Edward H. Piepmeier

Theory of laser saturated atomic resonance fluorescence

Abstract Rate equations are developed and used to calculate the monochromatic laser power densities (~100 kw cm 2 ) that are required to effectively saturate the excited atom population in typical hot gases (

Atmospheric Influences on Q-Switched Laser Sampling and Resulting Plumes:

When a 10–100-mJ single-spike Q-switched Nd laser pulse is focused on a copper sample, the presence of an atmosphere affects the spectra, the crater size, and the amount of sample vaporized. At 760 Torr the crater diameter (90 µ) and amount of sample vaporized (35 ng) remain relatively constant while at 1 Torr they both increase with increasing laser energy. Spatial changes in the spectra occur with changes in ambient pressure. The continuum intensity of the limited region just above the sample surface appears to be a better measure of the energy reaching the sample than does the energy of the laser beam. The experimental results appear to be caused by absorption of a large fraction of the laser energy in an atmospheric plasma. A radiation-supported shock-wave model is evaluated in detail and compared briefly with similar models as possible mechanisms for production of the atmospheric plasma. The analytical chemical implications of the experimental and theoretical results are discussed.

Fundamental studies of electrolyte-as-cathode glow discharge-atomic emission spectrometry for the determination of trace metals in flowing water

The fundamental characteristics and analytical performance were investigated of a new glow discharge emission source (GDES) for the determination of trace metals in flowing water. The application of an atmospheric glow discharge in argon gas between an electrolyte solution cathode and a platinum rod anode led to the development of a stable discharge. The intensity of the lines was found to depend strongly on the acidity of the water, the current and the discharge gap. The spectrum emitted from the tap water contained the basic atomic lines of the dissolved metals and OH band peaks, but no emission lines of argon from the discharge gas. The strong emission lines for an element and no emission lines from the discharge gas in the electrolyte-as-cathode glow discharge (ELCAD) were different from those in a solid-as-cathode discharge, suggesting a different excitation mechanism. Sub-parts-per-million detection limits of Al, Cd, Cr, Cu, Mn, Pb and Hg were obtained. The continuous-flow sample system demonstrates the possibility to develop a device for the continuous analysis of water and waste water solutions.

Line profiles emitted by Cu and Ca hollow cathode lamps pulsed to one ampere

Abstract Interferometrically measured emission resonance line profiles are presented for commercial copper and calcium hollow cathode lamps, pulsed at frequencies from 10 to 300 Hz with pulses having a constant current-duration product. Pulses ranged from 7.8 mA at 1280 μs to 1000 mA at 10 μs. Time integrated profiles show that self-reversal increases with repetition frequency and that the lines broaden with increased current. Time resolved profiles show that self reversal developes during a pulse. A simple transport model is proposed to account for the observed blue shifts in the emission profile and in the self-reversal dip of the copper resonance line. The analytical significance of the results is briefly discussed.

Influence of non-quenching collisions upon saturated resonance fluorescence

Abstract When an analyte atomic vapour is irradiated by a monochromatic laser beam with sufficient energy density, the excited atom population (and hence fluorescence intensity) may approach saturation. Non-quenching collisions that change the Doppler shift of an excited atom may decrease by one or two orders of magnitude the amount of energy density necessary to achieve a given degree of saturation. A steady state theory is developed that allows predictions to be made from collisional rate constants.

Atomic Absorption Measurements in a Q-Switched Laser Plume Using Pulsed Hollow Cathode Lamps:

Quantitative atomic absorption measurements on transient plasmas induced by a Q-switched laser beam were made using pulsed hollow cathode lamps. Calibration curves were obtained for 0.002 to 0.15% Cu in an aluminum alloy and 0.013 to 0.124% Mn in graphite pellets. The dependence of the atomic absorption characteristics upon changes in laser energy and upon atmospheric gas and pressure is discussed.

Time- and wavelength-resolved emission line profiles for pulsed Cu and Ag hollow cathode lamps

Abstract Time-resolved, line emission wavelength profiles were obtained for a copper and a silver pulsed hollow cathode lamp with the use of a piezoelectrically driven interferometer. A computer was used to control the timing of the sweep of the interferometer, the pulsing of the source, and the acquisition, averaging, sorting and display of data. Minor manipulation of the data stored in core facilitated the presentation of intensity vs wavelength emission profiles in a time-resolved form. Ten sequential line profiles spaced at 21 μsec time intervals were obtained. The copper hollow cathode lamp was driven at 100 Hz with pulse currents of up to 400 mA. Pulses lasted for up to 300 μsec. Line profiles changed dramatically during a pulse and showed extreme self-reversal of the resonance emission lines after 100 μsec. Variations of line profile with dc background current level and spatial position within the discharge were also investigated. Uncorrected line widths of each of the doublets of the Cu(I) 324.7 nm resonance line measured during the first 21 μsec of the discharge, where they were narrowest, ranged from 0.0012 to 0.0022 nm, including an instrumental broadening contribution estimated to be less than 0.0004 nm. The silver lamp showed extreme self-reversal even during the first 21 μsec.

Chemical and Physical Influences of the Atmosphere upon the Spatial and Temporal Characteristics of Atomic Fluorescence in a Laser Microprobe Plume

Atomic fluorescence observations of Li and Cu in a laser microprobe plume were made to study the chemical and physical influences of a reactive atmosphere of oxygen relative to an atmosphere of argon. Time and spatially resolved fluorescence observations show a region of relatively low concentration of Li in the center of the plume from a film emulsion sample, which is not present for Cu from an aluminum alloy sample. For both samples, the plume appears to rise as a ball above the sample, rather than fan out continuously throughout the atmosphere.

A Laser Microprobe System for Controlled Atmosphere Time and Spatially Resolved Fluorescence Studies of Analytical Laser Plumes

A laser microprobe system has been developed that provides for time and spatially resolved studies of the interaction between the material ejected by a laser from a sample surface and the gaseous species in a controlled-atmosphere chamber. A microsecond flashlamp pumped dye laser atomizes the sample, and a narrow band, accurately tunable flashlamp pumped dye laser is the light source for absorption and fluorescence measurements of atomic and molecular species. A sample wheel rotates and translates to provide a new surface for hundreds of shots before resurfacing is required. Stepper motors allow the laser plume to be positioned in three dimensions by manual or computer control. Examples are given of the types of measurements that are possible under computer control. In an observation region comprising 0.05% of the plume, 109 atoms of Li are detectable.

Chemical dynamics of a laser microprobe vapor plume in a controlled atmosphere

The vapor plume produced by the action of a focused laser beam on a solid sample of an aluminum alloy or zirconium was studied in a controlled atmosphere. The formation of binary compounds by chemical reaction between the vapor plume and the atmosphere was followed with the use of emission, absorption, and fluorescence detection methods. Most experiments were performed at 150 Torr in 100% O2, 50% O2/50% Ar, and 100% Ar. Both spatially and temporally resolved profiles of the metal monoxide formed in the plume were recorded. The experimental data are compared to kinetic calculations for the formation of AlO and AlO2. The initial concentration of the atmospheric gas in the region of the plume was found to be less than the average bulk concentration in the sample chamber. A “piston” mechanism of plume expansion is proposed to account for this. Complete mixing of the plume with the atmosphere took at least 100 µs. A significant quantity of metal monoxide was formed in an atmosphere containing only trace amounts of oxygen. The sample is believed to be the source of oxygen.