ESR resonator with field coils ESR power supply

Transcription

1 ESR resonator with field coils ESR power supply PHYWE Systeme GmbH & Co. KG Robert-Bosch-Breite Göttingen Germany Tel. +49 (0) Fax +49 (0) Operating Instructions Fig. 1: ESR-power supply Fig. 2: ESR-resonator with field coils CONTENTS 1 SAEFTY PRECAUTIONS 2 PURPOSE AND CHARACTERISTICS 3 FUNCTIONAL AND OPERATING ELEMENTS 4 NOTES ON OPERATION 5 HANDLING 6 TECHNICAL SPECIFICATION 7 MATERIAL 8 NOTES ON GUARANTEE 9 DISPOSAL risk of explosion. Before applying the mains voltage, ensure that the earth lead of the power supply is properly connected to the earth lead of the mains. Only plug the mains plug into a mains socket which has an earth lead. Do not cancel the protective effect by using an extension cable without an earth lead Check that the mains voltage stated on the type plate of your power supply matches that of your electric mains supply. Set up the experimental arrangement so that the power supply and instrument plug are freely accessible. Do not cover up the venting slots of the experimental set-up. Only use the experimental set-up for the use it is intended for. Do not open up anything in the experimental set-up. Do not connect any other pieces of equipment other than those specified to the instrument. Caution: The instrument must be separated from the mains before any cable connection is loosened, exchanged or removed! 1 SAEFTY PRECAUTIONS Carefully read these operating instructions completely before operating this instrument. This is necessary to avoid damage to it, as well as for user-safety. Only use the instrument for the purpose for which it was designed. Only use the instrument in dry rooms in which there is no 2 PURPOSE AND CHARACTERISTICS The Electron Spin Resonance power supply and the ESR Resonator has been designed for detecting the magnetic moment of the electron spin and for determining its numerical value. It can be used to show that the gyromagnetic ratio for the magnetic moment to the correspondig angular momentum for the spin moment is twice that for the linear moment. 1

2 2.1 Measuring principle The apparatus works on the principle of magnetic resonance. By absorption of a quantum of radiation, a transistion from one state to another will occur in an atom or molecule. The energy difference between the two states results from Zeeman splitting in a magnetic field. The magnetic moment, whose different orientation in the magnetic field produces the energy difference, results from the frequency of the radiation quantum and the flux density B of the magnetic field. In the specimen substance used this unit (diphenyl picryl hydrazyl), the moment of a stationary unpaired electron is of concerrn. Since the linear portion is practically eliminated by internal crystal fields, only the pure spin moment of the electron is left. If M its the magnetic quantum number of the electron state, L z = M. ħ is the projection of its angular momentum onto the field direction. This corresponds to a magnetic moment µ z= - g. µ B. M, the negative spin results from the negative charge of the electron. The quantity µ B is the Bohr magneton, and g is the so-called Landé splitting factor. For a pure linear moment g = 1, its value for a spin moment is given by the result of measurement. W 0 being the energy of the electron state in the absence of a magnetic field, its energy W at the magnetic flux density B is obtained according to the following equation: W - W 0 = - µ z. B = M g. µ B. B. (1) For the spin of the electron, M can assume only the two values, M = +1/2 and M = -1/2. Therefore, the energy difference between the two states is: W = g. µ B. B. (2) The transition from the lower to the higher energy level by absorption of radiation energy requires radiation quanta whose energy h. f ( within a certain line width) just barely corresponds to the difference between the two levels. If the frequency f of the radiation is definitely given, this condition can be fulfilled by adjusting a definite resonance flux density B r of the magnetic field. For this we have W = h. f = g. µ B. B r. (3) Which yields the Landé factor: g = h f µ B B r For calculating g, the following values are required: µ B = ħ. e 0 / 2 m e = 9, Am 2, mit: e 0 / m e = specific electron charge, ħ = h / 2 π, h = 6, Js f = Hz (± Hz) If these values are introduced in Eq. (3), we obtain g = 10,43 T. 1 Br In the ESR Apparatus, the magnetic field is produced in a Helmholtz arrangement of circular flat coils having turns w = 250 and a radius R = 0,054 m. If I is the coil current, we have: B = 0,7155 µ 0. w I R with the permeability of vacuum µ 0 = 1, Tm/A If these values for µ 0, w and R in Eq. (6), we have: (4) (5) (6) B = I. 4, T/A (7) If instead of the resonance flux density B r the corresponding resonance current I r is applied by using Eq. (7), the following is finally obtained: 2,507 A g = I r Example: I r = 1,23 A gives g = 2,04. The theoretical value for the Landé faktor of the electron spin is g = 2, The g value in a crystal, however, will usually deviate slightly from this. 2.2 Description The Operating Unit contains a quartz-stabilized oscillator (146 MHz), whose high frequency AC voltage is fed to a bridge circuit (Fig. 3) in the resonator. In one of the branches of the brigde, a tunable oscillating circuit of extremely high quality is connected; in the coil is a free radical (approx. 1 g diphenyl picryl hydrazyl) as a specimen substance. The substance is under the action of the variable magnetic field produced by a pair of Helmholtz coils. If resonance absorption occurs in the specimen substance at a resonance flux density B r, the complex oscillator circuit resistance and, therefore, the voltage across the diagonal branch of the bridge will change. The output voltage of the bridge is taken off a semi-conductor diode provided in this branch for rectification of the highfrequency AC and is supplied to an amplifier in the operating unit. For indicating the ESR signal, a DC voltmeter and an oscilloscope are connected to the output of the operating unit. For displaying the ESR signal by oscilloscope trace, a low-frequency alternating field (e.g. 50 Hz) must be superimposed on the constant magnetic field of the resonator. Fig. 3: Circuit diagram of the apparatus 3 FUNCTIONAL AND OPERATING ELEMENTS 3.1 ESR-power supply On-off switch with pilot light 2 High frequency output tob be connected (by BNC-cable) to the HF input 21 of the resonator. 3 Input for operating an internal phase shifter on 6V/50 Hz 4 Output of the phase shifter for external X deflection of the electron beam in the oscil- (8) 2

3 Fig. 4: ESR power supply loscope. Output 4 must be connected by a BNC cable to the X input of the oscilloscope connected to 7. 5 Input for the ESR signal supplied by output 22 of the resonator. After internal amplification the ESR signal is available from outputs 6 and 7. 6 Amplifier output for the ESR signal for connection of a 1V- voltmeter. When button 9 ("~") is depressed, the output is switched off. 7 Amplifier output for ESR signal, for connection of an oscilloscope (through BNC cable). 8 Push button "Brückenabgleich" (bridge balancing) Press if an approximately correct bridge balance is to be determined priror to measurement. 9 Push button "~" Press if oscilloscopic display o n l y is to be used. 10 Push button " " Press if oscilloscopic display and voltmeter indication are to be used simultaneously. 11 Control "Amplitude" for adjusting the height of the ESR signal. 12 Control "Nullpunkt" (zero) By means of this knob the working point of the internal amplifier is suitably adjusted ( provided that push button 8 or push button 10 has been depressed). It should be noted that the pointer should stay within the voltmeter range. If the range is exceeded towards negative or positive values, the amplifier is no longer operating under the required working conditions. 13 Control knob "Phase" for shifting the phase condition of the AC voltage across output 4 used for X deflection. 3.2 ESR-Resonator Input for the high-frequency ac taken from output 4 of the ESR Operating Unit. 22 Output for the ESR signal voltage fed to input 5 of the ESR Operating Unit for amplification. 3

4 23 Terminals of one of the Helmholtz field coil s (I max. 1,5 A) 24 Terminals of the second Helmholtz field coils 25 Control knob"c" for adjusting the capacitance of the oscillating circuit, i.e. for tuning the HF oscillating circuit. 26 Control knob "R" for adjusting the bridge resistance R 27 Capsule containing the absorbing specimen. After disengaging the bayonet lock, the capsule can be pulled out. research, educational and training facilities (schools, universities, institutes and laboratories). This means that in such an environment, no mobile phones etc. are to be used in the immediate vicinity. The individual connecting leads are each not to be longer than 2 m. The instrument can be so influenced by electrostatic charges and other electromagnetic phenomena that it no longer functions within the given technical specifications. The following measures reduce or do away with disturbances: Avoid fitted carpets; ensure potential equalization; carry out experiments on a conductive, earthed surface, use screened cables, do not operate high-frequency emitters (radios, mobile phones) in the immediate vicinity. Following a blackout failure, operate the on/off switch for a reset. After a total blackout, carry out a "Reset" (new start) of the complete system. This instrument corresponds to Class A, Group 1 of the EN Standard and can only be operated without limitation outside of residential areas. Should electromagnetic disturbances occur in surrounding residential areas although operation is limited to the technical room of a school or other training facility, then it can be demanded of the operator that he carries out adequate measures (e.g. screening, greater distance, reduction in the operating time) at his own cost. Fig. 5: ESR resonator NOTES ON OPERATION This instrument is only to be put into operation under specialist supervision in a controlled electromagnetic environment in 5 HANDLING Circuit arrangement and wiring are shown in Fig. 5 and 6. The Helmholtz coils of the resonator are joined in series by connecting a red socket with a blue one. The ripple-free dc operating voltage is taken from a stabilized power supply. (0...16V / 1,5 A). For display of the ESR signal on an oscilloscope screen, an ac voltage of 2 to 3 volts, taken from from the 2 V~ output of the power supply. A 3 A- ammeter is connected in the coil circuit for measuring the resonance current I r. Input 3 of the phase shifter is connected with the 6 V~ output of the variable transformer, and output 4 of the phase shifter is connected with the oscilloscope input for external X deflection; output 7 is connected to the Y input of the oscilloscope (amplification of mv/cm); output 6 is connected to a 1 V- voltmeter. 5.1 Tuning the HF bridge Prior to measurement the HF bridge must be tuned, whereby current should not be allowed to flow through the resonator coils. Knob 26 "R" on the resonator should be about in the middle position. With push button 8 depressed, knob 12 "Nullpunkt" of the operating unit should be adjusted so that Fig. 6: Experimental set-up 4

5 the pointer of the voltmeter is within the range ,2 V of the scale. Tuning of the bridge oscillating circuit is then effected by tuning knob 25 "C" of the resonator until the pointer of the voltmeter shows minimum deflection, the sensitivity being highest near the beginning of the scale. Usually the pointer deflection will exceed the scale zero to the left. The pointer must then be brought back into the scale range by control knob 12, which, however, has no influence on the tuning state of the oscillating circuit. After this, the tuning procedure must be continued with knob Measuring I r with a voltmeter After the HF bridge has been balanced (push button 8 depressed), measuring of the resonance current can be done by pressing button 10 and setting control knob 11 Amplitude at it extreme clockwise position. The voltage applied to the voltmeter will again at first be outside the range of indication. This can be corrected by turning knob 12, thereby bringing the pointer preferably into the middle portion of the scale. With push button 10 depressed, the sensitivity of adjustment is equal throughout almost the entire scale range(up to approx. 0,8V). It is recommended to now check the bridge balance by slightly varying the position of knob C. To measure I r, the coils must be energized by pure dc voltage, the ac source being 1,1 A und 1,3 A there is a stepp rise and fall of the pointer deflection dure to resonance absorption in the specimen. For varying the current, the control knob A on the Stabilized Power Supply is turned clockwise as far as it will go, and the voltage is varied by control knob V. The current producing the maximum pointer deflection is the resonance current I r, which is used according to Eq. (8) to determine the Landé faktor g. Note: It may be checked wether the pointer deflection due to the ESR signal increase when the position of the knob "R" is changed. For a different knob position the bridge balance must be readjusted (with coils dead). 5.3 Measuring I r with an oscilloscope After the HF bridge has been balanced (push button 8 depressed), press buttom 10 and set knob 11 "Amplitude" at ist extreme clockise position. The voltage now applied to the voltmeter will again be outside the measuring range. This is corrected by turning knob 12, bringing the pointer preferably into the middle portion of the scale. To enable the ESR signal to be displayed on an oscilloscope screen, a low voltage ( V) must be superimposed on the pure dc voltage used obtained from the V~ output of the variable transformer. The pulsating direct current, I = I~ + I- passes periodically through the resonance value B r (cp. Fig. 7). This obviously occurs twice during each ac cycle, i.e. once during rise and once during fall. Each time B = B r, the bridge circuit supplies an ESR signal U S. If a dc portion of suitable magnitude is used, B oscillates s y m m e t r i c a l l y about the resonance flux density B r, and the ESR signals coincide with the middle of the leading and trailing edges of the lux density curve. In this case the mean current I = I- indicated by an ammeter is equal to the resonance current I r. Fig. 7: Tuning the HF bridge The relation between the instantaneous values U S(t) and B(t) can be displayed by an oscilloscope trace, if the X deflection of the electron beam is effected externally by an alternating voltage which is in frequency and phase conformity with the alternating current portion I~ of the coil current. For this purpose, 6 V~ from the variable transformer applied via the phase shifter in the operating unit to the X-input of the oscilloscope. The sweep line thereby produced must be located horizontally and appear to be exactly symmetrical in relation to the ruled display area of the screen (oscilloscope control knob for horizontal displacement). For varying the direct current, knob A of the stabilized power supply is turned clockwise as fast as it will go, and the voltage is increased by means of control knob V until the ESR signal appears on the oscilloscope screen as a bell-shaped curve for both the forward trace and the retrace of the electron beam. The two signals may be slightly different in shape and amplitude. For determining I r, the mean lines of both signals are made to coincide by adjusting control knob 13 "Phasenschieber", and the signal traces are positioned exactly in the middle of the ruled display area by suitable adjustment must be repeated alternately several times. The oscilloscope trace can be used to achieve an accurate bridge balance by adjusting knob C until the ESR signal appears with optimum s y m m e t r y. If 5

6 the coinciding mean lines of the two signals are in coincidence with the middle line of the ruled oscilloscope display area, the current I = I- indicated by the ammeter equals the resonance current I r. 6 TECHNICAL SPECIFICATION ESR resonator with field coils Specimen substance Diphenylpicrylhydrazyl 1 g Resonator frequency ca. 146 MHz Quality factor of the resonator ca Radius of the coils (Helmholtz-coils) 5,4 cm Turns 250 ESR power supply Connecting voltage 230 V Sheet metal housing with carrying handle Tray area (mm) 225 x 232 x MATERIAL Description Art.-No. ESR resonator with field coils ESR power supply Digitalmultimeter Power supply, universal Variable transformer, 25 V~/20 V-, 12 A Oscilloscope 30 MHz, 2-channel Screened cable BNC, l = 750 mm (4x) Adapter BNC-socket/4-mm--plugpair Fig. 8: Modulation of the magnetic field Note If a measurement is made with push button 10 depressed, care should be taken to ensure that the value measured by the voltmeter does not exceed the range provided, otherwise the signal amplitude of the oscilloscope trace will be too small or will disappear entirely. If, however, push button 9 has been depressed, the correct working point of the amplifier will always be adjusted automatically. It may be tried whether the signal amplitude can be increased by changing the adjustment of knob R. If the absorbing substance is removed from the resonator, no ESR signal will be observe while the field current is being varied. 8 NOTES ON GUARANTEE We guarantee the instrument supplied by us for a period of 24 months within the EU, or for 12 months outside of the EU. Excepted from the guarantee are damages that results from disregarding the Operating Instructions, from improper handling of the instrument or from natural wear. The manufacturer can only be held responsible for the function and technical safety characteristics of the instrument, when maintenance, repairs and alterations to the instrument are only carried out by the manufacturer or by personnel who have been explicitly authorized by him to do so. 9 DISPOSAL The packaging mainly consists of environmentally-friendly materials that should be returned to the local recycling stations. Do not dispose of this product with normal household waste. If this unit needs to be disposed of, please return it to the address that is stated below for proper disposal. PHYWE Systeme GmbH & Co. KG Customer Service Robert-Bosch-Breite Göttingen Germany Tel. +49 (0) Fax +49 (0)