On the Combination of an Electron Spin Resonance Spectrometer and a Multichannel Analyser

Abstract

IN a recent communication, Kent and Mallard¹ pointed out that the versatility of an electron spin resonance-spectrometer can be considerably improved by combining it with an analogue digital converter and a commercial multichannel analyser as they are commonly used in nuclear spectroscopy. We have used a similar piece of equipment, and have already presented a preliminary report². The basic technique was introduced by Klein and Barton³. It should be noted that any real improvement is based on the fact that the noise seen in electron spin resonance-spectra is not ‘white noise’. That is to say that the probability for a certain frequency to occur in the noise-spectrum is not equal for all frequencies, but is enhanced for lower values³. It is therefore clear that no higher sensitivity can be achieved if the time constant of the amplifier and the recording time are enhanced above a certain limit being about 100 sec for the time-constant⁴. The sensitivity is restricted, for example, by instabilities of the whole equipment. This applies to relatively long times of measurement. If the recording time is short, a sampling procedure as described by Kent and Mallard¹ is not expected to bring any improvement if the time-constant is correctly adjusted. Thus the sub-division of one measurement into ten single ones appears to offer no advantage and the same result may be obtained by increasing the apparatus time constant or by repeating ten times the recording of the whole spectrum at a higher speed. The basic law of communication theory, namely, that any improvement in signal-to-noise ratio requires an increase in time of measurement, cannot be overcome by any of these methods.