Contains research objectives and reports on five research projects.Signal Corps Contract DA36-039-sc-73014Signal Corps Contract DA36-039-sc-7489

Collins, S. A., Jr.

Davis, C. F., Jr.

Faughnan, B. W.

Josephson, B., Jr.

Kyhl, Robert L.

McNicholl, J. J.

Park, D. J. M., Jr.

Strandberg, Malcom W. P.

Venkatesh, H. G. R.

Wolga, G. J.

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IV. MICROWAVE SPECTROSCOPYProf. M. W. P. Strandberg B. W. Faughnan J. J. McNichollProf. R. L. Kyhl J. G. Ingersoll I. J. MayerDr. C. F. Davis, Jr. B. Josephson, Jr. D. J. M. Park, Jr.Dr. B. V. Gokhale J. D. Kierstead W. J. SchwabeDr. G. J. Wolga S. Krongelb V. L. TeplitzS. A. Collins, Jr. R. D. Mattuck H. G. R. VenkateshD. H. Douglass, Jr. J. YacusRESEARCH OBJECTIVESThe microwave spectroscopy laboratory studies matter - gases, liquids, and solids -by examining the interaction of matter with radiofrequency and microwave-frequencyradiation. Much of the work at the present time uses paramagnetic materials, either asimpurities or as the total matter, to provide the link between the electromagnetic radia-tion and the matter.This work leads to an understanding not only of the energy levels of the system itself,but also of the coupling between the nearly commuting elements that form the total sys-tem. Thus, in paramagnetic solids we have been studying not only the appropriatedescriptions of the paramagnetic states of the solids, but also the appropriate descrip-tion of the coupling between the spin states and the lattice modes of vibration.Although considerable effort has been placed upon understanding the application ofparamagnetic crystals to the new field of quantum-mechanical amplifiers, particularlyupon understanding their operation and characteristics, such as gain-bandwidth productand noise figure, we have also investigated the operating properties of these devices.Operation of quantum-mechanical amplifiers, measurement, and development of afurther understanding of their basic properties will continue. The impact of this workon modern technology is so dramatically imminent that an examination of our work andthe work of others in this field demonstrates how short the time interval between basicresearch and technological integration can be made.M. W. P. StrandbergA. AMMONIA MASERThe parts of the maser were tested, and the microwave circuits and electronicequipment were set up for the detection of the extremely small signal from the ammoniabeam. The superheterodyne method of detection has been employed with a tuned ampli-fier for the intermediate frequency at 40 me. Highly attenuated 23.87-kmc power issupplied to the maser cavity and reflected from it. The local oscillator operates at23.91 kmc. The signals from the maser and the local oscillator are mixed in the crys-tal. The signal with the frequency difference of 40 mc is amplified and observed on anoscilloscope.The maser cavity has a resonant frequency of 23.88 kmc under normal conditions(temperature, 250 C). It is cooled inside the vacuum chamber to liquid-nitrogen tem-perature and electrically heated so as to resonate at the 3-3 inversion frequency ofammonia. Approximately 4. 5 watts are supplied to the cavity heater for this purpose.This work was supported in part by Signal Corps Contract DA36-039-sc-73014.(IV. MICROWAVE SPECTROSCOPY)The beam source inside the vacuum chamber is supplied with a power of approxi-mately 5 watts for heating, in order to prevent the ammonia from condensing on thechannel source. The ammonia beam is formed in the vacuum chamber by bubbling thegas through the Octoil at the rate of one bubble per second, so that there is a pressureof approximately 0.3 mm behind the channels of the beam source.Search for the emission line is being carried out.H. G. R. VenkateshB. PARAMAGNETIC -RESONANCE SPECTROMETERA paramagnetic -resonance spectrometer that can be made preferentially sensitiveto either the paramagnetic absorption or the associated paramagnetic dispersion hasbeen constructed. The conventional spectrometer that was previously used is insensi-tive to dispersion because the microwave oscillator is frequency-"locked" to the samplecavity. In this new spectrometer the microwave oscillator is "locked" to an auxiliarycavity. The absorption modulated signal from the matched sample cavity contains termsproportional to X' and x", the absorption and dispersion. Preferential detection ofeither of them is achieved by mixing the signal with a reinjected carrier of adjustablephase before detection by the microwave crystal.The theory of operation of this spectrometer has been worked out and is beingchecked experimentally. Its sensitivity as compared with the conventional spectrometerthat is in use will be obtained, and an absolute sensitivity determination for both systemswill be made.G. J. Wolga, B. Josephson, Jr.C. VERSITRON 1800 -PULSE TECHNIQUEPartial population inversion has been achieved on crystals of ammonium chromealum and gadolinium ethyl sulphate at liquid-helium temperature and at a frequency of9000 me. Since the resulting negative resistance was not sufficient to overcome cavitylosses, gain was not achieved. Emphasis has been placed on improving the short-pulsegenerator and the detection equipment.The short-pulse generator uses a resonant length of microwave transmission lineas a pulse-forming network. This resonator is discharged by means of a spark-gapswitch. The first version of this pulser used a switch that was triggered by a micro-wave signal from the same magnetron that excited the line resonator. The triggeringsignal was delayed by a microwave delay network. This first system was used for thefirst inversion runs. It offered economy of components but was never entirelyThis work was supported in part by Signal Corps Contract DA36-039-sc-74895.(IV. MICROWAVE SPECTROSCOPY)satisfactory. The three-point spark gap that was used was critical to adjust.The present short-pulse generator uses video triggering of the microwave spark.The gap terminals are hemispheres with a trigger pin inside one terminal. Reliableoperation was achieved with this system and the pulser is considered satisfactory.Although the microwave pulse length has not been measured, there is reason to believethat it is less than 10 msec.No further development of this short-pulse generator is planned. The minimumpulse length of such a pulser is determined by the spark rise time. Research on thisaspect of the problem might be undertaken at some future time if it seems necessary.A detailed description of the short-pulse generator follows.An extremely short (millimicrosecond) pulse is formed by storing microwave energyin a cavity and then discharging it rapidly to the load. A schematic diagram of the short-pulse generator is shown in Fig. IV-1. The cavity is formed from a length of waveguidewith a short circuit at one end and a magic tee at the other. Power from a magnetronis coupled in through an iris on the side of the cavity at a point located near the magictee. By construction, arms 3 and 4 of the tee are identical. The bridge thus formedis balanced, and an effective short circuit is provided at that end of the cavity. A photo-graph of the magic tee is shown in Fig. IV-2; a cross-section drawing, with a detail ofthe spark-gap arrangement, in Fig. IV-3.Energy is fed into the cavity until equilibrium fields are reached. This time coin-cides with the end of the magnetron pulse. A spark is fired at a point located at an oddnumber of quarter wavelengths from the end of one of the arms of the magic tee (arm 3in Fig. IV-2) and unbalances the bridge. Ideally, the tee (and load) with the gap, whenfired, presents a match to the resonant section of the line. The energy in the cavity,considered as two waves traveling in opposite directions, then flows out of arm 2 to theload. The pulse duration is equal to twice the time that it takes for a wave to travelfrom one end of the cavity to the other. Different pulse lengths are obtained by usingdifferent lengths of waveguide for the cavity.The spark in arm 3 is set off through a small hole in the gap by a trigger spark out-side the waveguide. The trigger spark is fired by a conventional trigger generator. Aniris is placed around the main spark gap to form a resonant slot.The sample cavity which was used for all of the runs is a cylindrical cavity withtwo orthogonal polarizations of the TM 1 1 0 mode. It is filled with fused quartz to bringdown its size to the dimensions of the cryostat. Two waveguides are used, with onecoupled to each mode. The crystal is mounted along the cavity axis, in which positionit is in the magnetic-field maximum. The microwave magnetic fields of the two modesare normal to each other and to the dc magnetic field. The mode used for measurementof susceptibility has a Qo of approximately 5000 at room temperature. The spin-flippingmode is loaded by the waveguide to a Q of 40 so that it can respond to very short pulses.MAIN SPARKOUT TO LOADMAGIC TEE (ARMS I AND 2 AND 3 AND 4ARE ELECTRICALLY EQUIVALENT)Schematic diagram of short-pulse generator.> FROM TRIGGER SPARK SOURCEFROM MAGNETRON-- TO LOADTO CAVITYFig. IV-2. Photograph of short-pulse generator (without cavity).POWER IN FROMMAGNETRONFLANGECAVITYFig. IV-1.~_f __ __ __ _FMAGNETRONSECTION A-AMAIN SPARK GAPTRIGGER SPARK GAPSECTION B-BFig. IV-3. Cross-section view of short-pulse generator, with detailof spark-gap arrangement.HELIUM DEWARNITROGEN DEWARCAVITYFig. IV-4. Schematic diagram of equipment.(IV. MICROWAVE SPECTROSCOPY)Dewars are arranged so that measurements can be made at liquid-helium temperature.Figure IV-4 is a schematic diagram of the experimental apparatus.The method of observation was that of making conventional impedance measurements.No modulation procedures are necessary, since a crystal which is capable of amplifi-cation must have an easily detectable paramagnetic resonance. Point-by-point meas-urement was found to be time-consuming and was made more difficult by line-lengthdrifts as the helium evaporated. To meet this problem, an electronic Smith-chartplotter was developed. Commercial Smith-chart plotters are not fast enough to followthe relaxation time T 1 of the crystal. The system developed here uses 30-mc circuitryand has a response time of a few microseconds. The finishing touches are being put onthe device; a detailed description will be given in a later report. A crudely constructedmodel has already demonstrated its worth by displaying some unsuspected features inthe spin-flipping behavior of chrome alum. Its operation will now be described.The output of a klystron is modulated at 30 mc - one sideband and the carrierbeing separated for use as the measuring signal and local-oscillator signal in asuperheterodyne detection system. The measuring signal is applied to the cavity,the reflected wave going to the mixer of the i-f system. The 30-mec signal fromthe i-f amplifier is split and detected with the in-phase and quadrature signalsfrom the 30-mec oscillator. These signals give the in-phase and quadrature com-ponents of the reflection coefficient, which are applied to the two axes of an oscil-loscope.This Smith-chart plotter has a much faster response time than existing commercialmodels. This speed is necessary for observing the rapidly varying spin population inpulsed paramagnetic experiments.Several flipping runs were made with the old pulser on gadolinium ethyl sulphateand ammonium chrome alum. The change in cavity Q immediately after the applicationof the flipping pulse indicated that an inverted population had been achieved. For gado-linium ethyl sulphate, the best run gave a negative temperature of approximately - 11 Kfor the paramagnetic transition that was being flipped, compared with the temperatureof -40 K which would result from complete population inversion. We estimate that atemperature of -4. 80 K will be necessary for oscillation with the spin concentration andcavity Q that were used.One run was made on the same chrome alum crystal with the new pulser andthe Smith-chart plotter. It indicated that a significant contribution to the suscep-tibility of the sample was coming from two neighboring lines of the paramagneticspectrum.The next objective is, clearly, to obtain paramagnetic crystals with narrower linesthan those of the crystals that we used in these first attempts.R. L. Kyhl, S. A. Collins, Jr., D. J. M. Park, Jr.(IV. MICROWAVE SPECTROSCOPY)D. SOLID-STATE QUANTUM-MECHANICAL AMPLIFIERAn S-band solid-state quantum-mechanical amplifier has been operated at 4. 20 Kwith a computed noise temperature of less than 4. 50 K. There are indications thatoperation at a still higher temperature is feasible and is potentially capable of yieldinghigher output powers. This device has operating characteristics similar (e.g., gain-bandwidth product) to the one previously built by McWhorter and Meyer (1).Considerable information was derived by noting the orientation and appropriateselection rules for the pumping frequency. McWhorter and Meyer saturated a line thatwas almost degenerate with the pumping transition; this saturated line, in turn, satu-rated the pumping transition. This effect can best be accounted for by demonstratingthat a narrow-band lattice-phonon saturation can effectively couple the two transitions.By analyzing the operation of another maser (2), it is possible to estimate that phononsaturation occurs in a band less than 600 mc wide.M. W. P. Strandberg, C. F. Davis, Jr., B. W. Faughnan,R. L. Kyhl, G. J. WolgaReferences1. A. L. McWhorter and J. W. Meyer, Phys. Rev. (to be published).2. H. E. D. Scovil, G. Feher, and H. Seidel, Phys. Rev. 105, 762 (1957).E. GADOLINIUM ETHYL SULPHATE SPECTRUMThe paramagnetic-resonance spectrum of the trivalent rare earth ion, gadolinium,modified by its crystalline surroundings, is being investigated. The ground state of thefree ion is S7/2 and the effect of a crystalline potential with hexagonal symmetry is tobreak down the eightfold degenerate ground state into four doubly degenerate states,these final degeneracies being removed by an external magnetic field.Expansion of the crystalline potential as a sum of spherical harmonics and assump-tion of a crystal symmetry that is characteristic of the C3v group enables us to describethe interaction of the free ion with its surroundings and with an external magnetic field,by means of the following "spin Hamiltonian"H = gPH + B2P 2 + B4P 4 + B4P 42 4 4 44oo 33 66+ B6P 6 + B6P + B6P 66This work6 6 6 6in part by Signal Corps Contract DA36-039-s6This work was supported in part by Signal Corps Contract DA36-039-sc-74895.(IV. MICROWAVE SPECTROSCOPY)3 3The effect of including the off-diagonal elements in the energy matrix, the P4' P6'and P6 terms, should become apparent when the magnetic field is normal to the crystalsymmetry axis. We hope to make a quantitative estimate of the magnitude of theseterms by comparing an exact diagonalization of the energy matrix with the observedspectrum.J. J. McNicholl

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Microwave Spectroscopy

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