Joseph J. Tiee

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.

Derivation of water vapor fluxes from Lidar measurements

Two techniques are described by which the flux of water vapor can be derived from concentration measurements made by a Raman-Lidar. Monin-Obukhov similarity theory and dissipation techniques are used as the basis for these methods. The resulting fluxes are compared to fluxes from standard point instruments. The techniques described are appropriate for measuring the flux of any scalar quantity using Lidar measurements in the inner region of the boundary layer.

Quantitative chemical identification of four gases in remote infrared (9–11 µm) differential absorption lidar experiments

A combined experimental and computational approach utilizing tunable CO(2) lasers and chemometric analysis was employed to detect chemicals and their concentrations in the field under controlled release conditions. We collected absorption spectra for four organic gases in the laboratory by lasing 40 lines of the laser in the 9.3-10.8-mum range. The ability to predict properly the chemicals and their respective concentrations depends on the nature of the target, the atmospheric conditions, and the round-trip distance. In 39 of the 45 field experiments, the identities of the released chemicals were identified correctly without predictions of false positives or false negatives.

Lifetime of the A 2Σ+, v’=0 level of HS measured using the Hanle effect

H2S was photodissociated using an ArF excimer laser at 193 nm to form HS photofragments. LIF spectra of HS in the region of 324 nm were obtained using a pressure tuned dye laser with improved resolution of the R1 and RQ21 branches in the A 2Σ+←X 2Π3/2 transition. The zero field linewidths and the Zeeman splitting of the RQ21 (1.5) line in a magnetic field were obtained. The latter was used to verify the g values expected for a Hund’s case (b) 2Σ+ upper state and Hund’s case (a) 2Π lower state. Depolarization of fluorescence in a magnetic field using the RQ21 (3.5) line with σ‐polarized LIF excitation was used to determine an estimated lifetime of 0.15–0.3 ns for the lowest vibrational level of the 2Σ+ state. A lower limit on the lifetime of 0.17 ns was determined from the measured adsorption linewidths. No rotational dependence on the linewidth was observed.

Quenching of C2H emission produced by vacuum ultraviolet photolysis of acetylene

Excited C2 H* is produced by vacuum ultraviolet photolysis of acetylene using a frequency tripled laser. Time‐dependent emission is measured from 400–940 nm. The use of a coherent photolysis source produces an excitation spectrum in which the rotational band contour is resolved. The absorption spectrum of C2 H2 taken in the same apparatus closely resembles the excitation spectrum indicating a homogeneous predissociation. Time‐dependent quenching of the C2 H* emission by Xe, Kr, Ar, He, N2, H2, D2, and C2 H2 is measured. The rapid quenching rates and lack of strong dependence on atomic weight suggest a spin‐allowed process is involved in this channel of C2 H2 photolysis. Quench rates are compared with several theoretical models.

Two‐photon pumped CO B‐A laser

An optically pumped CO laser that involves a two‐photon excitation of the B state is described. A photon conversion efficiency of 15% is determined for the 0‐1 and 0‐2 vibraonic transitions of the B‐A system. It appears that the major portion of the emitted light is the result of amplified spontaneous emission due to the high gain of the transition.

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.

Gas temperature and density of UF(6) determined by two-wavelength UV absorption.

The new technique of Wampler and Gentry for determining the temperature and density of a gas by measuring UV gas absorption at two wavelengths [F. B. Wampler and R. A. Gentry, J. Appl. Phys. 52, 1583 1981)] has been applied to gaseous UF(6). At 266 nm the absorption cross section sigma of UF(6) appears to be constant, sigma = (1.15 +/- 0.01) 10(-18) cm(2) from 0-100 degrees C. The absorption cross section at 245 nm over the same temperature range may be represented with the empirical polynomial sigma = [1.37 +/- 0.05 + (9.7 +/- 1.5) 10(-3). T - (4.2 +/- 1.1)10(-5). T(2)] 10(-18) cm(2), where T is in degrees Celsius. Differences in (dsigma/dT)lambda at 266 and 245 nm allow both UF(6) temperature and density to be determined by UV absorption measurements. The strengths and limitations of this technique are discussed.

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.

Atmospheric effects on CO2 differential absorption lidar sensitivity

The ambient atmosphere between the laser transmitter and the target can affect CO2 differential absorption lidar (DIAL) measurement sensitivity through a number of different processes. In this work, we will address two of the sources of atmospheric interference with CO2 DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. Measurements of atmospheric extinction under different atmospheric conditions are presented and compared to a standard atmospheric transmission model (FASCODE). We have also investigated the effects of atmospheric turbulence on system performance. Measurements of the effective beam size after propagation are compared to model predictions using simultaneous measurements of atmospheric turbulence as input to the model. These results are also discussed in the context of the overall effect of beam propagation through atmospheric turbulence on the sensitivity of DIAL measurements.