Contains reports on three research projects

Lazarus, D.

McAllister, W. W.

Peter, M.

Wolga, G. J.

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IV. MICROWAVE SPECTROSCOPYProf. M. W. P. Strandberg J. D. Kierstead W. W. McAllisterDr. M. Peter S. Krongelb J. J. McNichollS. A. Collins, Jr. D. Lazarus P. W. SieckJ. G. Ingersoll R. D. Mattuck H. G. R. VenkatesB. Josephson, Jr. G. J. WolgaA. PARAMAGNETIC RESONANCEIn previous Quarterly Progress Reports preliminary notes were given on the modifi-cation of the paramagnetic resonance spectrograph to the higher modulation frequencyof 6 kc and 100 kc. This phase of the work is essentially completed, and exploratorymeasurements were made in order to give an indication of the realized sensitivity. Adescription of these measurements and of the experimental equipment follows.1. General Description of ApparatusThe signal source is an X-band klystron (2K25 or VA-203) whose frequency is lockedto the microwave cavity resonant frequency by a feedback-stabilization circuit designedin the Microwave Spectroscopy Laboratory. The microwave absorption is detected asa change in reflected power from the microwave cavity. The absorption is modulatedat 6 kc (in the future it can be 100 kc) by superimposing a sinusoidally varying magneticfield of amplitude 0-40 gauss peak-to-peak upon the dc Zeeman- splitting magnetic field.The modulated absorption is detected in a 1N23B crystal that operates as a lineardetector. The output of the detector is amplified in a 6-kc, tuned (twin-T) amplifierand is then synchronously detected (at 6 kc) and filtered by the 90-cycle remodulation-demodulation scheme that is now being used in microwave spectrographs in thislaboratory. The amplification, detection, and filtering are performed with the 6-kcamplifier and filter amplifier designed in this laboratory for microwave spectroscopy.The filtered output is suitable for oscilloscope or recording-meter presentation. The100-kc modulation apparatus uses a tuned amplifier and a filter amplifier similar tothose described, except for modification to pass 100-kc signals. Details of the equip-ment follow.2. Klystron StabilizerThis unit uses a Foster-Seeley microwave frequency discriminator. The signaloutputs of two 1N23B crystals are superposed in proper phase to yield the discrim-inator characteristic. Each crystal receives the microwave power reflected froma highly dispersive microwave cavity and microwave power with frequency-insensi-tive phase, so that the linear difference in the crystal outputs is a frequency-dis-criminator pattern against a dc background. This signal, phased properly, is usedas the basis of a correction voltage which is applied to the reflection electrode of the(IV. MICROWAVE SPECTROSCOPY)klystron. The microwave Iower splitting is accomplished by means of matched magic-Tdirectional couplers. With this stabilization system, the klystron frequency variationis kept within the order of 10-20 kc, which is well within the linewidth of expectedmicrowave absorptions.The advantages of this stabilization system over the Pound stabilizer, previouslyused in the 50-cycle modulation system, are manifold. The avoidance of i-f circuitry,which is accomplished by our dc system, affords much greater simplicity in the useand maintenance of the system. The physical size and complexity of both microwaveand electronic circuitry is markedly reduced.3. ModulationSinusoidal amplitude modulation of the magnetic field is accomplished by two smallHelmholtz coils mounted securely on the magnet pole faces on either side of the micro-wave cavity. The modulation field is parallel to the dc field, and both, in turn, are per-pendicular to the axial microwave cavity field. The power source for the modulation isa 40-watt power amplifier which is fed by a 6-kc (100-kc) oscillator designed and builtin this laboratory.4. Microwave CavityThe problem of introducing modulation fields of 6-kc and 100-kc frequency into ahigh-Q metallic cavity was solved by designing cavities of very thin (approximately0. 001 inch) metallic wall thickness. They are made of fused quartz with a thin silvercoating. The 0. 001-inch thickness of the silver is large with respect to the microwaveskin depth but small with respect to the skin depth at the modulation frequency. Thecavity used in the exploratory measurements described below operates in the TE011cylindrical mode at 9408 mc and has a measured loaded Q of approximately 7800. Thisparticular cavity has its silver coating on the outside of the cavity with some resultinglosses in the quartz. Higher Q' s are obtainable if the coating is put on the inside of thecavity. The cavity is iris-coupled to the waveguide from one of the flat end surfaces.5. Exploratory MeasurementsIn order to determine the sensitivity of the 6-kc spectrograph, the paramagneticresonance spectrum of oxygen at X-band has been observed, and the results were com-pared with results previously obtained in this laboratory by M. Tinkham. His work(Technical Report 281) gives the detailed field-dependent spectrum as well as calculatedand measured line intensities. The estimate of attainable sensitivity is derived fromthe intensity calculations. The 1/Q increment caused by absorption in a gas-filled cavityis equal to 4 rX" or to (c/w)a, where X" is the imaginary part of the complex suscepti-bility of the gas, and a is the absorption coefficient of the gas for a plane wave. In(IV. MICROWAVE SPECTROSCOPY)paramagnetic resonance the absorption is proportional to X"; in microwave spectros-copy, where a waveguide transmission cell is used, a is the meaningful quantity. Theoxygen line observed in the following calculations was the K = 5; J = 6 - 4; M = -3 -, -2-8line. Tinkham gives 1/Qij = 1. 36 X 10 - 8 for the K = J = 1; M = -1 -+ 0 line. Our valueswill be based on this calculation. The signal-to-noise ratio observed is now, at 6 kc,Sn 5, with a filter amplifier time constant of 0. 01 sec. Tinkham gives 5. 49 for thenrelative intensities of the two lines, with our line the weaker. Therefore I/Qi =(1. 36 10 )/5. 49 = 2. 48 10- 9 . This decrement is larger than the minimum detectabledecrement at 6 kc by the factor (3/0. 01)1/2 SO = 17. 3 S0, since a three-second time-n nconstant filter is available to narrow the bandwidth, and we assume that the criterionfor detectability is a signal-to-noise ratio S = 1. Thusnnmin-92m 48 X 10 -9 = 2. 87 X 10 11inQ 17. 3 X 5 87 10at 6 kc. This corresponds to-12 -12 -1Xmin= 2. 3 X 10 ; a min = 9 X 10 cm6. Measurements with the 100-kc SystemThe crystal noise power is proportional to 1/v, where v is the modulation fre-quency. Therefore, we should expect a gain in signal-to-noise of (100/16)1/2 = 4. 1.Therefore, at 100 kc, the values given above are smaller by a factor 1/4. 1 = 0. 244.G. J. WolgaB. MICROWAVE EMISSION SPECTROSCOPEA state selector added to the molecular-beam microwave spectroscope has increasedthe sensitivity of the device by a factor kT/hv - Z, where Z is the selector efficiency.The upper and lower states of the desired transition are separated in an inhomogeneouselectrostatic field, and the upper states alone remain in the beam. The field is zero inone direction normal to the beam. In this direction, the selector will therefore imposeno velocity component on the molecular beam, and doppler-width reduction is stillachievable.The emission spectrum of ammonia was seen with the spectroscope (Fig. IV-i). Thespectroscope is being adapted for observation of alkali halides.M. Peter(IV. MICROWAVE SPECTROSCOPY)Fig. IV- 1. Emission spectrum of the ammonia 3-3 line.C. SPIN ECHOESEquipment, consisting of a double-pulse generator, a 30-mc gated oscillator, aprobe circuit, an amplifier, and a detector, is being constructed to study spin echoes,especially those of nuclei. The pulse circuit was designed and constructed and it isnow in working order. The gated oscillator is being constructed in accordance with acircuit described in RCA Report 185-10. The amplifier is in working order. The probecircuit has not yet been constructed.D. Lazarus, W. W. McAllister13 I- --

Microwave Spectroscopy

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Microwave Spectroscopy

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