assumption that the "excited" beam consisted entirely of excited molecules. While it was argued that, on the basis of the almost resonant charge transfer, the excited N 2 A S Z should be highly preferred, the possibility that some groundstate N 2 molecules are produced cannot be eliminated. It should be noted, however, that corrections for this effect would make the excitedstate ionization-cross-section and emission coefficient even larger. These arguments contain the implicit assumption that the N 2 + ions were in the ground state. In order to verify that excited N 2 + ions did not dominate the results, the ionizing electron energy was varied from 18 to 24 eV in the charge-transfer cross-section measurement (35-eV ion beam) and from 19.5 to 24 eV in Several workers 1 " 3 have reported the influence of electric fields upon the scattering of light by cholesteric liquid crystals. In this Letter, we wish to report a phase transition which was observed in cholesteric liquid crystals subjected to high dc electric fields. To our knowledge, a change of phase in these materials due to electric fields has not been previously identified.Results were obtained as follows. A cholesteric liquid crystal was put between, and in contact with, tin-oxide-coated glass plates separated by 0.2 mm. The liquid-crystal materials were mixtures of cholesteryl chloride, nonanoate, and oleyl carbonate with typical proportions of 30, 56, and 14% by weight, respectively. 4 The normal appearance of the layer was that of a milky white, opalescent sheet from which light was diffusely scattered. When plane-polarized light was normally incident on the layer, it was impossible to extinguish the field of view with a linear analyzer. When an average electric field of «10 5 V/cm was applied, the liquid crystal assumed a clear, colorless appearance. Normally incident, plane-polarized light now emerged plane-polarized and thus could be extinguished. However, the material was not optically isotropic. Plane-the ionization cross-section measurement (35-eV neutral excited beam). No change outside ±5 and ±10%, respectively, could be detected up to 22-eV electron energy. Above 22 eV, however, both cross sections increased markedly. This is interpreted as being due to the production of long-lived N 2 + excited ions, which transfer charge with NO to produce highly excited metastable N 2 molecules. 3 N. G. Utterback and G. H. Miller, Phys. Rev. 124, 1477 (1961), and 129, 219 (1963. 4 N. G. Utterback and T. Griffith, Jr., Rev. Sci. Instr. 37, 866 (1966). 5 This result has immediate practical importance in calculation of bulk ionization rates through integration of cross sections over velocity distributions. It has been shown (C. F. Hansen, private communication) that if this result holds generally, bulk ionization rates can be dominated by collision between excited particles even though relatively few of these are present. It would then be essential to know the cross sections for collisions between all the excited-state possibilities in order to calculate reliab...

An electrically controllable diffraction mode in liquid crystals is described. The variable grating is a stable single layer of small uniform birefringent spherulitic domains imbedded in a homeotropic matrix. The size of the domains is changed by small dc or ac voltages.

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Diffraction effects have been observed in ferrofluids in the presence of a magnetic field. The diffraction pattern from a laser beam normal to a thin layer of ferrofluid and normal to a magnetic field is confined to a plane also normal to the layer and normal to the field, indicating alignment of ferrofluid particles.

A novel electrically reversible storage effect in doped nematic liquid crystals is described. The ``off state'' is homeotropic nematic and the ``on state'' a single layer of densely packed spherulites. Both states are stable in time. The effect was observed in a narrow range of pitch and cell thickness combinations, and is interpreted as a nematic-cholesteric phase transition.

not Available.

Optical scattering data have been used to investigate molecular order in certain cholesteric mixtures. When films of these materials are deposited on a reflecting substrate, their colors can be shifted by several thousand angstroms for any condition of illumination and observation by a mechanical disturbance. The dispersive color reflections in both the undisturbed and disturbed films can be attributed to Bragg-type crystallite sites embedded in a material with refractive index n. When the films are mounted on a reflective substrate, there are effectively two angles of incidence. One angle corresponds to the actual angle of incidence and the other angle corresponds to the reflected beam. Accordingly, for some fixed combination of incidence and observation angles, two wavelength peaks should be observed for an isotropic distribution of scattering sites. Scattering data in both reflection and transmission indicate that the distribution is far from isotropic and that the effect of disturbance is to reorient the sites such as to produce a large increase in the intensity of light scattered from the direct beam and a corresponding decrease in the intensity of visible light scattered from the reflected beam. There is evidence that the basic helical pitch parameter is not changed significantly by the mechanical disturbance.

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