Wilbur Kaye

Low angle laser light scattering-absolute calibration.

A new method for the calculation of the Rayleigh factor from low angle light scattering measurements is developed. This method does not require a uniform intensity illuminating beam, hence efficiently utilizes all the beam from a focused laser source. Scattering volume is then very small, reducing sample volume and interference from contaminant particles. All the parameters necessary for the calculation of the Rayleigh factor (including the exact dependence on sample refractive index) are measurable, hence absolute calibration is possible. Over-all error is estimated to be less than 2.3% under specified conditions. Measurements are possible at scattering angles as small as 2 degrees obviating the need for angular extrapolations in the determination of molecular weight of most dissolved samples. Rayleigh factors at 22 degrees C, 633 nm, and a scattering angle of 4 degrees for water, methanol, benzene, and toluene, were found to be, respectively, 0.907, 2.56, 12.15, and 13.45 x 10(-6) cm(-1).

Low-Angle Laser Light Scattering—Rayleigh Factors and Depolarization Ratios

The Rayleigh factors and depolarization ratios at 6328 A have been measured for carbon disulfide, toluene, benzene, chloroform, carbon tetrachloride, cyclohexane, butanone, acetone, methanol, and water using an improved model of the low-angle laser light-scattering (LALLS) photometer. Small errors in some of the Rayleigh factors published using an earlier model of this instrument have been revealed. Depolarization ratios are measured by a new method involving detection of the full cone of forward scattered light.

Far Ultraviolet Spectroscopy. I. Modification of the Beckman DK Spectrophotometer for Automatic Transmittance Recording in the Region 1700-2000 A

Abundant opportunities exist for analytical and molecular structure studies using the far ultraviolet region of the spectrum since almost all compounds in the vapor state possess intense, characteristic, and often detailed absorption spectra. Unfortunately, the experimental difficulties in working in this region have discouraged its widespread use Beckman Model DK-1 and DK-2 spectrophotometers have been modified to perform to 1700 A for the automatic recording of relative transmittance spectra This performance has been accomplished through the use of very high quality quartz and fused silica for prisms and windows. The absorption of the atmosphere in the optical path has been eliminated by purging with nitrogen. The resolution achieved is remarkably good, because of the high dispersion of quartz in this region. Except for the necessity of purging, the experimental difficulties of working to 1700 A are little different from those in near ultraviolet spectroscopy.

Far Ultraviolet Spectroscopy. II. Analytical Applications

One of the prime requirements for the use of a spectral region for analytical purposes is a library of reference spectra. Since no adequate libraries exist, the literature since 1941 has been indexed and references to spectra are listed by empirical formula. The few published analytical applications of far ultraviolet spectroscopy are discussed. With the commercial spectrophotometers that are now available it seems certain that the analysts will make frequent use of this region in the future

A Universal Spectrophotometer

By placing the sample between the monochromator of a prism-grating spectrophotometer, considerable versatility is obtained. Absorption, emission, fluorescence, excitation, polarized fluorescence, polarized excitation, phosphorescence, light scattering, and other types of spectral data may be obtained. Readily interchanged accessories fitting into the sample compartment are utilized. The instrument may be used in three modes—single-beam, double-beam time-shared, or double-beam space-shared. The monochromators may be used independently or simultaneously. High-brilliancy mercury and xenon sources are provided in addition to the standard hydrogen and tungsten lamps.

Resolution and stray light in near infrared spectroscopy

The slit function at 1064 nm for a double monochromator spectrophotometer has been obtained with sufficient dynamic range to quantitatively account for the resolution and stray light over a wide range of resolutionv and absorbance levels. Conventional stray light tests are found to underestimate seriously that portion of the nearby stray light that most influences errors in ir absorbance determinations, and improved tests are suggested. A revision of the definition of stray light is suggested in order to minimize the variation of stray light with resolution. Convolutions of the observed slit functions with the transmittance function for the 1691-nm band of chloroform agree with observed absorbance measurements over a wide range of resolution and absorbance levels. The ratio of true to observed absorbance levels for chloroform is significantly less influenced by resolution than the literature, using triangular or Gaussian slit functions and Lorentzian absorbance functions, predicts.

Molecular Orientation in Twisted Liquid Crystal cells

Abstract A useful model of the molecular orientation within twisted liquid crystal cells has been developed by Berreman (1). This model predicts a surprisingly large homogeneous components of orientation at fields at which a typical display is “on”. It also predicts the retention of twist at voltagaes considerably above that normal “turn-on” voltage. At normal operating voltage the molecular configuration of partially homogeneous molecules with twist cofined to the center of the cell can produce considerable retardation of off-axis polarized light. This degrades readability of the display. A method of verifying this predicted molecular structure has been developed utilizing the interference fringes formed by reflections at the internal electrode-liquid crystal interfaces. Using HeNe laser light, these interference fringes are of considerable amplitude and permit an accurate measure of refractive index and birefringence. In 90[ddot] twisted cells, these interference fringes exhibit a 90[ddot] asymmetry. Th...

Sensitive Measurement of Twist in LCD’s

The contrast between on and off states of twisted liquid crystal displays (LCD’s) can exceed 105 under certain circumstances.1 This contrast is far in excess of that usually encountered in LCD’s and an understanding of the controlling parameters has obvious technological significance. It suggests nondisplay applications such as light modulation. Numerous papers have appeared discussing the parameters controlling contrast. This paper is primarily concerned with the significance and measurement of cell twist.

Interpretation and Nomenclature for the Transmittance vs. Voltage Curves for LCDs

Abstract Like other fields that rpidly change from academic to commercial interest, therest, there is a need for standardization of nomenclature in liquid crystal technology. A case in point involves the interpretative features of the transmittance vs. voltage curves for 90[ddot] twisted field effect LCDs. It will be shown that a semi-logarithmic plot is far superior to the linear plot. Considerations must be given to the pronounced interferences that are introduced by internal reflection at the electrodeliquid crystal interfaces when using monochromatic light. Measurements should be made with polars both crossed and parallel. The features of these plots that have offered varying definitions and measurement details are: (1) zero field transmittance, (2) threshold or critical voltage, (3) interference fringes, (4) 90% turn-on voltage, and (5) saturation voltage. The usual practice of attributing threshold voltage solely to elastic constants and dielectric anisotropy overlooks the dominating factor of surfa...

Polarization Modulated Excitation Spectroscopy

An accessory has been designed for the Beckman Model DK spectrophotometers for automatic recording of polarization modulated excitation spectra. The polarizer is continuously rotated while the excitation spectrum is scanned with the monochromator The absorption, fluorescence, excitation and polarization spectra of rhodamine B in glycerol are used to illustrate performance.