Constant acceleration mode

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

Fig. 3.2 Triangular velocity reference signal top) and drive error signal bottom) of a Mossbauer drive operating in constant acceleration mode. The error signal is taken from the monitor output F of the drive control unit (see Fig. 3.1). Usually it is internally amplified by a factor of 100. Here, the deviations, including hum, are at the 2%o level of the reference. The peaks at the turning points of the triangle are due to ringing of the mechanical component, induced by the sudden change in acceleration (there should be no resonance line at the extremes of the velocity range)...

Fig. 3.10 Variation of the spectrometer aperture as a function of the source motion for Mossbauer spectrometers operated in constant acceleration mode with triangular velocity profile, and the resulting nonlinear baseline distortion of the unfolded raw spectra. For simplicity a point-source is adopted, in contrast to most real cases (Rib mm active spot for Co in Rh)...

Fig. 11. MBssbauer spectrum in the constant-acceleration mode. Zero velocity is with respect to 57Co in copper source. Figure according to Boudart el til. (215).

The Mossbauer transmission spectra were recorded in the constant acceleration mode with an Elscint Mossbauer drive unit and a model MFG 3A Elscint function generator, an MVT-3 linear velocity transducer and an MD-3 transducer driving unit, y-ray detection was done with a Reuter-Stokes Kv-CH4 proportional counter driven by an Ortec 401A/456 high voltage power supply. Voltage pulses were introduced into an Ortec 142 PC preamplifier and an Ortec 571 spectroscopy amplifier. Data were collected on a Tracor-Northern NS-701A multichannel analyzer. The data were later analyzed on an IBM 360/370 computer. 

Fe Mossbauer spectra were taken in an in situ cell at 300 K. A Co/Cr source was used in constant acceleration mode. The isomer shifts are relative to metallic a-Fe. The spectra of the as prepared as well as the pretreated (evacuation/623 K/N2 or eva-cuation/623 K/N2/H2, duration of each step 1 hour) samples were registered. 

S7Fe Mossbauer spectra were recorded in constant acceleration mode. Lorentzian line shape was presumed for decomposition of separate lines, and no positional parameters were constrained in the fits. The accuracy of positional data is ca. +- 0.03 mm/s. The isomer shift values are related to metallic a-iron. 

In addition some of the samples were studied by Fe-57 Mossbauer spectroscopy. The spectrometer was an Apple Ile-controlled Ranger Scientific MS-900 operated in constant acceleration mode, using Co-57 in a matrix of rhodium as source. Spectral data were accumulated in 1024 channels. 

Mossbauer spectra have been recorded in a conventional transmission geometry in the constant acceleration mode. The temperature of the sample could be varied from 4.2 K up to room tenq>era-ture by means of a liquid helium cryostat, the source (57co in Rh) being kept at room temperature. The Mossbauer experiments were carried out in the "Laboratoire d Interactions Hyperfines at the CEN-Grenoble. 

Conversion electron Mossbauer spectra (CEMS) of the samples were measured at 295 K with the aid of the same Haider Mossbauer spectrometer capable of operating in conventional constant acceleration mode using a backscatter-type gas flow detector. 

We will present the Mossbauer data to show its usefulness in monitoring various stages of the process. Transmission Mossbauer spectroscopy was carried out on powder samples at room temperature with a Co (Rh) source in constant acceleration mode using a thin iron foil for calibration. 

Even more interesting is a new spectrometer that has been built with a novel moving system that synchronizes the movement of the radioactive source with that of the detector (206). This system allows the accumulation of Mossbauer spectra either in constant acceleration or constant velocity modes, therefore giving better signal-to-noise ratios and also reducing the time necessary for accumulation of a spectrum. Narrower line widths were also obtained with this equipment design. 

Accelerated deactivation tests in hydrocracking have been reported (38), where a constant conversion mode was run at much higher space velocity (and hence temperature) than under actual operation conditions. Differences in deactivation were measured that were later substantiated in commercial operation (38). Although all these approaches aim at accelerating the catalyst deactivation reaction in Equation 7, such tests should obviously not be applied to catalyst systems that — at the high space velocity — operate at such high temperatures that very high polyaromatics concentrations prevail. 

The majority of Mdssbauer experiments that are conceived to elucidate problems of chemical structure make use of velocity modulated spectrometers. Most frequently, these are of the constant acceleration type, in which equal time intervals are spent in equal velocity increments. The primary information storage device for such spectrometers is commonly a multichannel analyzer operated in the multiscaler mode. Since the minimum observable line widths (2rnat= hf ) for S7Fe and 1,9 Sn are 0.1946 and 0.6193 mm/s, respectively, and since on the order of at least 5-10 data points on each side of the resonance maximum are... 

The normal experimental technique is to scan rapidly through the velocity range and repeat this scan many times imtil data of the required accuracy has been accumulated. The Doppler motion is provided by an electromechanical drive system controlled by a servo -amplifier. Usually, the source is attached to the drive shaft and driven either in a saw-tooth or a triangular constant acceleration wave form. The transducer is coupled to a multichannel analyser operating in the multiscaler mode, and the servo-amplifier is controlled by the channel advance frequency. The dwell time in each channel, corresponding to a specific velocity increment, is 100 ps, and while the channel gate is open it accepts pulses from the detector. 

A mode] study has demonstrated the pathways shown in Scheme 4,17. The first cyclization step gave predominantly five-membered rings, the second a mixture of six- and seven-membered rings.155 Relative rate constants for the individual steps were measured. The first cyclization step was found to be some five-fold faster than for the parent 5-hexenyl system. Although originally put forward as evidence for hyperconjugation in 1,6-dienes, further work showed the rate acceleration to be sterie in origin.113-I3j... 

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