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Easy Q-vand ESR spectra at liquid nitrogen temperature
JOURNAL
OF MAGNETIC
RESONANCE
73, 539-540 (1987)
Easy Q-Band ESR Spectra at Liquid Nitrogen Temperatures WILLIAM
C. TIMMER,* WILLIAM S. MILLMAN, AND GEORGE W. KEULKS
Department of Chemistry and Laboratory for Surface Studies, Vniversity of Wisconsin, Milwaukee, Wisconsin 53201 Received January 16, 1987
We describe a simple and efficient technique to record Q-band (35 GHz) ESR spectra at liquid nitrogen temperatures (77 K). Q-Band cavities, such as the Varian E-266, have small dimensions which preclude a Dewar insert. This limitation can be overcome by directing a constant stream of liquid nitrogen (LNp) onto the outside cavity surface. Upon cooling to 77 K the internal cavity volume contracts by about 2%. Because cavity volume is proportional to Q it is necessary to retune the cavity once it has equilibrated. Ordinarily this is not a problem. Retuning is normally achieved by adjusting the microwave bridge frequency. However, the Varian E- 110 microwave bridge operates at 35.0 GHz. Retuning at 77 K must be performed by adjusting the cavity tuning rod. A pressurized Dewar with a length of connective tubing delivers LN2 within the narrow confines between the magnet pole face and modulation coil (Fig. 1). Copper tubing has been found to work best. One end of the tubing is connected to the Dewar with Swagelock fittings while the other end is placed next to, but not directly on, the cavity at its vertical midpoint. Both the copper tubing and the cavity/waveguide assembly should be secured from vibration. Two experimental procedures are possible. If the sample cannot be warmed above 77 K it is best to pretune the cavity first. An empty evacuated quartz sample tube can be used. When the LN2 flow is started the cavity begins to cool and the resonant frequency moves off the oscilloscope screen. Equilibration at 77 K occurs when LN2 drips from the bottom of the tuning rod. The cavity is now ready to be retuned. Retuning at 35.0 GHz is accomplished by rotating the tuning rod while LN2 is flowing over it. Once the cavity is retuned the sample can be inserted. If the sample can be examined at temperatures above 77 K the pretuning step can be omitted. After cavity equilibration it is necessary to retune at 35.0 GHz. The advantage of this technique is that once the cavity is thermally stabilized it is easy to obtain spectra. The temperature inside the cavity was measured via a thermocouple and was determined to be 77 to 80 K. A typical spectrum obtained by this method is presented in Fig. 2. * To whom correspondence should be addressed. 539
0022-2364187 $3.00 Copyright 0 1987 by Academic Press, Inc. Au rights of Ivnduaion in any form I-eseNed.
540
NOTES
\
a
/
b
b
\
/
FIG. 1. Experimental setup: (a) Waveguide and cavity; (b) magnet pole faces; (c) modulation coil unit; (d) cavity tuning pole; (e) liquid nitrogen delivery. (Figure not drawn to scale).
An alternate procedure consists of immersing the entire cavity in a Dewar containing LN2. This however requires knowledge of the cavity resonant frequency at 77 K. Additionally, the cavity tuning pole must be removed which would not allow for small corrections when tuning.
IOOG
1
FIG. 2. Spectrum of defect centers on polycrystalline Al*O,. Experimental conditions: power, 22 dB; modulation amplitude, 2 G; gain, 4000; scan time, 8 min; time constant, 0.25 s. Recorded at 77 K.
OF MAGNETIC
RESONANCE
73, 539-540 (1987)
Easy Q-Band ESR Spectra at Liquid Nitrogen Temperatures WILLIAM
C. TIMMER,* WILLIAM S. MILLMAN, AND GEORGE W. KEULKS
Department of Chemistry and Laboratory for Surface Studies, Vniversity of Wisconsin, Milwaukee, Wisconsin 53201 Received January 16, 1987
We describe a simple and efficient technique to record Q-band (35 GHz) ESR spectra at liquid nitrogen temperatures (77 K). Q-Band cavities, such as the Varian E-266, have small dimensions which preclude a Dewar insert. This limitation can be overcome by directing a constant stream of liquid nitrogen (LNp) onto the outside cavity surface. Upon cooling to 77 K the internal cavity volume contracts by about 2%. Because cavity volume is proportional to Q it is necessary to retune the cavity once it has equilibrated. Ordinarily this is not a problem. Retuning is normally achieved by adjusting the microwave bridge frequency. However, the Varian E- 110 microwave bridge operates at 35.0 GHz. Retuning at 77 K must be performed by adjusting the cavity tuning rod. A pressurized Dewar with a length of connective tubing delivers LN2 within the narrow confines between the magnet pole face and modulation coil (Fig. 1). Copper tubing has been found to work best. One end of the tubing is connected to the Dewar with Swagelock fittings while the other end is placed next to, but not directly on, the cavity at its vertical midpoint. Both the copper tubing and the cavity/waveguide assembly should be secured from vibration. Two experimental procedures are possible. If the sample cannot be warmed above 77 K it is best to pretune the cavity first. An empty evacuated quartz sample tube can be used. When the LN2 flow is started the cavity begins to cool and the resonant frequency moves off the oscilloscope screen. Equilibration at 77 K occurs when LN2 drips from the bottom of the tuning rod. The cavity is now ready to be retuned. Retuning at 35.0 GHz is accomplished by rotating the tuning rod while LN2 is flowing over it. Once the cavity is retuned the sample can be inserted. If the sample can be examined at temperatures above 77 K the pretuning step can be omitted. After cavity equilibration it is necessary to retune at 35.0 GHz. The advantage of this technique is that once the cavity is thermally stabilized it is easy to obtain spectra. The temperature inside the cavity was measured via a thermocouple and was determined to be 77 to 80 K. A typical spectrum obtained by this method is presented in Fig. 2. * To whom correspondence should be addressed. 539
0022-2364187 $3.00 Copyright 0 1987 by Academic Press, Inc. Au rights of Ivnduaion in any form I-eseNed.
540
NOTES
\
a
/
b
b
\
/
FIG. 1. Experimental setup: (a) Waveguide and cavity; (b) magnet pole faces; (c) modulation coil unit; (d) cavity tuning pole; (e) liquid nitrogen delivery. (Figure not drawn to scale).
An alternate procedure consists of immersing the entire cavity in a Dewar containing LN2. This however requires knowledge of the cavity resonant frequency at 77 K. Additionally, the cavity tuning pole must be removed which would not allow for small corrections when tuning.
IOOG
1
FIG. 2. Spectrum of defect centers on polycrystalline Al*O,. Experimental conditions: power, 22 dB; modulation amplitude, 2 G; gain, 4000; scan time, 8 min; time constant, 0.25 s. Recorded at 77 K.