The advantages of the four-level laser are described, especially the maintenance of an exceptionally high population inversion because the ground final state is maintained at zero population. The intrinSiC intramoleCular excited-state proton-transfer fluorescence is described as conforming to the four-level laser requirement. Examples are presented which function efficiently, with amplified spontaneous emission gain factors in the range 12 —14 vs. normal laser dye gain factors in the range 7 —9. The specific paradox is described of cases having good proton-transfer fluorescence and no laser potentiaL The spectroscopic criteria necessary are indicated. The extension of the present studies of flavonol proton-transfer cases to one-ring and multi-ring aromatics promises to yield powerful CW lasers from the UV to the JR range.

Molecularions play an important and often dominant role In the gaseous media in which they occur, even when present In very small quantities. Ion densities are typically lower than densities of excited neutrals, often by orders of magnitude. Conventional laser spectroscopy must deal with the strong Interfering signal from excited neutrals. One widely used technique for detecting Ions Is velocIty-modulated spectroscopy. A gas-phase dIscharge is driven by an audlofrequency power supply, Impressing a modulation upon the ion velocity distributIon. A laser beam traverses the discharge and falls upon a detector, whose signal Is demodulated by a lock-In amplifier. The velocitymodulation technique enhances the Ion signal and suppresses the interfering signal. However, the technique is plagued by a coherent background signal, arising from electrical pick-up or fluorescence from the discharge. Variation in the background can obscure the desired signal. This paper describes an Improvement in the velocity-modulated technique that suppresses the background. A doublemodulation technique Is used, in which low-frequency amplitude modulation of the laser beam Is combined with conventional velocity-modulation spectroscopy. Two lock-in amplifiers are used In series. This suppresses the background by several orders of magnitude.

Degenerate four-wave mixing (D4WM) is presented as a simple nonlinear laser method with spectral resolution high enough to resolve isotope/hyperfine lines and detection sensitivity sufficient for sub-ppb or sub-attomole detection in various atomizers and detection cells. Two counterpropagating pump beams and a probe beam are mixed inside the nonlinear medium that consists of analyte atomic or molecular species, and the phase conjugate signal beam is generated as the wavefront-reversed replica of the probe beam. Optical alignment is relatively simple, and optical signal collection is highly efficient since the signal is a laser beam. Continuous sample introduction is possible using an air-acetylene flame as the atomizer. When a relatively low-pressure demountable discharge cell is used as the atomizer, Lorentzian (pressure) broadening is also minimized in addition to cancellation of Doppler broadening. Degenerate four-wave mixing is also effective in detecting trace amounts of analytes in the condensed phase at room temperature using continuously flowing capillary cells that are suitable for interfacing to liquid chromatography and capillary electrophoresis. In addition to pulsed lasers, such as excimer- and Nd:YAG-pumped dye lasers, one can also use relatively low-power continuous-wave lasers such as argon-ion lasers or single-frequency ring dye lasers.

We report the detection of short-lived species on electrode surfaces utilizing the enhancement of the Raman intensity afforded by proximity to silver electrodes. Using gated multiplexed detection we identify an photoproduct of flavinmononucleotide after excitation with 337nm radiation from a pulsed nitrogen laser. The product decays on the 700nsec time scale and is identified as the radical anion of the nucleotide.

The importance of techniques to sense and monitor the environment are becoming increasingly more important with the intensifying presence of groundwater and soil contaminations. Our research and development effort is aimed at producing a commercial, low cost, field portable instrument for the field screening/in situ monitoring of contamination from organic solvents based on the principle of combining spectroscopic, electrochemical, and fiber optic techniques. Some of the advantages of this technique for monitoring a contamination site are cost, small size of sampling probe, real-time analysis, the capability of sensing in adverse environments, and the ability of using a central detection facility. The technique has an advantage over current integrating fiber optic chemical sensing methods in that the sensing only takes place when the electrochemical device is turned on. This should enable long-term monitoring of a site to be accomplished with only one probe/instrument system.

Fluorescence quenching of benzopurpurin (BP) by HC1 in methanol is investigated. The observed extremely efficient quenching is attributed to the formation of a non-fluorescent exciplex between BP and HC1. Other pollutant gases, such as H2S and NH3, were found not to effect the fluorescence of BP. Possibilities of using this finding to develop a fibre optic HG sensor are discussed.

Raman spectroscopy produces highly characteristic responses from all molecular species. It uses visible or near-IR radiation and thus is well suited for use with relatively inexpensive low-loss fiber optics. Because it is a scattering measurement, it is readily applied to samples in a variety of forms, including aqueous solutions, solids and slurries, and to samples in hostile environments. Potential problems relating to sensitivity and reproducibility have been addressed by selection of appropriate instrumentation and data treatment strategies. This report describes considerations affecting the use of fiber optic components in Raman measurements for quantitative analysis. Both bidirectional single fiber probe systems and unidirectional multifiber probes have been considered. Some results for in situ monitoring are reported. Reactants and products have been monitored during the synthesis of a conducting polymer. Spectra have been measured and quantitation carried out for a mixture of azo dyes in a flowing stream. Measurements have also been made on mixtures of solids and on organic films on a metal surface. Problems relating to representative sampling of nonuniform materials are considered. A Hadamard transform approach to obtain energy throughput and spatial averaging advantages is described.

A long open path Fourier-transform infrared (FTIR) system operable at pathlengths up to 1 kilometer has been constructed for the simultaneous detection and measurement of trace pollutant gases in the Taiwan urban air. Good mid-IR spectra at 0.5 cm1 resolution covering most pollutant gases of greater than a few ppbv are recored hourly on full 24 hours bases. The spectral absorptions in the region due to HCOOH, C₂H₄, H₂CO and O₃ are investigated. A good correlation among HCOOH, C₂H₄. and 0₃ is found. On the basis of the kinetics rate information, most formic acid is not formed directly through the oxidation of C₂H₄. by O₃ in the environment of high humidify. This organic acid in the urban air is the dominant form.

The surface of aluminum alloy sheet often is chemically modified to promote wetting, corrosion resistance, and adhesion of polymeric coatings. A detailed description of the molecular composition of such pretreated surfaces usually is unknown, although this knowledge is vital in designing better surfaces. Both reflection-absorption infrared spectroscopy (RAIRS) and surface-enhanced Raman spectroscopy (SERS) have been employed to charscterize thin (few tens of angstroms) films on aluminum. The results from these studies on chromate-phosphate coatings (chromium phosphate conversion coatings) and phosphonic acid anodized layers on aluminum have provided a me detailed understanding of the structure of these engineered surfaces. The nature of interaction between the pretreatment species and the aluminum substrate has been determined, and a possible orientation of these species with respect to the substrate is suggested. The strengths and limitations of both techniques will be discussed within the confines of investigating rough aluminum surfaces.

A portable Raman instrument is presented which measures spectra simultaneously from 10 separate fiber-optic probes. The instrument is being developed for a number of applications including multipoint process monitoring and characterizing mixed waste tanks.

The principles of intracavity laser spectroscopy (US) together with the experimental conditions which permit the detection of absorption with extremely high sensitivity are briefly described. Special attention is given to the optical parameters which need to be controlled in the U.S laser and to the observational conditions under which quantitative measurements can be made. Applications of ILS to the in situ monitoring of chemical vapor deposition of thin film materials and the quantitative characterization of absorption lines (e.g., line strengths and collisional broadening coefficients) are described. Recent results on the design and performance of solid-state ILS lasers operating in the infrared also are presented.

Pulsed infrared laser excited photothermal spectroscopy is used to study the nonlinear infrared absorption character of ethylene. Ethylene exhibits apparent optical saturation when high irradiance CO₂ lasers are used for excitation. In these experiments a pulsed TEA-CO₂ laser tuned to the 1OP(14) line at 949.49 cm^-1 is used to excite the v7 transition of ethylene and photothermal lens spectrometry is used to monitor the resulting thermal perturbation. The data are obtained as the thermal lens signal versus excitation irradiance, linearized to account for nonlinearities in the apparatus response, and processed to yield 'apparent' absorption coefficient versus excitation irradiance. Apparent absorption coefficient data are then modeled based on the vibrational energy level structure and the interaction of the Gaussian excitation source with the nonlinear system. Assumptions are used to derive a reasonable vibrational state model based on probable excited states involved in the kinetic saturation. Saturation irradiance and related parameters for ethylene in Ar, He, and N2 buffer gases are given. The saturation irradiance results suggest a vibrational relaxation up to 4 orders of magnitude faster than shown in previous work. This high relaxation rate is probably due to a break down in the steady state aproximation used to solve the rate expressions.

Absorption and fluorescence spectroscopy has proven to be a valuable analytical tool for environmental and process monitoring. Several publications have addressed different spectroscopic applications related to process monitoring. Since most chemicals absorb in the UVIVis part of the spectrum, the majority of laser applications utilize this shorter wavelength region. Nevertheless, the utilization of the longer wavelength part of the electromagnetic spectrum may be advantageous due to its relatively low interference. The environmental and process monitoring applications of this spectral region may be especially advantageous if semiconductor lasers are utilized as light sources. Laser diodes have all the properties of other types of lasers with the added benefits of compactness, low power consumption, low cost and long lifetime. However, to utilize this spectral region for environmental or process monitoring applications, appropriate near-infrared (NIR) absorbing probe molecules need to be employed. These probes may be used to determine analytical properties important for environmental or process monitoring applications, e.g., pH, oxygen concentration, metal ion determinations, solvent hydrophobicity, just to mention a few. These NIR probes may be incorporated into polymers to form a stable probe arrangement for convenient monitoring using semiconductor lasers. The utility can be further enhanced using fiber optics. In this paper the use of MR absorption and fluorescence spectroscopy for monitoring applications will be demonstrated.

Several recent technological developments have greatly improved the sensitivity and simplicity of Raman spectroscopy, and have reduced interference from fluorescence. These developments include fiber optic sampling, diode laser excitation, holographic filters, compact spectrographs, and charged coupled device detectors. Spectrometers based on these developments are simple to use and can detect part-per-million levels of strong Raman scatterers in solution. Examples will be presented.

A novel method was developed for tomographic combustion diagnostics using tunable infrared laser absorption spectroscopy. In this method complete spectral profiles are reconstructed rather than at one or a few frequencies. When compared to existing techniques, the new method needs much less prior infonnation about the sample gas and the spectroscopic behavior of the candidate spectral lines. Acklitionally, the new method is much more immune to interference and random noise. The method is demonstrated by CO2 measurements in an axisymmelric flame.

The development of new infrared transmitting optical fibers with low optical losses, sufficient mechanical strength, and temperature range to meet the demanding conditions of many process environments and the availability of improved, ruggedized low-cost FTIR spectrometers have made in situ FflR measurements possible. This paper discusses the development of in situ fiber optic remote FT spectroscopy and its application to the characterization of polymer resin synthesia

GAMMA-METRICS and Oak Ridge National Laboratory (ORNL) have developed a prototype portable Toxic Chemical Analyzer (TCA) for environmental screening using surface enhanced aman scattering (SERS) technology. The focus is on detection of anthropogenic chemicals such as polycyclic aromatic compounds. In this instrument a laser illuminates a small sample of analyte on a substrate of silver coated particles. Light scattered from the illuminated spot is analyzed to determine the chemicals in the analyte. The development process involved miniaturizing the instrument from a large laboratory table-top device to a small portable package, then ruggedizing the components and the packaging to withstand field conditions. A reference design for a commercial instrument has been developed. The instrument employs internal direct optics or, optionally, an external, in-situ probe connected by fiber optics to internal components. A substrate dispenser is used to refresh the substrate in preparation for a new measurement. Measurement results to date show that high quality spectra can be obtained with the portable system. A prototype instrument is currently undergoing field trials. Data from these trials are used to refine a reference instrument design. Measurements with the prototype show that the design is capable of screening for a wide variety of environmental contaminants. Trends toward miniaturization of components and increased sensitivity of measurement procedures lead to growth and increased optimization of the TCA.

Laser-induced fluorescence with fiber optic light delivery and collection has been applied for remote analysis of aromatic hydrocarbons. We report laboratory and field data acquired with a transportable and fully wavelength tunable laser system.

A remote detection system for in-situ monitoring of the bioluminescence produced by a microbial "reporter" strain grown in rotary film bioreactors has been developed. Two systems were developed: one using a low-temperature photomultiplier (PMT), and one using an avalanche photodiode. The photodiode system was preferred for its sensitivity and portability. Testing was performed on a recombinant constitutive light producing strain. The results indicate that the optical monitor is sensitive enough for direct microbial detection for hazardous waste application.

Absorption of p-polarised light by the metal electrode of metal-semiconductor Schottky barrier photodetectors is considerably enhanced through the excitation of surface plasmon polaritons (SPPs) at the metal-air interface. Using a prism-air gap-sample arrangement SPPs have been excited at the Al-air interface of Al-GaAs diodes, giving rise to acorresponding enhancement in device photoresponse. By adjusting the air gap dimension the coupling efficiency between incoming light and SPPs may be varied and, for large air gaps (> 1 gm), it is shown that a number of reflectance dips occur (as a function of internal angle of incidence in the prism) due to excitation of leaky guided wave modes; there are accompanying peaks in the photosignal. The potential use of SPP enhanced Schottky diodes as sensitive chemical sensors is discussed in the context of results, both experimental and computer modelled, relating to surface contamination of silver films.

A new sampling technique is described, where volatile organic compounds are extracted from an aqueous phase into the gas phase using a spray process. The formation of droplets during this spray process enormously increases the total interfacial area between the liquid and the gaseous phase. Using a spray extractor, volatile organic compounds dissolved in water can be sampled to perform. gas chromatographic / mass spectrometric analysis.

We have developed a system for the in situ determination of petroleum hydrocarbons in soils. This system uses a pulsed N₂ laser coupled with a photodiode array detectorto make fluorescent measurements via optical fibers. The measurement is made through a sapphire window on a probe that is pushed into the ground with a truck-mounted cone penetrometer. This is the first reported direct optical detector for contaminants in soils. Remote in situ fluorometric measurements over long lengths of optical fibers give rise to several complications not encountered with conventional laboratory fluorescence measurements. The effects of these issues on the calibration and response of the optical detector are discussed. While we specifically discuss calibration of measurements of diesel fuel marine (DFM) by UV fluorescence, we believe that the calibration techniques and optical issues we are addressing will be germane to most if not all in situ optical measurements of contaminants in soils. In orderto improve the in situfluorescent quantitation of petroleum hydrocarbons, we have calibrated the fluorescent response of fuels as a function of soil type and conditions. The fluorescent response of DFM vanes by an order of magnitude or more as a function of soil type. Experiments to determine the causes of this variability have shown that the controlling variable is surface area of the substrate, although there are secondary effects as a function of grain size, mineralogy, and degree of soil aggregation. We have found that normalizing contaminant concentration to available soil surface area allows for a much more predictable response factor. It should be noted, however, that the variation in fluorescent response of OEM in three of four EPA soils tested is relatively small, with only one showing a large divergence. Soils with mixtures of grain types and sizes fall in a relatively tight response range, while soils such as very pure sands or clays diverge significantly. Preliminary studies on the effects of moisture content on the fluorescent response of OEM in soils suggestthat the addition of waterto the matrix begins to exclude DFM from the grain surfaces, forcing the fluorophore into the grain interstices, and greatly decreasing the difference in fluorescent response between soil types. The moisture effect is the smallest in mixtures of sand and clay. We are currently developing specific fluorescence calibration algorithms as a function of soil type, and correlating actual soil types to soil classifications derived from cone penetrometer strain gauge data.

New approaches have been developed for the detection and quantification of low levels of bioactive compounds that may be introduced into the environment. There is a challenge to scientists to develop analytical methods which are rapid, inexpensive, highly sensitive (e.g., ppb and ppt), have a capacity for high throughput, indicate very few false positives or negatives, and can be applied to any sample matrix relevant for the material to be detected. The basis of the system is the selective responses of sensitive biodetectors whose chemical and molecular interactions with a wide variety of toxic chemicals have been previously determined and reported. The test system assays for the differential growth rate of mutants and wild type Bacillus subtilis strains which will respond to toxic or nutrient substances according to the chemical species and bioavailability. The extent of bacterial growth is determined by the differential light scattering of a laser beam at multiple angles. The intensities at multiple angles and input to photodetection are integrated with a computerized system that collects and analyzes data. Preliminary fmdings indicate that bioactive compounds can be assayed in water, soil, or vegetation matrices; therefore, the laser-bacterial assay appears to be a rapid and inexpensive analytical tool for screening chemicals in a complex matrix.

We have developed a new technique for detecting the concentration of Cl in aqueous solution. The concentration of CF is determined by measuring the absorbance of C1₂ which is formed by ArF laser irradiation. The absorption maximum of C1₂ is located at 340 nm. A linear relationship between the absorbance and the concentration on a log-log scale was confirmed. The detection limit of 1x10^-5 M has been achieved. This method is applicable to real-time and in-line measurement, and to other halogen ions.

A brief review of the development of various optical chemical sensors which can be applied in environmental analysis is presented. Only those devices which make use of the immobilized reagent phase are discussed. Immunosensors and generic techniques, such as surface plasmon resonance, are not included. Current limitations of the technology and future trends are discussed. Activities at Cranfield on environmental optical diagnostics are presented.

The development of an information-theoretic image measure for sensor evaluation, under contract to the United States Air Force, is described. Although current approaches are based on human perception models, a need exists for evaluation of sensors for automatic target cuing/automatic target recognition (ATC/ATR) systems. Such an evaluation should be performed in terms of the probabilities of detection/identification and false alarms, independent of the idiosyncrasies of the specific ATC/ATR algorithms. Such an approach based on the information-theoretic content of images for the target versus background separability is being developed and applied to evaluating sensors using the tower test data collected at the Wright Laboratories.

Fluorescence quenching of 9-amino acridine hydrochloride (9-AAH) by ammonia was found to be extremely efficient. Quenching data were analyzed using the Stem-Volmer relation. The method offers a limit of detection of 0.1 ppm over a useful range of 0-10 ppm. Fluorescence of the indicator 9-AAH showed a little or no mtereference from acidic gases, such as SO₂, HC₁ and H₂S. The fmding is considered to be of potential utility for the development of a fibre optic ammonia sensor and other methods of its quantitative determination.

A number of fluorophores were investigated for their suitability to be employed as indicators in SO₂ measurement. Several indicators, including PAll's, 5(and 6) carboxy-4'-5'-dimethyl fluorescein, new fuschin, hydrazine hydrochloride and chioropyridine hydrochloride, showed decrease in the fluorescence intensity with increase in SO₂ concentration. Fluorescence quenching of benzopurpurin by 502 is found to be extremely efficient, with a little or no interference from the other toxic gases NH₃ and H₂S. These results are analyzed using Stem-Volmer relation. The fmding is considered to be of potential utility for the development of an improved fibre optic SO₂ sensor/probe.

Current technology for monitoring personal exposures to vapor-phase toxic materials is both instrument and labor intensive. It is desirable to devise a system for monitoring such exposures using simpler equipment coupled with more rapid quantitation. Immunochemical sensors provide an avenue for developing such vapor monitoring systems because of their high sensitivity and selectivity, and the ease with which they provide quantitative information on analyte concentrations. Immunochemical personal exposure monitors (PEMs) are currently being developed for assaying pentachiorophenol (PCP) sampled from ambient air. These devices will demonstrate the feasibility of direct, antibody based sampling of analytes from air with subsequent quantitation of the analyte by enzyme immunoassay.

A portable device is described, which continuously measures ammonia gas concentrations. The detection is based on reversible changes in the optical transmittance of a dye-film coated on a glass support. The design of the analyzer is presented and response characteristics are discussed.

The quenching of the fluorescence of thionine by hydrogen suiphide is reported. The extremely efficient quenching process is attributed to reversible reduction of the thionine dye both in the ground and the excited state leading to the formation of a non-fluorescent semi/leuko thionine. Other toxic gases, namely SO₂, HC1 and NH₃, were found to have no effect on thionine. Potential analytical applications of this discovery, including development of a fiberoptic H₂S sensor, are discussed.