Seonkyung Lee

EOIL power scaling in a 1-5 kW supersonic discharge-flow reactor

Scaling of EOIL systems to higher powers requires extension of electric discharge powers into the kW range and beyond with high efficiency and singlet oxygen yield. We have previously demonstrated a high-power microwave discharge approach capable of generating singlet oxygen yields of ~25% at ~50 torr pressure and 1 kW power. This paper describes the implementation of this method in a supersonic flow reactor designed for systematic investigations of the scaling of gain and lasing with power and flow conditions. The 2450 MHz microwave discharge, 1 to 5 kW, is confined near the flow axis by a swirl flow. The discharge effluent, containing active species including O2(a1Δg, b1Σg+), O(3P), and O3, passes through a 2-D flow duct equipped with a supersonic nozzle and cavity. I2 is injected upstream of the supersonic nozzle. The apparatus is water-cooled, and is modular to permit a variety of inlet, nozzle, and optical configurations. A comprehensive suite of optical emission and absorption diagnostics is used to monitor the absolute concentrations of O2(a), O2(b), O(3P), O3, I2, I(2P3/2), I(2P1/2), small-signal gain, and temperature in both the subsonic and supersonic flow streams. We discuss initial measurements of singlet oxygen and I* excitation kinetics at 1 kW power.

Diode laser monitor for singlet molecular oxygen

Monitoring singlet molecular oxygen (1O2) produced by photodynamic therapy (PDT) can lead more precise and effective cancer treatment. Physical Sciences Inc. (PSI) has developed a singlet oxygen monitor based on a pulsed diode laser technology. In this paper, we present results of singlet oxygen detection in the solution phase and in a rat prostate cancer cell line, as well as PDT mechanism modeling. We describe an improved detection approach for singlet oxygen monitoring that employs a fiber-coupled optical set-up and fast data acquisition system.

Kinetics of oxygen discharges and I(2P1/2) excitation for EOIL

The electric oxygen-iodine laser (EOIL) concept uses an electric discharge plasma to generate an effluent flow containing singlet oxygen, O2(a1&Dgr;), and atomic oxygen, O, which react with I2 to excite the atomic iodine laser transition at 1.315 &mgr;m. This chemically rich system has unique characteristics, whose understanding requires systematic chemical kinetics investigation under carefully selected conditions to isolate the key reaction mechanisms. We describe a series of reacting flow measurements on the reactions of discharge-excited active-O2 with I2, using a comprehensive suite of optical emission and absorption diagnostics to monitor the absolute concentrations of O2(a1&Dgr;), O2(b1summation), O(3P), O3, I2, I(2P3/2), I(2P1/2), small-signal gain, and temperature. These multispecies measurements help to constrain the kinetics model of the system, and quantify the chemical loss mechanisms for I(2P1/2).

Rotational energy transfer in NO (A 2Σ+,v′=0) by N2 and O2 at room temperature

State-to-state rotational energy transfer (RET) rate coefficients for NO (A 2Σ+, v′=0, J=5.5, 11.5, 17.5) were measured for N2 and O2 at room temperature using a pump-probe method. The NO A 2Σ+ state is prepared by 226 nm light and the RET is monitored by fluorescence from the D 2Σ+ v′=0 state, following excitation by a time-delayed laser at ∼1.1 μm. Additionally, total collisional removal and final state distributions were measured exciting in the Q1+P21 band head, to simulate an NO laser-induced fluorescence atmospheric monitoring scheme. Time-resolved modeling is used to understand relaxation mechanisms and predict relaxation times in ambient air. H2O at atmospherically relevant concentrations does not affect the degree of RET in ambient air.

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.

Production of metastable singlet oxygen in the reaction of nitric oxide with active oxygen

Predictive modeling of the performance of EOIL laser systems must address the kinetics of the active oxygen flow, including the production of O3 from recombination of O and O2, and the effects of NO as an additive to remove O and promote O2(a) formation. This paper describes experimental measurements of the reaction kinetics for active-oxygen flows generated by microwave discharge of O2/He mixtures at 3 to 10 torr. The concentrations of O2(a1Δ), O, and O3 were directly measured as functions of reaction time in a discharge-flow reactor. Both the O removal rate and the O3 production rate were observed to be significantly lower than expected from the widely accepted three-body recombination mechanism for O3 production, indicating the existence of a previously unknown O3 dissociation reaction. Addition of NO to the flow well downstream of the discharge resulted in readily detectable production of O2(a) in addition to that generated by the discharge. The observed O2(a) production rates were remarkably insensitive to variations in total pressure, O2 concentration, and NO concentration over the ranges investigated. The mechanism for this O2(a) production remains to be identified, however it appears to involve a hitherto undetected, metastable, energetic species produced in the active-oxygen flow.

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.

Re-examination of the role of O2(b) in the I2 dissociation mechanism

Dissociation of I2 by O2(b) is a process that is potentially important in iodine laser systems that are driven by discharge and chemical singlet oxygen generators. Recent work on the quenching of singlet oxygen by I2 suggests that the accepted upper bound of <0.2 for the branching fraction for O2(b) + I2 → O2(X) + 2I may be too low. New measurements of the branching fraction have been carried out using transient diode laser absorption technique to monitor I atom formation. The results indicate that the branching fraction may be as high as 0.6.

Ultrasensitive diode-laser-based monitor for singlet oxygen

In this paper we present results from experiments to develop a real-time, optical monitor for singlet molecular oxygen produced during photodynamic therapy. Using a pulsed diode laser and a sensitive photomultiplier tube, we have obtained signals from singlet oxygen during and following pulsed laser excitation. Several photosensitizers were used, and we obtained strong signals even in the presence of protein laden environments. Values obtained for the lifetimes of the singlet oxygen state and the photosensitizer triplet state are compared to literature values.

Studies of an Advanced Iodine Laser Concept

We discuss experimental results from spectroscopic and kinetic investigations of the reaction sequence starting with NCI3 + H. Through a series of abstraction reactions, NCI (a1Δ) is produced. We have used sensitive optical emission diagnostics and have observed both [NCI(a1Δ)]and [NCI(b1Σ)] produced by this reaction. Upon addition of HI to the flow, the reaction of H + HI produced iodine atoms that were pumped to the excited I(2P1/2) state, and we observed strong emission from the I atom 2P 1/2 -> 2P3/2 transition at 1.315 μm. With a tunable diode laser we probed the I atom transition and observed significant transfer of population from ground state (2P3/2) to the excited state (2P1/2) and have observed optical transparency within the iodine atom energy level manifold.