Douglas E. Hof

Development of a scanning, solar-blind, water Raman lidar

The need for an instrument capable of measuring water-vapor fluxes over mixed canopy and large areas has long been recognized. Such a device would greatly enhance the study of evapotranspiration processes and has great practical value for water management. To address this problem, a scanning water Raman lidar has been designed and constructed. Analytical methods have also been developed to take advantage of the type of information that this lidar can generate. The lidar is able to measure the absolute water content and calculate the evaporative flux quickly over relatively large areas. This capability provides new opportunities for the study of microscale atmospheric processes. The variogram data indicate that the spatial sampling size must be of the order of 10 m if fluxes and scalars are to be properly represented. Examples of data are presented.

Observation of line shifts and line profiles in an inductively coupled argon plasma

Abstract Shifts in spectral line positions of argon, iron, barium, calcium and strontium are addressed in this preliminary study of the effect of pressure, and electron density on wavenumber position in the 27.12 MHz inductively coupled plasma (ICP). We report line shifts for three sets of data in an argon ICP. The first set of data shows the effect of power (1.1, 1.5, 1.9 kW) on shifts of argon (21 lines) and iron (28 lines) lines in the ultraviolet and visible. The second set of data gives shifts of 25 Ar lines in the near infrared (i.r.) and far visible at 1.1 kW. The third set of data concerns prominent ion lines of calcium, barium and strontium. Commercial hollow cathode lamps (HCL) were used to obtain unshifted line positions. The Los Alamos Fourier Transform Spectrometer was used to obtain the data.

Observations of coherent structures from a scanning lidar over an irrigated orchard

Abstract The exchange of mass and energy is a turbulent process that often occurs in coherent periods of time and in discrete regions of space. Prior to the development of volume imaging lidars, the study of coherent structures in the atmosphere was limited, for the most part, to time-series analysis of point-instrument data. This paper describes the use of the Los Alamos National Laboratories scanning Raman lidar to observe both temporal and spatial coherent structures, such as plume and ramp-like features, that developed over a Green Ash orchard. Most of the ramp structures identified from lidar data were between 20 and 30 m in size and had transit lifetimes of between 20 and 30 s. The validity of these results was confirmed by comparison with previously collected point-instrument data. An analysis of the multi-dimensional lidar images was also able to relate discrete spatial features, such as plumes to ramp patterns, found at the base of plumes in both the temporal and spatial domains. A further finding supports the concept that ramp development is a function of shear-scale and roughness length. The lidar represents a new tool to gain a deeper understanding of the mechanisms underlying the turbulent exchange process.

Characterization of several ultraviolet–visible emission lines from a lead hollow-cathode lamp

The Fourier-transform spectra (0.015-cm−1 resolution) of the nonresonant 405.8-, 368.4-, and 364.0-nm lines and the resonant 283.3-nm line from a lead hollow-cathode lamp are reported. The splittings for the various isotopic and nuclear hyperfine transitions for all four lines are found to be consistent with previous measurements and assignments. At the recommended lamp operating current of 4 mA, all peak shapes within all four lines studied were satisfactorily fitted with Gaussian functions corresponding to the same Doppler temperature of 750 ± 30 K, indicating no significant self-absorption of the 283.3-nm resonant line. At a much-elevated lamp current the peaks for the nonresonant lines are increased in intensity and slightly broadened, whereas the peaks for the resonant line have approximately the same intensity as at 4 mA but are no longer Gaussian shaped.

A Nonflowing, Variable-Gas Inductively Coupled Plasma as a Light Source for High-Resolution Spectroscopy

Ten self-contained 27.12-MHz Inductively Coupled Plasmas (ICP) are generated in a static torch. The torch offers spectroscopists a closed-system, low-pressure (0.1-100 Torr), variable-power (currently up to 1 kW) plasma. Doppler temperatures (Na D) measured are approximately 5 times those of electrodeless discharge lamps (EDLs) or Hollow Cathode Lamps (HCLs) operating under normal conditions. This torch has improved noise considerations, compared with those for the flowing ICP traditionally used.

SUPPLEMENTAL DATA ON LINE SHIFTS AND LINE PROFILES STUDIES IN AN INDUCTIVELY COUPLED ARGON PLASMA

Abstract Line profiles of 10 Ar I lines emitted from an ICP ( T = 6800 K) are carefully evaluated with the Los Alamos Fourier Transform Spectrometer. In contrast to a previous paper, which reported measurements of 6p-4s, 5p-4s, and 4p-4s transitions (all 4s ground state), this Note deals with shifts for the transitions with 4p ground states. The shifts in line positions are on the order of 1 cm −1 . These shifts are accompanied by line profiles which have large contributions of pressure broadening. Previous work on a -parameters of elements in the analytical zone are on the order of 0.3–0.6 [1]. This note reports a-parameters as great as 39.03. Our HCL emission source could not be used as a standard because it lacked the excitation energy needed to adequately populate the higher energy levels. This results in a poor SNR for these lines. Argon I data reported by L i and H umphery [4] have been used for the unshifted values. It is the authors belief that with a large data set of experimentally determined line shifts from the ICP at a given set of operating conditions, a correlation between the magnitude of the line shift and the quantum numbers can be obtained for Ar.

Time-resolved fluorescence measurements of KrF emission produced by vacuum ultraviolet photolysis of KrF2 and Kr/F2 mixtures

Vacuum ultraviolet (VUV) light radiation was used to produce electronically excited KrF excimers (in D-, B- and C-states) by the photolysis of KrF2 and F2/Kr mixtures at various excitation wavelengths. The excited KrF photoproduct quantum yield was measured over the excitation wavelength range of 120 to 200 nm, and a quantum efficiency of 0.11 was estimated at the peak absorption wavelength of 159 nm for KrF2. The collision-free fluorescence lifetime of the B-X transition near 248 nm was determined to be 9.5 ± 0.6 ns when the KrF2 was excited with the 159 nm light. Near gas kinetic rate constants were measured for the quenching of KrF B-X emission by KrF2 and CO2. Using the threshold wavelength needed for the production of excited KrF photofragments, an upper bound for the bond dissociation energy of KrF2 was determined to be 1.03 ± 0.05 eV.

Photofragment Fluorescence As A Sensitive Probe For Gas-Phase Alkali Compounds And Their Photochemistry

Sensitive techniques are needed for the detection of highly corrosive gas-phase alkali compounds in coal gasifier gas turbine streams. We report on the use of photofragment fluorescence as a very sensitive, selective probe for alkali compounds. Photodissociation of a gas-phase alkali compound using a laser at suitably short ultraviolet (uv) wavelengths produces an electronically excited alkali atom. Detection of fluorescence from these excited atoms allows sensitive and quantitative density measurements of a compound while signal strength as a function of dissociation laser wavelength allows differentiation of compounds. We present here an evaluation of this approach based on the results of experiments to study the photodissociation of the alkali compound KC1. The KC1 vapor was contained in a heated quartz cell and irradiated at 193 nm with an ArF laser and at other Raman-generated wavelengths. Emission at 766 nm was observed from atomic potassium (42P°) produced in the photodissociation process. The spectral dependence for the production of excited potassium atoms is distinct enough that discrimination from other compounds, such as KOH, appears likely. The atomic emission intensity quantitatively tracks the KC1 density over at least 5 orders-of-magnitude. As little as 4 x 107 KC1 molecules/cc, or a 0.5 ppb KC1 concentration, can be measured on a single laser shot, making this a very sensitive diagnostic technique. Experiments on other alkali compounds are now in progress.

Optically induced lattice dynamics probed with ultrafast X-ray diffraction

We have studied the picosecond-dynamics of optically pumped hexagonal LuMnO/sub 3/ using ultrafast X-ray diffraction. Experimental data are compared with a simulation based on dynamical diffraction theory modified to account for the hexagonal structure of LuMnO/sub 3/.

An Ultrafast X‐Ray Diffraction Apparatus for the Study of Shock Waves

The use of X‐ray diffraction for the study of shock physics has been pursued for decades. Conceptually, changes in the diffraction line, including broadening and shifts, provide details about the nature of compression, plasticity, phase, and kinetics of the phase transition for the material being shock‐loaded. In practice, X‐ray source brightness, sample preparation, and turn‐around times have limited the applicability to a few crystalline systems. We report our development of an ultrafast X‐ray diffraction instrument suitable for studying rapid phase changes, including both solid‐solid and solid‐melt, in shock‐loaded materials. Due to the relatively small sample sizes needed and to the ability to conduct thousands of shock physics experiments with these small samples, we can build up the statistics required to study elastic‐plastic transitions, the kinetics of phase changes, as well as the mechanistic details of phase changes in nearly all materials, including high‐Z samples. An overview of the technique...