This overview addresses three general areas: spectroscopy; molecular interactions and reaction dynamics; interrelation of chemistry and laser development. The various topics and specific chemical examples contained in the overview are by no means exhaustive, but are chosen simply to give the reader a flavor of the role of lasers in chemistry.
Following a brief summary of the present theoretical picture for infrared multiple photon absorption, a number of recent examples of infrared laser-induced unimolecular reactions are discussed. The results for the laser-induced reactions of allene, ethyl vinyl ether and cyclopropane appear to indicate that the vibrational energy deposited by the laser is distributed non-statistically in these molecules.
The laser induced decomposition of HC Cℓ F2 was studied under collisional conditions using time resolved probing of the dominant reaction products. The reaction was shown to be non-thermal, and a lower bound of 43% of the absorbed laser energy is channeled into decomposition.
Several examples are presented of the laser photolysis/infrared fluorescence technique for measuring atom-molecule reaction rates. Atom production is achieved by multiphoton dissociation using a TEA CO2 laser or by direct dissociation using a dye laser, its harmonic, or a uv excimer laser. Reactions of ground state H and F atoms as well as of excited 1(52P-1/2) atoms are illustrated. The information provided by these studies is useful for the prediction and modeling of chemical lasers.
A promising new technique for studying vibrational and low-lying electronic states of radicals has been developed. Infrared emission from vibrationally excited radicals is observed following ultraviolet photolysis of several small polyatomics with a rare gas-halide excimer laser. Strong emission from the C-H stretches of the CH2I radical is directly detected following photolysis of CH2I2 at 248 nm. Photolysis of CH2I2 at 308 nm and CH3I at 248 nm does not produce excitation of the C-H stretches in the radicals generated. In all three cases, strong emission from excited I*(2P1/2) atoms is observed. With the techniques described, infrared fluorescence is monitored as a function of both wavelength and time, allowing information to be obtained about the initial vibrational state distribution of radicals following photolysis. Subsequent vibrational deactivation, energy transfer, and reaction kinetics may also be studied. By using low resolution, continuously variable interference filters, measurements of the gas phase vibrational frequencies of the radical can be obtained, since the temporal resolution allows radical emission to be readily distinguished from that of the parent molecules.
The effect of CO2 laser radiation on the Pt-catalyzed decomposition of HCOOH has been investigated in a static reactor and in a low-pressure matrix isolation system. HCOOH is known to decompose on Pt by two reaction paths, one leading to CO2 + H2, and the other to CO + H2O. It was found in this study that the laser caused either a decrease in the rate of formation of both CO and CO2, or selectively decreased the yield of CO, depending upon the condition of the Pt surface. In the case of a clean Pt surface, a laser line that is strongly absorbed by HCOOH caused a decrease in the rate of formation of both CO and CO2. A nonabsorbed line had no effect. In the case of a Pt surface that had been partially poisoned by reaction products, the strongly absorbed line caused a selective decrease in CO formation, thus enhancing the CO2/CO product ratio by as much as 50%. The activation energy measured for HCOOH decomposition in the matrix isolation experiments is 3.5 ± 0.2 kcal/mole using a clean Pt surface under the low pressure conditions. Additionally, the 0=C-0-H radical has been identified by the use of deuterium substitution. The present results have demonstrated the possibility of combining the unique properties of both catalysts and lasers to drive chemical reactions in selected synthetic routes and also the utility of the matrix isolation technique for heterogeneous catalytic studies.
The design of future combustion systems will require a thorough understanding of the combustion processes. This understanding can only be obtained through the characterization of experimental combustion media. Although there are various types of diagnostic techniques available for this characterization, optical techniques are more attractive because they often permit in situ measurement--with spatial and temporal resolution--of such parameters as gas temperature, species concentration, and flow velocity, without introducing any disturbance to the process being analyzed. Such optical techniques include laser Raman scattering, laser-induced fluorescence, coherent anti-Stokes Raman scattering, laser Doppler velocimetry, and absorption and emission spectroscopy. Of these, the first three techniques--laser Raman scattering, laser-induced fluorescence, and coherent anti-Stokes Raman scattering--will be examined in depth at this seminar. These three techniques, while somewhat different in principle, can all be used for the measurement of temperature and species concentration. Included in the discussion will be an overall tutorial review of the theoretical background, various experimental methods, and the capabilities and limitations of these techniques. Also discussed will be the relative merits of these techniques as they relate to combustion diagnostics. In addition, the requirements for the instrumentation and laser sources which are needed to fully utilize these methods will be addressed.
Coherent anti-Stokes Raman spectroscopy (CARS) is a promising diagnostic technique for the probing of flames and practical combustion environments. In this paper, the results of CARS investigations at UTRC in a variety of flames will be described. CARS thermometry has received the major emphasis in this work, but some species sensitivity studies will be reported as well. The CARS spectra are produced by mixing a 10 pps, frequency-doubled neodymium laser with a spectrally broadband, laser-pumped dye laser. In this approach, which avoids the need to frequency scan the dye laser, the entire CARS spectrum is generated in a single pulse, permitting "instantaneous" measurements of medium properties. A new crossed-beam phase-matching technique, termed BOXCARS, has been demonstrated for the first time and is described. This approach, which will be extremely useful in studies of stratified flames, leads to greatly enhanced and unambiguous spatial resolution, in contrast to the conventional collinear phase-matching approaches. Utilizing this technique, CARS temperature measurements are reported in a variety of structured flames including sooting, hydrocarbon-fueled diffusion flames. Moderate resolution CARS spectra from hot N2 obtained by scanning the spectrum in pre-mixed laminar flames show excellent agreement with computer generated spectra. The spectra display a number of very interesting features, such as the appearance of individual Q-branches and hot band structure arising from the overlap of the v = 0 →4 and 1 → 2 bands. Low resolution CARS spectra have been obtained in a single pulse using an optical multichannel analyzer which also display good agreement with predicted spectra and which demon-strate the feasibility of single pulse thermometry. CARS species sensitivity limitations have been explored in an investigation of CO detectability levels in flames.
The main reason for doing spectroscopy in a sooty diffusion flame, rather than in a well characterized and controlled environment, is that such a flame is thought to be a good model for the early stages of many jet combustors where particle concentrations, temperature, and possibly turbulence are likely to be similar. However, there are important differences. For example, the total pressure in a practical combustor is several atmospheres and it is pressure rather than an excess of fuel which causes soot formation in that case.
At Sandia Laboratories a number of non-linear optical spectroscopies are being applied to combustion-related problems. A principal long term goal is to obtain the temperature and concentration of major species in highly luminous, particulate-laden combustion environments. With the use of pulsed lasers and crossed beam geometries, these techniques can provide excellent temporal, spatial, and species resolution. In addition, these methods can generate relatively strong signals with high rejection of background radiation. Thus, non-linear spectroscopy promises to provide valuable information in hostile environments where spontaneous Raman scattering is of limited utility or not feasible. Recently the detection of stimulated Raman gain/loss signals produced by a single (10 ns) laser pulse in gaseous methane and nitrogen has been demonstrated. Implications of these preliminary results on combustion research are discussed.
Rayleigh scattering from a hydrogen-air laminar jet diffusion flame in combination with a numerical model of the flame has been used to determine temperature profiles. The model predictions of species concentration are used to calculate a mean Rayleigh cross-section which is used to relate the Rayleigh scattered intensity to temperature. Using an argon ion laser producing 7.5 watts at 488 nm and an optical multichannel analyzer (OMA), the scattered light was imaged into a spectrometer. The OMA was rotated 90 degrees to its normal orientation, allowing scans to be taken along the spectrometer exit slit. This resulted in a spatially resolved Rayleigh signal along the laser beam through the entire flame. Spatial resolution of 0.18 mm on each of the 500 detector elements with good signal-to-noise ratios was achieved even with integration times of only 0.03 second. Since the entire profile is made simultaneously, particulate perturbed profiles are easily recognized and discarded. Transverse profiles are presented to show flame structure. Axial profiles are compared to radiation corrected thermocouple measurements.
Pulsed Pb-salt tunable diode lasers operating at 77°K in the 10 μm spectral region have been used to characterize the response speed of wide bandwidth (3 GHz) photovoltaic HgCdTe infrared detectors. Detector output risetimes and falltimes of the order of 100 picoseconds have been observed. Detector bandwidth vs detector bias has been inferred from the falltime of the detector output at the termination of a short (25 nsec) optical pulse generated by the tunable diode laser.
Tunable diode lasers have significant and potentially widespread applications to analytical measurement and control in a variety of scientific and industrial areas. This paper describes a number of recent new developments and approaches in this emerging technology. Specific new measurement systems described include a defect sensor for multilayer film webs, a nondestructive quartz halogen lamp analyzer, an instrument for analyzing nearly opaque liquids and an improved longpath air pollution monitor. The use of digital data processing techniques to obtain improved performance in tunable diode laser instruments will be described.
A review of the photochemical methods that are applicable to purification of the rare earths in the liquid state is presented. Of the types of processes possible in solution, photoredox has been applied to the isolation of both europium (with separation factors exceeding 1000) and cerium from mixtures of lanthanides. Photosubstitution is the method of choice for those lanthanides displaying only one stable oxidation state in solution and has been demonstrated for the first time using chelates of europium.
Ultraviolet and infrared spectra are presented for CO2, CH2O, CD2O, CC12F2, C2H3C1, CH3OH, and 0s04 dissolved in liquified Xe, Kr, Ar, or air, and SeF6 dissolved in solid Xe. The infrared absorption features in the liquid solutions are narrower than the corresponding gas phase vibrational bands but broader than rotational lines. Absorption bands in the solids are narrower than in the liquids. Absorption bands in SeF6 exhibit the Se-isotope splitting. The frequencies of the absorption features are observed to shift with solvent, temperature, and isotopic composition. High solute densities (1017 to 1019 molecules/cm3) are obtainable. Some unstable compounds, such as CH2O monomer at 1018 molecules/cm3, form stable cryogenic solutions. Cryogenic solutions are shown to be useful for spectroscopic detection of parts per million to parts per billion impurities in gases and for photochemically induced isotope separation.
Recent measurements of the vibrational kinetics and spectroscopy of simple cryogenic liquids and of molecular dopants in these liquids are presented. Current and potential applications of these liquids as laser media, as hosts for laser photochemistry and as infrared nonlinear optical media are discussed.
During the past several years laser techniques for spectroscopic detection of trace atmospheric species have been proposed, developed, and implemented in field-demonstration programs. The spectral region utilized has ranged from the near ultraviolet to the mid-infrared. Measurements have been performed at ground level and above to the upper atmosphere. The operating principles have included direct absorption, resonance scattering, resonance fluorescence, and Raman scattering. This paper concentrates on the application of absorption spectroscopy to the detection of molecular pollutants, stressing some of the important properties of longpath and laser-radar systems.
The design and performance of laser and flash lamp fluorescence NO2 monitors are compared with respect to sensitivity, size, specificity, reliability and economy. A detectability of ppb for a one minute integration time in ambient air was achieved for both units. The sensitivity of the laser prototype can be increased to achieve 0.1 ppb detectability. A variety of tests, including use of smog chambers, has shown that our NO2 fluorescence monitors are free of interferences; namely, they are specific for NO2. Results obtained in monitoring ambient NO2 levels at our location (El Segundo, California) with the prototype instruments are presented and discussed.
A provision in the Clean Air Act of 1970 requires the States to implement plans for maintaining the air quality on a regional basis and for enforcement of National Ambient Air Quality Standards (NAAQS). Airports must be included in the emission inventory, pollution levels must be monitored, and their impact on air quality assessed. Also, the location of new airports or alteration of existing ones requires the preparation of environmental impact statements. Up to the present time this information was based on mathematical pollution dispersion models and point values obtained from conventional point sampling devices, both of which have inherent difficulties in terms of data acquisition and subsequent interpretation. Remote laser systems offer the potential of overcoming previous difficulties. A comprehensive study was undertaken to determine which laser systems could be most useful in the airport environment. The performance of the different methods was investigated, as they relate to airport monitoring, to determine their capability to measure air pollutants within the requirements of the NAAQS. It was found that of the different laser methods under development, for measuring gaseous pollutants, only the differential absorption by scattering (DAS) and the long-path transmission (LPT) methods appear to be useful, while the Raman scattering and fluorescence methods do not have sufficient sensitivity and, hence, are range limited. The advantages and disadvantages of the DAS and LPT are discussed in detail. A critical performance analysis shows that the DAS method has just enough sensitivity under the constraints of the HEW eye safety regulations to measure the primary pollutants within the levels of the NAAQS up to distances of several hundred meters. In contrast, the LPT method has a greatly improved sensitivity above DAS, but is limited in its three-dimensional application and lacks the ability of range resolution. It was further found that for measuring particles, lidar systems are applicable, provided that a satisfactory relationship between optical parameters and mass density can be established.
A technique is proposed here for the on-board measurement of an aircraft's speed and altitude by measuring the doppler shift and linewidth of laser radiation which is resonantly scattered from carbon dioxide molecules in the air. The system employs a small, low power, tunable laser diode which gives a small, lightweight system with low power consumption. The technique promises to work very well at high altitudes (100,000 ft) and speeds (Mach 10).
Experiments in 1976 and 1978 indicated that laser induced fluorescence of plants offers a means to determine plant maturity and stress, as well as identifying varietal differences within a crop. A laser fluorosensor lasing at 410 nm in the 1976 and 1978 experiments and a 337.1 nm laser in the 1976 experiment were assembled in a mobile spectrolaboratory at the Research Branch, Agriculture Canada and attached to a greenhouse where 4 different varieties of lettuce were grown under a controlled environment. The lettuce plants were excited with a 410 nm 18 mW HeCd or with a 20 mW 337.1 nm Nitrogen laser, and their fluorescence quantum yield as a function of wavelength measured and compared. The experiment demonstrated the usefulness of laser fluorometry, but not conclusively.