<p> Crystallographic image processing (CIP) is a technique first used to aid in the structure determination of periodic organic complexes imaged with a high-resolution transmission electron microscope (TEM). The technique has subsequently been utilized for TEM images of inorganic crystals, scanning TEM images, and even scanning probe microscope (SPM) images of two-dimensional periodic arrays. We have written software specialized for use on such SPM images. A key step in the CIP process requires that an experimental image be classified as one of only 17 possible mathematical plane symmetry groups. The current methods used for making this symmetry determination are not entirely objective, and there is no generally accepted method for measuring or quantifying deviations from ideal symmetry. Here, we discuss the crystallographic symmetries present in real images and the general techniques of CIP, with emphasis on the current methods for symmetry determination in an experimental 2D periodic image. The geometric Akaike information criterion (AIC) is introduced as a viable statistical criterion for both quantifying deviations from ideal symmetry and determining which 2D Bravais lattice best fits the experimental data from an image being processed with CIP. By objectively determining the statistically favored 2D Bravais lattice, the determination of plane symmetry in the CIP procedure can be greatly improved. As examples, we examine scanning tunneling microscope images of 2D molecular arrays of the following compounds: cobalt phthalocyanine on Au (111) substrate; nominal cobalt phthalocyanine on Ag (111); tetraphenoxyphthalocyanine on highly oriented pyrolitic graphite; hexaazatriphenylene-hexacarbonitrile on Ag (111). We show that the geometric AIC procedure can unambiguously determine which 2D Bravais lattice fits the experimental data for a variety of different lattice types. In some cases, the geometric AIC procedure can be used to determine which plane symmetry group best fits the experimental data, when traditional CIP methods fail to do so.</p>

The theory as given by J. J. Thomson for the drop of potential at plane electrodes has been modified by allowing for recombination in the layer and a similar theory for cylindrical electrodes has been worked out. The equation for plane cathodes is V2=32pi 75k1 12x32 2 , and for cylindrical cathodes is V2=2i 3k1&parl0;r22-r2 0&parr0;2 12 r32 logr2+r 22-r20 r0-r2 2&parl0;r22-r20 &parr0;12-&parl0; r22-r20&parr0; 323 , where V2 = the potential drop across the sheath, i = the current density, x2 = the sheath thickness at the plane cathode, r0 = the radius of the cylindrical cathode, r2 = the radius of the cylindrical sheath about the cathode, and k1 = the velocity of the positive ions for a gradient of 1.0 volt per cm. The experimental results for platinum electrodes immersed in pure NaCl flames agree well with the theoretical equations given. It is found that the drop in potential at the cathode occurs in a sheath of uniform thickness, which completely surrounds the electrode. By plotting the gas potential at various points in the flame against distance from the cathode it is possible to estimate the thickness of the sheath. Over 95% of the potential drop takes place across the sheath at the cathode provided it is of clean platinum. If the cathode is not clean electrons are emitted which partially neutralize the accumulation of positive ions and thus reduce the sheath thickness. By measuring v2, i, x2 or r0 and r2, and making the proper substitutions in the above equations, the mobility k 1 of the positive ions is found to average 12.4 for a pure flame and 8.1 cms per sec. for one volt per cm for a NaCl flame. A study of the characteristic current-voltage curve for a wire probe in a flame makes it possible to measure the voltage correction to be applied to the measured probe potential in order to obtain the true gas potential adjacent to the test probe. The correction is of the order of +1.0 volts. It is found that the current density at the surface of the cylindrical sheath is constant for any given flame conditions, and size of cathode. Thus one can measure the current density existing in the small uniform gradient just outside the sheath. The current density at the sheath surface in a pure flame varies from 1.0 to 2.5 microamperes and for the NaCl flame from 5.0 to 11.0 microamperes.

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Two methods for the accurate determination of high vacuum pressure are reviewed with particular attention paid to the 10$\sp{-6}$ to 10$\sp{-5}$ Torr range. The first method is characterized by calculating the pressure generated from a knowledge of the characteristics and geometry of the vacuum chamber. Static and dynamic expansion techniques are examined. The second method establishes a pressure in the vacuum chamber by balancing an input of gas against a pumping speed and measuring the resulting pressure with a suitable gauge. McLeod, spinning rotor, hot-filament ion, and capacitance diaphragm gauges are discussed. Additionally, new results for ion and capacitance diaphragm gauges are presented including an extension of the range of capacitance diaphragm gauges down to 10$\sp{-6}$ Torr through the use of averaging and a new technique for the direct calibration of capacitance diaphragm gauges between 10$\sp{-6}$ and 10$\sp{-4}$ Torr which uses gravity to deflect the diaphragm and generate apparent pressures.

The temperature dependence of conversion of He(2$\sp3$S$\sb1$) metastable atoms to He$\sb2$(a$\sp3\Sigma\sp+\sb{\rm u}$) metastable molecules in the three-body reaction$$\rm He(2\sp3S\sb1)+2He(1\sp1S\sb0) \to He\sb2(a\sp3\Sigma\sp+\sb{u})+He(1\sp1S\sb0)$$has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the He(2$\sp3$S$\sb1)$-He(1$\sp1$S$\sb0$) interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient $\delta$(T) is accurately described by$$\rm \delta(T) = \lbrack 87.4\ T\ exp({-}750/T){+}4.1T\ exp({-}200/T)\rbrack \times 10\sp{-37}\ cm\sp{-6}sec\sp{-1}.$$The first activation energy, 750 $\pm$ 70K(63 $\pm$ 6meV), is equal to the known He(2$\sp3$S$\sb1)$-He(1$\sp1$S$\sb0$) repulsive barrier height. The second activation energy is 17 $\pm$ 2 meV.

A miniaturized pulsed laser-vaporization supersonic cluster beam source was developed and mounted on the magnetic axis of a Fourier transform ion cyclotron resonance apparatus (FT-ICR) so that the clusters were directly injected into the ICR ion trap. Mass-selected positive silicon cluster ions were levitated in the FT-ICR, and probed in chemisorption reactions with ethylene and ammonia. Clusters in the 36-51 atom size range were effectively annealed to unique structural forms by excitation with a XeCl excimer laser followed by cooling through infrared radiation and collisions with argon. Reactions on the annealed clusters showed the variation in reactivity with cluster size was enhanced. To our knowledge this is the first reported example of a successful annealing experiment on gas phase clusters of any kind.

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