Demonstration of a high-temperature fiber-optic gas sensor made with a sol-gel process to incorporate a fluorescent indicator.

To make a gas sensor suitable for use at high temperatures, we have used a sol-gel-processing technique to bond a copper-exchanged zeolite fluorescence indicator onto the end of an all-silica optical fiber. Experimental results from single-fiber prototype sensors show they can be used to measure either the oxygen concentration or the equivalence ratio for gas mixtures containing weak or strong reductants, respectively.

Fluorescence characteristics of Cu-ZSM-5 zeolites in reactive gas mixtures: mechanisms for a fiber-optic-based gas sensor.

In situ measurements of the fluorescence spectra, intensity, and response time were made at elevated temperatures (~500 degrees C) for samples of Cu-ZSM-5 exposed to dilute mixtures of O(2) in N(2) and to various O(2)-reductant combinations in N(2). The results of these experiments are interpreted with a physical model of the oxidation/reduction kinetics of the copper ions. The fluorescence signal depends strongly on the gas composition, providing a mechanism for a gas composition sensor. A prototype sensor configuration is described that uses a visible (488-nm) excitation source and a fiber-optic geometry to generate and detect the fluorescence. The results indicate that for a weakly reducing gas the fluorescence signal correlates well with the oxygen concentration, whereas for strongly reducing gases the signal correlates more closely with the reductant-to-oxidant ratio.

Evanescent-wave scattering by electrophoretic microparticles: a mechanism for optical switching.

The total internal reflection of light occurring at the interface between glass and a low-index liquid containing suspended microparticles can be electrically controlled. The particles are charged and the glass is coated with a thin, transparent conductor. When the conductor is biased to attract the particles, they scatter and absorb light from the evanescent optical field near the interface, thus reducing the reflectivity. When the conductor is biased to repel the particles, total internal reflection is achieved. Experimental results are given for the time, voltage, and angle-of-incidence dependence of the reflectivity at the interface between an In-Sn-oxide-coated glass surface and a suspension of 0.47-µm-diameter silica particles in acetonitrile. The switching is found to be fast (~ 100 ms) and reproducible. In certain conditions the on/off ratio for a single reflection can be as large as 2:1. A simple theoretical model is developed to interpret these experiments. The model gives a reasonable fit to the data and allows one to extract information such as the particle mobility and the particle density in the evanescent-wave region.

Loss mechanisms in optical light pipes.

A geometrical optics approach is used to develop a theoretical model for analyzing loss mechanisms in optical light pipes. Five mechanisms are identified: intrinsic absorption, bulk scattering, losses that are due to roughness at the core-cladding interface, losses that are due to large-scale defects at the core-cladding interface, and losses that are due to absorption in the cladding material; and the effects of each of these on light-pipe transmission are considered. An approximate model appropriate for slightly rough surfaces is used to estimate the loss that is due to interface roughness. Optical experiments on commercially available light pipes are done to quantify the various loss processes. These experiments indicate that the interface effects play an important role in limiting the transmission in high-quality light pipes. From the optical measurements a rms interface roughness height in the 30-70-A range is deduced, and these values are confirmed by direct surface profilometry with an atomic force microscope.

Application of high-performance thin-layer chromatography and automated multiple development for the identification and determination of pesticides in water.

The combination of high-performance thin-layer chromatography (HPTLC) with automated multiple development (AMD) allows full automation of the separation step. This provides both a separation efficiency that is considerably better than that in conventional TLC and reproducible gradient elution on the thin layer. Reliable identification of trace amounts of pesticides (e.g., 28 ppt of atrazine) in subsoil, surface and drinking water is clearly demonstrated by AMD-HPTLC using a polarity gradient based on dichloromethane. A second universal elution gradient with changed selectivity on which the substances have different relative migration distances, identification by the multi-wavelength response correlation and the option of recording UV spectra in situ constitute three independent methods for reliable and rapid verification. Using 100- instead of 200-microns HPTLC silica gel layers and reducing the running distance increments from 3 to 2 or 1 mm increase the sensitivity, the linearity and the speed of the method.

Identification of forbidden transitions in the nu(4) band of (14)NH(3) by intracavity laser-Stark spectroscopy.

Numerous perturbation-allowed transitions in the 6-microm nu(4) NH(3) band are assigned by laser-Stark spectroscopy. The transitions are forbidden in the sense that they follow the rules DeltaK = +/-2, a ? s, in contrast to the allowed transitions in nu(4), which follow the rules DeltaK = +/-1, a ? a, s ? s. These lines indicate the presence of a significant perturbation, which should be included in the analysis of nu(4) and which, through combination relations with allowed transitions, will provide a basis for precisely determining K-dependent energy intervals in NH(3).

Optical electric-field enhancement at a metal surface arising from surface-plasmon excitation.

Surface plasmons can be excited on a metal by an external plane-wave beam through the use of prism or grating couplers or by introducing roughness. This can enhance the electric field at the surface compared with that in the incident beam, and it has been suggested that this effect is an important aspect of surface-enhanced Raman scattering. A general upper limit for this field enhancement is derived on the basis of energy conservation. Numerical results are given for Ag, Au, and Cu. With a perfect coupler on a Ag surface, the maximum increase of the square of the electric field in the 2-3-eV range is congruent with300. On randomly roughened surfaces, the estimated enhancements are of the order of unity.

Sub-Doppler double-resonance spectroscopy of NH(3) using a tunable-diode laser and a CO laser.

Results are given for a Stark-tuned double-resonance experiment, using a CO-laser pump and a diode-laser probe. The CO laser, operating on the 13-12 P(15) line at 1775.2588 cm(-1), is locked by Lamb-dip stabilization to one of the Stark components of the a(R)R(9, 9) nu(4) line of NH(3). The diode laser probes the aQ(9, 9) nu(2) line at 921.255 cm(-1), revealing a complex spectrum of sub-Doppler features, the narrowest of which are 5.3 +/- 0.3 MHz wide. As well as the resonances associated with population depletion of the common lower levels, we see line-narrowing effects that are due to two-quantum Raman-type processes and collision-induced resonances arising from state-changing collisions that preserve the molecular velocities. The zero-field a(R)R(9, 9) nu(4) line is established to be 264.1 +/- 5.0 MHz above the CO-laser line.

Materials for luminescent greenhouse solar collectors.

Luminescent greenhouse solar collectors are potentially useful for concentrating sunlight onto photovoltaic power cells. Measurements of the performance of small-scale collectors made of two commercially available materials (Owens-Illinois ED2 neodymium-doped laser glass and rhodamine 6G-doped plastic) are presented. The results are encouraging, but they indicate a need for further spectral sensitization and for reduced matrix loss coefficient. The measurements with monochromatic illumination agree with the predictions of a mathematical model developed to take account of reemission following the absorption of luminescence. Under solar illumination, the model predicts photon flux concentrations of about 15 for optimized full-scale collectors made of the materials studied and concentrations of 110 for reasonably improved glass.

High-Resolution Stark Spectroscopy in the nu(4) Band of NH(3) Using a Thin-Film Diode Laser.

Thin-film PbTe diode lasers are used for Doppler-limited Stark spectroscopy on the nu(4) vibration-rotation band of NH(3) in the 1545-1595-cm(-1) region. The lasers operate cw below 20 K and are frequency tuned by varying the diode current. The nu(4) excited state dipole moment of NH(3) is found to be about 1% less than the ground state value.