metastables, N(2D) and N(2P), produced in a discharge-flow apparatus, are presented which show markedly different behavior between microwave-excited He/N2 and Ar/N2 lamps. The implications of these effects for design application of resonance absorption/ fluorescence diagnostics are illustrated.