Absorption coefficient data are reported for acrolein, styrene, ethyl acrylate, trichloroethylene, vinyl bromide, and vinylidene chloride at up to seventy-two CO2 laser wavelengths each. These compounds are toxic industrial substances for which improved ambient air detection methods are desired. Absorption data for these compounds are needed to determine their detectabilities by CO2 laser-based photoacoustic, long-path absorption, and laser radar (lidar) techniques. The absorption data obtained for these compounds indicate that sub parts-per-billion (ppb) level, interference-free detection limits should be possible for these compounds by the CO2 laser photoacoustic technique. CO2 laser photoacoustic detectabilities of 40 ppb or less should be possible for these compounds in the presence of expected ambient air concentrations of water vapor and other anticipated interferences. In addition, absorption data on the first four compounds are needed to assess the capability of using CO2 laser spectroscopic techniques to detect low levels of the toxic hydrazine-based rocket fuels in air samples containing these compounds as interferences. The absorption data obtained for these four compounds indicate that the hydrazine-fuels should be detectable by the CO2 laser photoacoustic technique at concentrations below proposed workplace standards for hydrazines as low as 30 ppb in the presence of expected airborne concentrations of these compounds together with other expected interferences.

In conjunction with conventional techniques intracavity dye laser photoacoustic spectroscopy, an extremely sensitive technique for probing highly forbidden vibrational transitions in the visible spectrum (10,000-25,000 cm-1), has provided new insights into the nature and dynamics of the C-H stretch vibrational manifold of gaseous cyclopropane. Observations of strong coupling of the manifold to low frequency CH2 deformation modes and linebroadening of high energy vibrational states are discussed in terms of intramolecular vibrational energy relaxation processes and related to infrared multiphoton chemistry of cyclopropane.

The sensitivity and dynamic range of the photoacoustic effect enables an investigation of the gas phase kinetics and of the adsorption kinetics onto heterogeneous surfaces for the chemical system formed by anhydrous nitric acid vapor and ammonia. By studying the laser absorption spectroscopy of these gases and corresponding photoacoustic responses, the photoacoustic signals can be interpreted in terms of absolute concentrations. By illuminating the cell at different CO laser frequencies that are uniquely absorbed by one component and by slowing the reaction by using low concentrations of the reactants, a gas phase rate of 2X10-18cm3/sec is found. A total pressure of 50 Torr using nitrogen as a buffer gas allowed sufficient signal without excessive pressure broadening of the component spectral lines. Wall effects were included by studying the adsorption of ammonia and nitric acid onto cell surfaces. Present capabilities will allow measurement of bimolecular reaction rates approximately four orders of magnitude greater than the one already determined.

In situ aerosol absorption spectroscopy was performed using two novel photothermal detection schemes. The first, based on a photorefractive effect and coherent detection, called 'phase fluctuation optical hetero-dyne'(PFLOH) spectroscopy, could, depending on the geometry employed, yield particle specific or particle and gas absorption data. Single particles of graphite as small as 1 μm were detected in the particle specific mode. In another geometrical configuration, the total absorption (both gas and particle) of submicron sized aerosols of ammonium sulfate particles in equilibrium with gaseous ammonia and water vapor were mea-sured at varying CO2 laser frequencies. The specific absorption coefficient for the sulfate ion was measured to be 1 be 0.5 m2/g at 1087 cm -1. The absorption coefficient sensitivity of this scheme was less than or equal to 10 cm -1. The second scheme is a hybrid visible Mie scattering scheme incorporating photothermal modulation. Particle specific data on ammonium sulfate droplets were obtained. For chemically identical species, the relative absorption spectrum versus laser frequency can be obtained for polydisperse aerosol distributions directly from the data without the need for complex inverse scattering calculations.

Low concentration pollutants in the atmosphere can be detected by their infrared absorption spectrum. We use a diode laser spectrometer in a dual beam configuration for this purpose. The laser source is frequency modulated to provide the sensitivity enhancement associated with derivative spectroscopy. One of the laser beams is passed through a reference cell containing the gas to be detected in order to lock the laser frequency to the center of the absorption line. The other beam passes through a White cell with 64 in absorption path length. Sample air is sucked through this cell at a pressure of about 100 mbar. Although the pressure reduction reduces the density of absorbing molecules by a factor of ten, the increase in absorption cross section due to the norrowing linewidth nearly compensates this effect and drastically reduces interference from other gases. The absorption is observed as a modulation of the laser intensity at twice the modulation frequency. The intensity modulation is proportional to the second derivative of the absorption line. The spectrometer was used in a field experiment on board a research vessel in the North Sea for the measurement of HC] in the plume of incineration ships. An HC1 detection sensitivity of 100 ppb1/Hz was achieved.

A pulse amplified cw dye laser has been developed which produces a peak power of 50 to 100 kilowatts with a bandwidth of about 80 MHz. Frequency doubling this output produces power levels of up to 1 kilowatt, sufficient for observing two-photon processes in noble gases. Doppler-cancelled two-photon resonant, three-photon ionization spectra of several states of xenon and one state of krypton have been recorded which clearly show hyperfine splittings in odd isotopes. State assignments and fitted hyperfine constants have been determined where appropriate.

In this paper we describe molecular detection methods based on Multi-Photon Ionization (MPI) spectroscopy that provide high selectivity in addition to excellent detection limits. Prior work based on one-wavelength resonance enhanced 2-photon ionization spectroscopy is reviewed. Highlights include improvement of detection limits for aromatic hydrocarbons by a factor of one-thousand and adaptation of 2-photon ionization for spectrally selective detection in gas chromatography. Two alternative approaches for highly selective MPI detec-tion are discussed. Time-of-Flight (TOF) ion analysis in conjunction with MPI provides mass spectral signatures in addition to MPI spectra. Single ion sensitivity can be approached in this configuration. In a purely optical approach, selectivity can be improved by using two-color excitation. The new two-color method of Ion Dip Spectroscopy (IDS) can be used to label specific intermediate states involved in the MPI process and it is capable of generating Raman-type spectra as a supplement to the MPI spectra. The ultimate detection limit of the IDS method is expected to be about 104 molecules/ cm3, which is comparable to the detection limit that we have obtained for naphthalene by one-color MPI.

In laser induced inertial confinement fusion the understanding of the physical process taking place in the plasma atmosphere surrounding the target is extremely important because it is here where the energy is absorbed and transported to the core region. Optical probing of the target atmosphere can provide information about this important region by using laser diagnostics such as interferometry, Faraday rotation and Thomson scattering. Interferometry is used to determine electron density distributions in the plasma and has permitted the direct observation of effects due to ponderomotive forces in the plasma. Thomson scattering is another diagnostic which can help understand some of the interaction process taking place in plasma corona. This technique, apart from providing electron and ion temperature inside the plasma, can provide direct information about wave-wave decay process, ion turbulence and other collective effects. It has been successfully used to isolate some non-linear wave phenomenon in 10.6 4m laser produced plasmas.

In the drive toward achieving controlled thermonuclear fusion using magnetic confinement of high temperature plasmas, there is a critical need for detailed knowledge of essential plasma parameters such as electron density and temperature. Lasers have made significant contributions to the magnetic confinement fusion program by providing nonperturbing measurements of almost all desired parameters with high spatial and temporal resolution. A wide variety of lasers have been utilized with wavelengths covering the range from the ultra-violet (- 1200 A) to millimeter regions of the spectrum. To illustrate the impact that lasers have made in plasma diagnostics, this paper reviews the selected techniques of laser interferometry, Thomson scattering, and laser resonance fluorescence together with the relevant laser requirements. The current state-of-the-art is described and future needs are projected. The successes already achieved will insure an expanding role for lasers in the fusion energy program.

Current algorithms employed for the inversion of infrared laser heterodyne measurements of tenuous gases in the stratosphere by solar occultation are the onion peel technique and the spectral inversion technique. One of the basic differences between the two techniques is that the onion peel algorithm requires data obtained by an instrument pointed at a number of altitudes from the Earth's limb whereas the spectral algorithm can use data obtained by an instrument pointed at only one altitude above the Earth's limb. An experiment approach and inversion technique are discussed which optimize the retrieval of concentration profiles by incorporating the onion peel data collection scheme into the spectral inversion technique. A description of an infrared heterodyne spectrometer and the mode of observations for solar occultation measurement are given. The results of inversions of some synthetic Ck0 spectral lines corresponding to solar occultation limb-scans of the stratosphere are presented, indicating considerable improvement in the accuracy of the retrieved profiles. The effects of noise on the accuracy of retrievals are discussed for realistic situations.

This paper describes measurements of pollutants and water vapor in the troposphere using the DIfferential Absorption Lidar (DIAL) technique. Emphasis is placed on research that has been conducted at the NASA Langley Research Center (LaRC) with ground-based and airborne lidar systems. Measurements of sulfur dioxide in power plant plumes have been made with a mobile DIAL system operated near 300 nm. Comparisons of sulfur dioxide concentrations determined with the DIAL system and in-stack monitors were found to be in agreement to within 18 percent. Vertical water vapor profiles have been measured with a ground-based lidar system using the water vapor absorption line at 724.3 nm. Rawinsonde and DIAL water vapor profile data were shown to agree within 10 percent to an altitude of 2.5 km. A multipurpose airborne DIAL system was recently developed at LaRC to investigate a wide range of tropospheric gases. The first remote measurements of ozone profiles from an aircraft were made in May 1980. Subsequently, the airborne DIAL system participated in a major field study with the EPA to investigate elevated pollution episodes. Results from these experiments are presented in this paper. Simulations are also discussed for airborne DIAL measurements of sulfur dioxide, water vapor, and nitrogen dioxide. An extensive study of the scientific rationale and potential experiments with a Shuttle Lidar System has recently been completed by NASA. The application of a spaceborne lidar to uniquely measure tropospheric gases in the late 1980's is addressed in this paper.

Highly sensitive atomic detection can be achieved from the combination of CW laser excitation and saturated absorption. We have used CW saturated fluorescence methods for single-atom detection and for background rejection in flames. With the development of CW tunable UV dye lasers dye lasers the methods can be applied to the detection of most metallic elements. Because the photodetector can be physically isolated from the laser excited volume, these methods are applicable to elemental analysis with all types of atomizers, and for diagnosis of combustion products, flow processes, and plasmas.

Using a digital correlator, we record the burst of photons emitted by a single atom as it passes through a resonant laser beam. A high efficiency light collection system allows us to register many counts from each atom. The transit time of the atom across the laser beam is computed from the displayed autocorrelation function. By averaging over many atoms, we determine diffusion coefficients for ground state sodium in helium, neon and argon. We have also used this technique to measure rapid flow speeds of gases.

The laser-induced fluorescence technique has been employed to detect hydroxyl radicals (HO) present in the decomposition of water (H20) and the oxidation of hydrogen (H2) by several oxidants over polycrystalline platinum surfaces. The activation energy and the internal energy content of the HO formed in the desorption process: H* + O* -- HO* + * -- HO(g) + 2* have been quantitatively determined for the first time. The mechanism for the catalytic oxidation of H2 and the decomposition of water as well as the dynamics of HO radical production will be reviewed in detail.

In response to recent interests in laser applications to monitoring the role of radical species in combustion and atmospheric chemistry several new techniques have been developed. In this paper we discuss a laser-pump laser-probe technique utilized in our lab to obtain spectroscopic data for such in situ or long range studies and kinetic data on mass transport, vibrational and rotational relaxation, and chemical decay. The work utilizes a pulsed photolysis (excimer, YAG, CO2) laser to generate the radical in a well defined spatial region and a second probe (tunable dye) laser delayed in time. Applications of this technique to relaxation processes in CF2 and to new spectroscopic data on OCH3 will be discussed.

Fluorescence line narrowing spectroscopy is a low temperature solid state technique for eliminating site inhomogeneous broadening. Rotationally cooled fluorescence is a gas phase technique for eliminating thermal and rotational broadening. The principles of the two techniques and an evaluation of their sensitivity and selectivity in analytical applications is presented.

The sharpline absorption spectra (FWHM of 1 to 10 cm 1) exhibited by polynuclear aromatic hydrocarbons (PAHs) in frozen Shpol'skii hosts facilitates selective excitation of the luminescence spectra through tunable dye laser excitation. In this communication we present observations that document the general utility of laser excited Shpol'skii spectroscopy (LESS) for the direct qualitative and quantitative characterization of complex mixtures of PAHs, including multialkylated isomers.

An enzyme-linked sandwich immunoassay for insulin is described. Horseradish peroxidase is employed as an enzyme label for antibody, and enzyme activity is measured via the fluorogenic substrate, p-hydroxyphenylacetic acid. The product is detected by excitation of fluorescence with the 325 nm line of a cw helium-cadmium ion laser on-line with reverse phase high performance liquid chromatography. The method requires a total incubation time of 45 minutes, and the limit of insulin detection is 1.1 μU/mℓ (6.6 pM). This assay is applicable to the analysis of human serum samples.

A 100-mJ/pulse, 10-Hz KrF laser was used to reduce vibrational Raman detection limits to the low parts-per-million level for a number of molecules of interest in pollution monitoring, combustion diagnostics, chemical vapor deposition, and reactive plasma etching. No inter-ferences from stimulated Raman scattering or multiphoton photodissociation were observed for the molecules investigated. Limitations to the use of spontaneous Raman scattering, such as poor discrimination against fluorescence, are greatly minimized because of the combination of large signal levels and relatively narrow wavelength band containing the Raman-shifted lines. Even though present results are limited by the 1.2 nm bandwidth of the KrF laser, it was possible to detect the 2 to 4 parts-per-million CH4 present in room air with a signal-to-noise of ~20.

The advent of the laser has unlocked several new Raman processes which have provided new approaches to many important diagnostic problems. These applications include biochemistry, high resolution spectroscopy, combustion engineering, structural chemical analysis, chemical kinetics, surface chemistry, etc. Included in these techniques are: Coherent Anti-Stokes Raman Spectroscopy (CARS) , Raman Induced Kerr Effect (RIKES), and Stimulated Raman Gain. A brief review of these methods and their utility are described with the main emphasis on CARS. Variations of the techniques, e.g., BOXCARS, are also discussed and their use in making temperature and concentration measurements in hostile environments such as turbulent media are presented.

Sensitive and rapid measurements of the absorption and dispersion associated with narrow spectral features are accomplished using frequency modulation (FM) spectroscopy. The absorption or dispersion is measured by detecting the heterodyne beat signal which occurs when the FM spectrum is distorted by the spectral feature of interest. Experimental results are presented.