Daniel E. Murnick

Intracavity optogalvanic spectroscopy. An analytical technique for 14C analysis with subattomole sensitivity.

We show a new ultrasensitive laser-based analytical technique, intracavity optogalvanic spectroscopy, allowing extremely high sensitivity for detection of (14)C-labeled carbon dioxide. Capable of replacing large accelerator mass spectrometers, the technique quantifies attomoles of (14)C in submicrogram samples. Based on the specificity of narrow laser resonances coupled with the sensitivity provided by standing waves in an optical cavity and detection via impedance variations, limits of detection near 10(-15) (14)C/(12)C ratios are obtained. Using a 15-W (14)CO2 laser, a linear calibration with samples from 10(-15) to >1.5 x 10(-12) in (14)C/(12)C ratios, as determined by accelerator mass spectrometry, is demonstrated. Possible applications include microdosing studies in drug development, individualized subtherapeutic tests of drug metabolism, carbon dating and real time monitoring of atmospheric radiocarbon. The method can also be applied to detection of other trace entities.

Efficient, stable, corona discharge 172 nm xenon excimer light source

We demonstrate that corona discharges in xenon gas can be an efficient source of 172 nm Xe2* excimer vacuum ultraviolet (VUV) radiation. Conversion efficiencies of electrical power into VUV light greater than 50% have been observed. A model describing the light production mechanism in the discharge region including the influence of water vapor content in the 10 ppm region is presented. A prototype large area lamp consisting of 21 corona discharges operating in parallel has been built with 38 mW/cm2 VUV output power per area at the lamps surface. Based on the model and experimental results achieved, a continuous wave large area Xe2* 172 nm excimer light source with 130 mW/cm2 output and a possible wall plug efficiency close to 48% is proposed.

Lyman-alpha emission via resonant energy transfer

Very intense hydrogen Lyman- light emission is observed from neon gas near atmospheric pressure containing small admixtures (per mil) of hydrogen when this gas mixture is excited by ionizing particle beams. A DC beam of 15 keV electrons or a pulsed beam of 100 MeV ions were used in different experiments for excitation. A collisional energy transfer rate constant from neon to of has been measured using time-resolved optical spectroscopy on the Lyman- line. Conversion efficiencies of particle beam power into Lyman- light of the order of 10% have been observed. No other significant radiation was emitted in the entire VUV, UV and visible spectral region. In particular, no other hydrogen lines are observed under these conditions. The selective excitation of the H(2p) level is interpreted as arising from a resonant energy transfer between excimers and hydrogen molecules.

Influence of Water Vapor on Photochemical Ozone Generation with Efficient 172 nm Xenon Excimer Lamps

The influence of water vapor on photochemical ozone generation has been investigated. Tests of a coaxial ozone generator driven by an efficient, tubular, 172 nm xenon excimer lamp revealed that ozone saturation concentration strongly depends on moisture concentration in the process gas. In order to adequately model the data, catalytic ozone destruction by OH and HO2 radicals formed by reactions with trace amounts of water vapor in the process gas had to be included in the photochemical ozone production rate equation system. Based on the model, optimized ozone photoreactor designs for ambient air, dry air and dry oxygen are described.

Lasers in dense gases pumped by low-energy electron beams

The use of low-energy (≈15 keV) electron beams for pumping laser systems in dense gases with high specific power deposition is described. Thin (300 nm) SiNx ceramic foils used as entrance window in a transverse geometry for the electron beam allows pressure differentials of several atmospheres with low percentage energy loss in transmission. The 1.73 μm XeI (5d[3/2]1–6p[5/2]2) infrared laser was used for a first demonstration of this concept. The laser operated between 130 and 650 mbar. A threshold pumping power of 5.3 W and a maximum output power of 6 mW were observed. The system can be scaled to high pumping power (≈MW/cm3) and short wavelength.

Use of the optogalvanic effect (OGE) for isotope ratio spectrometry of 13CO2 and 14CO2

The use of isotopic carbon dioxide lasers for determination of carbon (and oxygen) isotope ratios was first demonstrated in 1994. Since then a commercial device called LARA™, has been manufactured and used for Helicobacter pylori breath tests using 13C-labelled urea. The major advantages of the optogalvanic effect compared with other infrared absorption isotope ratio measurement techniques are its lack of optical background and its high sensitivity resulting from a signal gain proportional to laser power. Continuous normalisation using two cells, a standard and sample, lead to high accuracy as well as precision. Recent advances in continuous flow measurement of 13C/12C ratios of CO2 in air and extensions of the technique to 14C, which can be analysed as a stable isotope, are described.

Energy flow and excimer yields in continuous wave rare gas–halogen systems

A stable, continuous wave (cw), electron beam at 14 keV has been used to study energy flow, reaction rates, and radiative decay in rare gas plus halogen systems at high pressure. Steady state solutions to rate equations were used to isolate parameters which affect both the transient and steady state yield of 193 nm radiation from ArF* and 157 nm radiation from F2*. The scaling of pumping power density to the inverse 4.25 power and cube of the pressure allowed a wide range of reaction times, from nanoseconds to seconds, to be considered. The spectra and yields as a function of partial pressures were used to obtain energy transfer efficiencies near 10% for 193 nm in a Ne:Ar:F2,1:0.008:0.0004 mixture and near 5% for 157 nm in a Ne:F2,1:0.002 mixture at 2–3 bar pressure. Scaling to high brightness lamps and near cw lasers is possible.A stable, continuous wave (cw), electron beam at 14 keV has been used to study energy flow, reaction rates, and radiative decay in rare gas plus halogen systems at high pressure. Steady state solutions to rate equations were used to isolate parameters which affect both the transient and steady state yield of 193 nm radiation from ArF* and 157 nm radiation from F2*. The scaling of pumping power density to the inverse 4.25 power and cube of the pressure allowed a wide range of reaction times, from nanoseconds to seconds, to be considered. The spectra and yields as a function of partial pressures were used to obtain energy transfer efficiencies near 10% for 193 nm in a Ne:Ar:F2,1:0.008:0.0004 mixture and near 5% for 157 nm in a Ne:F2,1:0.002 mixture at 2–3 bar pressure. Scaling to high brightness lamps and near cw lasers is possible.

Non-thermal Doppler-broadened Lyman-α line shape in resonant dissociation of H2

The detailed Lyman-α emission line shape after resonant dissociation and excitation of H2 via collisions with Ne excimer molecules has been determined (Ne*2 + H2 → 2Ne + H + H*(n = 2)). Pressure effects due to the high Ne pressure, required for efficient excimer formation, decrease the effective lifetime of the hydrogen excited state. The reduced effective lifetime significantly reduces the average number of elastic collisions experienced by the excited hydrogen atom before radiative decay. Excess energy imparted to the excited-state hydrogen atom by the Ne excimer, in the dissociative excitation collision, produces a non-thermal Doppler-broadened emission line shape. The full width at half maximum (FWHM) is 3.85 pm and the effective lifetime of the n = 2 state is estimated to be 14 ps at a Ne pressure of 1 atm. Results are obtained from pressure-dependent measurements of the Balmer-α absorption line shape using laser spectroscopy.

Excimer formation using low-energy electron-beam excitation

Excimer formation in dense gas targets is studied using a 10 to 20 keV electron beam for the excitation of rare gases. A SiNx ceramic foil, only 300 nm thick but strong and completely vacuum tight is used as an entrance window. The electrons loose only 4 to 6% of their energy in this foil. The low electron energy allows a table top setup without high voltage or x-ray radiation hazards. First applications of the system for excimer physics research and the development of novel vacuum ultraviolet light sources are described.

Light Sources using Energy Transfer from Excimer to Line Radiation

Efficient energy transfer between neon excimer molecules and hydrogen has been found. A small, high gap density light source has been developed, emitting entirely on the hydrogen 2p-1s transition at 121.567 nm (Lyman-(alpha) ). Light output densities of 10 W/cm2 are obtained. Electron beam energy conversion efficiencies of approximately equals 10% have been measured.