High-resolution solid-state *'Si nuclear magnetic resonance (n.m.r.) spectra were recorded at 79.80 MHz, with rapid spinning of the sample at the magic angle to the external magnetic field, for a series of phase-pure s nthetic faujasites with Si/AI ratio in the range 1.19-2.75. On the basis of previous work which related Si chemical shift to environment, signal intensities corresponding to Si(nA1) structural units (n = 0, 1, 2, 3, 4) could be quantitatively determined by accurate deconvolution of the spectrally well-resolved Gaussian peaks. Even though n.m.r. spectra do not by themselves provide direct evidence for Si, A1 ordering beyond the first tetrahedral coordination shell, from the observed ratio of intensities of Si(nA1) units and from arguments based on crystal symmetry and electrostatic energy, a series of Si, A1 ordering schemes was constructed. These schemes offer greater insight into the structure of the aluminosilicate framework than can be achieved at present by other techniques. Inter alia, they reveal that the Loewenstein rule, which forbids A1 atoms to occupy neighbouring framework tetrahedral sites, is strictly obeyed for zeolites of the faujasite structure. Our results and conclusions can also explain observed discontinuites in the plot of the (cubic) unit-cell parameter against aluminium content in terms of Si, A1 ordering within the framework. Moreover, th,e magic-angle-spinning nuclear magnetic resonance (m.a.s.n.m.r.) spectra provide an independent method of determining Si/Al ratios: ratios determined by X-ray fluorescence or analytical electron microscopy agree within a few per cent with those derived from m.a.s.n.m.r., thus lending further credence to the value of solid-state n.m.r. in studies of aluminosilicates. The effect of second-nearest (tetrahedral) neighbours on chemical shifts, as well as the spectral and structural characteristics of faujasites possessing "non-ideal" Si/Al ratios, are also discussed. X9

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Frequency stabilization of mid-IR quantum cascade (QC) lasers to the kilohertz level has been accomplished by use of electronic servo techniques. With this active feedback, an 8.5-microm QC distributed-feedback laser is locked to the side of a rovibrational resonance of nitrous oxide (N(2) O) at 1176.61cm (-1) . A stabilized frequency-noise spectral density of 42Hz/ radicalHz has been measured at 100 kHz; the calculated laser linewidth is 12 kHz.

Lasing characteristics were evaluated for distributed-feedback quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the QC lasers' suitability for use in high-resolution spectroscopic applications, including Doppler-limited molecular absorption and pressure-limited lidar applications. By use of a rapid-scan technique, direct absorbance measurements of nitric oxide (NO) and ammonia (NH>(3)) were performed with several QC lasers, operating at either 5.2 or 8.5 microm. Results include time-averaged linewidths of better than 40 MHz and long-term laser frequency reproducibility, even after numerous temperature cycles, of 80 MHz or better. Tuning rates of 2.5 cm(-1) in 0.6 ms can be easily achieved. Noise-equivalent absorbance of 3 x 10(-6) was also obtained without optimizing the optical arrangement.

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The application of reflective Nomarski differential interference contrast microscopy for the determination of quantitative sample topography data is presented. The discussion includes a review of key theoretical results presented previously plus the experimental implementation of the concepts using a commercial Nomarski microscope. The experimental work included the modification and characterization of a commercial microscope to allow its use for obtaining quantitative sample topography data. System usage for the measurement of slopes on flat planar samples is also discussed. The discussion has been designed to provide the theoretical basis, a physical insight, and a cookbook procedure for implementation to allow these results to be of value to both those interested in the microscope theory and its practical usage in the metallography laboratory.