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29. On-line gas electron diffraction identification of gas chromatography effluents (GC-GED).

Author

Ewbank, John D., Paul, David W., Schäfer, Lothar, Siam, Khamis, Monts, David L., and Faust, Walter L.

Subjects
*DETECTORS, *ELECTRON diffraction, *GAS chromatography
Abstract

Experiments are described which document the utility of gas electron diffraction (GED) as a means of detecting analytes emerging from the column of a gas chromatograph (GC). In these experiments electron diffraction patterns from GC effluents are made visible on a phosphor screen, and the intensities of each GC peak are recorded on line by a 1024-channel linear photodiode array detector. The technique not only indicates the presence of elutants but also provides auxiliary information on their molecular structures under favorable conditions. Minimum sample sizes of about 1 µl per component are currently required for GC-GED, but indications are that further detector enhancements will allow the recording of electron intensities from smaller samples. [ABSTRACT FROM AUTHOR]

Published
1988
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30. Improvements in real-time data acquisition for gas electron diffraction.

Author

Ewbank, John D., Schäfer, Lothar, Paul, David W., Monts, David L., and Faust, Walter L.

Subjects
*ELECTRON diffraction, *PHOTODIODES, *INFORMATION storage & retrieval systems
Abstract

Several improvements are described of a recently reported prototype real-time data-acquisition system for gas electron diffraction (GED), in which the scattered intensity distribution is recorded by a photodiode array. The use of a computer-designed and photographically produced "butterfly" slit in the optical track of the instrument, which simulates the effects of the rotating sector of conventional GED, as well as a number of other modifications, now makes it possible to record, for the first time by this technique, scattered electron intensities in a region around s = 300 nm[sup -1]. Modeling of several test molecules shows that structural parameters can be determined accurately by the novel procedure, but systematic inaccuracies still exist in the determination of mean amplitudes of vibration. Data evaluation is based on a novel ratiometric analysis procedure, whose essential features are briefly discussed. [ABSTRACT FROM AUTHOR]

Published
1986
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32. Multichannel Densitometry of Gas Electron Diffraction Patterns

Author

Benston, O. Joel, Ewbank, John D., Paul, David W., Klimkowski, V. Joseph, and Schäfer, Lothar

Abstract

The use of a multichannel linear photodioide array in obtaining diffraction intensities from photographic gas electron diffraction (GED) records is described. A commercially available RETICON 1024-channel linear array together with its programmable controller (EG & G models 1412 & 1218) combined with a MINC DECLAB-23 have been tested in this novel procedure of GED plate densitometry. One complete radial intensity distribution (plate image) is obtained every 16 ms from either a spinning or manually adjusted plate and can be dynamically viewed on an oscilloscope or digitally graphed on the DEC system monitor. This rapid data acquisition presents powerful and efficient means whereby plate centering, curve averaging, parametric refinement, and other data management can be conveniently conducted. Test analyses document that results obtained by this method are accurate and reproducible. The success of this new approach suggests the use of photodiode arrays in dynamic data acquisition systems of GED requiring no photographic intermediary, either by direct exposure of the diode array to scattered electrons, or by immediate acquisition of optical intensities from a fluorescent screen illuminated by electron impact.

Published
1984
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33. Processor Controlled Multichannel Microdensitometry for Gas Electron Diffraction Patterns

Author

Paul, David W., Ewbank, John D., Benston, O. Joel, and Schäfer, Lothar

Abstract

The design of a novel type of densitometer for evaluating photographic plates from gas electron diffraction is described. A commercially available linear diode array has been interfaced to a PDP-11/23 minicomputer via the DRV11 parallel interface. The diffraction pattern from a photographic plate is optically focused onto a solid state linear photodiode array. The position of each diode in the array provides the radial location information and the radial intensity of each diode is digitized with 14-bit resolution. Software development provides for control over the scan parameters for the array as well as for on-line manipulation of the collected data. The new instrument thus retains the advantages of digital data filtering described in a previous procedure, but allows for faster data recording since the total pattern is determined in onescanning measurement without the mechanical stepping process which is needed in single channel instruments to move the diffraction pattern through the measuring beam. Rapid acquisition of data with on-line transgeneration should prove to be valuable in the application of the new procedure as a real time data acquisition system for gas phase electron diffraction without photographic intensity recording.

Published
1983
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34. Real-Time Electron Diffraction. Part III: Image Transfer via Fiber Optics

Author

Ewbank, John D., Paul, David W., Schäfer, Lothar, and Bakhtiar, Reza

Abstract

Improvements are described in photodiode-array real-time data recording for gas electron diffraction (GED). When the conventional glass window and lens optics in a previously reported detector configuration are replaced by fiber optic components, two significant effects arise: (1) detector gain is enhanced to the extent that it is now possible to detect nanoliter samples in combined GED-GC (gas chromatography) experiments, and (2) for the first time since the development of the real-time recording scheme, molecular mean amplitudes of vibration are within error limits of literature values. Thus the method now affords full molecular structure determinations, including bond distances and angles, and their associated mean vibrational properties.

Published
1989
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35. The Molecular Structures of Cis- and Trans-1, 2-Dichloroethene: A Real-Time Gas Electron Diffraction and AG Initio Study

Author

ARKANSAS UNIV FAYETTEVILLE DEPT OF CHEMISTRY AND BIOCHEMISTRY, Shafer, Lothar, Ewbank, John D, Siam, Khamis, Paul, David W, Monts, David L, ARKANSAS UNIV FAYETTEVILLE DEPT OF CHEMISTRY AND BIOCHEMISTRY, Shafer, Lothar, Ewbank, John D, Siam, Khamis, Paul, David W, and Monts, David L

Abstract

The molecular structures of cis- aand trans-1,2-dichloroethene were studied in the gas phase by electron diffraction, using our new real-time data acquisition technique. For cis-dichloroethene the structural parameters are (r sub a distances (Angstroms) and angles (deg), with 3sigma in parentheses): C-C1:1.717(2); C=C:1.337(4); C1-C=C: 124.0(2); and H-C=C:120.3(3). For trans-dichloroethene: C-C1:1.725(2); C=C:1.332(8); C1-C=C:120.8(6); and H-C=C:124.0(3). The experimental work was augmented by ab initio gradient geometry refinements of the two compounds and of cis- and trans-difluorothene. The 4-21G, 5-31G**, and 3-3-21G basis sets were used in various series of calculations. Without any exception, the calculated structural trends for the cis- and trans-configurations are in excellent agreement with the experiments.

Published
1985

36. Recent Improvements in Real Time Gas Electron Diffraction

Author

ARKANSAS UNIV FAYETTEVILLE DEPT OF CHEMISTRY AND BIOCHEMISTRY, Ewbank, John D, Schafer, Lothar, Paul, David W, ARKANSAS UNIV FAYETTEVILLE DEPT OF CHEMISTRY AND BIOCHEMISTRY, Ewbank, John D, Schafer, Lothar, and Paul, David W

Abstract

A 40 KeV focused electron beam intersects an effusive gas jet and the scattering pattern generated is displayed on a fluorescent screen. Currently the screen is optically coupled by a transfer lens to the photodiode array sensor, where as many as 1024 independent and equidistant channels record the angular dependence and intensity profile of the scattered electrons. Thus, in a matter of minutes, entire scattering cross sections are available for analysis. In contrast to the feedback latency inherent in photographic practice, this rapid data collection and interpretation allows for immediate on-line optimization of experimental conditions.

Published
1985

37. Improvements in Real-Time Data Acquisition for Gas Electron Diffraction

Author

ARKANSAS UNIV FAYETTEVILLE, Ewbank,John D., Schafer,Lothar, Paul,David W., Monts,David L., Faust,Walter L., ARKANSAS UNIV FAYETTEVILLE, Ewbank,John D., Schafer,Lothar, Paul,David W., Monts,David L., and Faust,Walter L.

Abstract

Several improvements are described of a recently reported prototype real-time data-acquisition system for gas electron diffraction (GED), in which the scattered intensity distribution is recorded by a photodiode array. The use of a computer-designed and photographically produced 'butterfly' slit in the optical track of the instruments, which simulates the effects of the rotating sector of conventional GED, as well as a number of other modifications, now makes it possible to record, for the first time by this technique, scattered electron intensities in a region around s = 300/nm. Modeling of several test molecules shows that structural parameters can be determined accurately by the novel procedure, but systematic inaccuracies still exist in the determination of mean amplitudes of vibration. Data evaluation is based on a novel ratiometric analysis procedure, whose essential features are briefly discussed. Keywords: Molecular structure data.

Published
1986

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