provide a conclusive demonstration of the principle of the enhancement of nonlinear optical response through the supramolecular engineering of polymers. It is expected that considerably larger enhancement can be observed in structures with better alignment of chromophores, such as certain biopolymers and derivatives of helical poly(tripheny1methylmethacrylate)~ (1 6).

Global distributions of CH 4 in the mesosphere and stratosphere have been measured continuously since October 11, 1991, by the Halogen Occultation Experiment (HALOE) onboard the UARS. CH 4 mixing ratio is obtained using the gas filter correlation technique operating in the 3.3-grn region. Since measurements are made during solar occultation in the 57 ø inclination orbit, data are collected 15 times daily for both sunrises and sunsets. This provides coverage of one hemisphere in a month period. One complete hemispheric sweep (from equator to -80 ø latitude) is made during the spring and summer seasons of two hemispheres, and a partial sweep (from equator to around 50 ø latitude) is made during the fall and winter seasons of two hemispheres. HALOE CH 4 measurements are validated using direct comparisons with correlative data and internal consistency checks using other HALOE-measured tracers, HF, and aerosols. It is estimated for the 0.3-to 50-mbar region that the total error, including systematic and random components, is less than 15 % and that the precision is better than 7%. The CH4 gas filter channel does not depend significantly on the Pinatubo aerosol extinction. An experimentally accurate measurement of CH 4 is very important because CH 4 is a primary interfering gas in the HALOE HC1 channel and, subsequently, can cause HC1 measurement error. Simultaneous measurements of CH4 and other HALOE species (03, H20, NO, NO2, HC1, HF, and aerosol extinction coefficients) provide important information on atmospheric dynamic and chemical processes, since CH4 can be used as a tracer and an indicator of atmospheric transport processes. Several new pieces of infolsnation on previously unreported HALOE-observed features are also presented. Introduction Atmospheric methane, CH 4, naturally produced and released from the surface, has an important role in the greenhouse effect, causing a global warming, and in atmospheric chemistry, by removing hydroxyl radicals in the troposphere and in the stratosphere by converting reactive chlorine atoms to HC1 and producing hydrogen species to form water vapor by oxidation (see summary [World Meteorological Organization (WMO), 1982]). Methane can be used as a tracer to study atmospheric transport processes when enough measurements are made on a regional or global scale. For example, the features of CH 4 and other species observed by the Halogen Occultation Experiment (HALOE) were reported by Russell et al. [1993a] for the springtime Antarctic and by Park and Russell [1994] for the summertime polar region. Both papers used CH 4 as a tracer, and in the latter paper it was used to separate dynamical effects and to assess the importance of chemical processes for ozone deficiency in the polar regions. Methane measurements in the stratosphere before the Upper Atmosphere Research Satellite (UARS) [Reber, 1993] include the 1979 global measurements by stratospheric and mesospheric sounder (SAMS) on Nimbus 7 [Rodgers et al., 1984] and otherwise sparse individual profiles obtained by in situ sampli...

Revised cross‐section values for the excitation of the O I(3s ³S°‐2p ³P; λ1304 Å), O I(3d ³D°‐2p ³P; λ1027 Å), and O I(3s′ ³D°‐2p ³P; λ989 Å) resonance transitions by electron impact on atomic oxygen are presented from threshold to 300 eV. These results are smaller than the excitation cross sections used in some airglow models by a factor of ∼2.8. The revised values are in good agreement with recent quantum‐scattering calculations. The downward revision is required by new laboratory studies in which the direct and dissociative cross sections for λ1304 Å excitation were normalized with small probable error to the O and O2 ionization cross sections. The results also reflect new advances in VUV optical calibration techniques. A number of outstanding airglow problems are simplified by these revisions.

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A simple, high-performance, electrostatically focused electron gun, which is useful over an energy range extending from a few electron volts to at least 500 eV, is described. The current is delivered in a beam ∼1 mm in diameter, with <50 mrad angular divergence and typically has a current over 1 mA at 100 eV. The design is small, compact, and rugged. This electron gun has been used successfully in both laboratory and sounding rocket experiments.