Contains reports on nine research projects.United States Department of Energy (Contract DA36-039-sc-73014

Davis, C. F., Jr.

Josephson, B., Jr.

Krongelb, S.

Mattuck, R. D.

McNicholl, J. J.

Peter, M.

Venkatesh, H. G. R.

Wolga, G. J.

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IV. MICROWAVE SPECTROSCOPYProf. M. W. P. Strandberg D. H. Douglas, Jr. R. D. MattuckProf. R. L. Kyhl J. G. Ingersoll J. J. McNichollDr. C. F. Davis, Jr. B. Josephson, Jr. R. E. Norris, Jr.Dr. M. Peter J. D. Kierstead H. G. R. VenkateshS. A. Collins, Jr. S. Krongelb G. J. WolgaD. M. LazarusA. MOLECULAR-BEAM MICROWAVE SPECTROSCOPEA resonant cavity was added to the molecular-beam spectroscope in order to allowmore efficient matching of the microwave power into the sodium chloride beam.The inner dimensions of this cavity (see Fig. IV-1) are: a = 3. 850 inches,b = 5 inches, c = 28 inches. It has a loaded Q of 5000 and is critically coupled to theinput. The two end surfaces of the cavity (a X b) are open to permit entrance of thetwo beams. Mode stability is obtained by means of damping slits, D, in the top andbottom surfaces and by excitation of the proper mode by means of the antenna systemdescribed earlier (1). This antenna excites the surface, A, which is a distributediris consisting of a copper-plated perforated steel plate, 1/16 inch thick, with 3/16inch round holes, whose centers are 5/16 inch apart and staggered. The neoprenespacers in the damping slits D can becompressed by the mounting screws, S,and thus a tuning range of 0.05 per centis obtained. Across the side surfaces,A and B, a homogeneous electric field,E, can be applied for Stark modulation,since the bars, C, are isolated. SurfaceB is shown in Fig. IV-2. The six darklines are created by the 12 molecularD beams coming from the two sources thatwere shown earlier (2).-4 BThe cavity was installed and pre-liminary measurements have been made.b , This system was further improved byadding an i-f amplifier of wider passband0a and better noise figure than the earliermodel had, and by adding a 2K50 klystronthat operates at 26, 050 mc, which wasFig. IV-1. Perspective view of large kindly selected by the Bendix Red BankK-band cavity. Division. A calibrated K-band noiseThis work was supported in part by Contract DA36-039-sc-73014.(IV. MICROWAVE SPECTROSCOPY)Fig. IV-2. Collimation of molecular beam as shown by sodium chlorideplated on one side wall.source that will soon be available will enable us to obtain the noise figure of the systemby direct measurement. This noise figure, together with the study of the moleculartrajectories made by H. G. R. Venkatesh (see Sec. IV-B), will allow a more reliablecalculation of the expected signal-to-noise ratio than has hitherto been possible.M. PeterReferences1. M. Peter, Ph.D. Thesis, Department of Physics, M.I.T., May 16, 1955.2. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T.,Jan. 15, 1957, p. 22; see Fig. IV-1.B. MOLECULAR BEAMS OF ALKALI HALIDESA first step in the development of a technique for the study of molecular energystates, especially the close splittings from nuclear quadripole interaction with orbitalelectrons, is to secure beams of sufficient intensity, predominantly in one quantumstate. A detectable signal from induced emission from excited states could then beobtained.Toward this end, the dispersal of a "white" beam of sodium chloride molecules inan electrostatic field was investigated, taking into account the angular divergence. Thefield was produced by a lattice of charged rods. The field has the property ofbeing exponentially cut off at the fringes of the lattice. Analytical expressions havebeen obtained for the fringe fields and the core field, and for the accelerations and dis-placements of the molecules in several quantum states. Of the nine tables being pre-pared for the numerical evaluation of the formulas, the last two give the accelerations,the actual trajectories, and the directions of emergence of the molecules in differentquantum states.In the calculation of the trajectories the charts for energy of the molecule versusthe field intensity, calculated from quantum theory by Strandberg and Peter-I I L I -- - -- -- -- -- - C-(IV. MICROWAVE SPECTROSCOPY)(to be published), are of the most vital use.An alternative field in a different experiment is being investigated. Two significantproperties of the Strandberg-Peter charts for electric dipolar molecules are important:the vanishing of the force on the molecules at certain fields, and the linearity of theenergy-field relation for a wide range of fields.H. G. R. VenkateshC. VERSITRONWe are testing materials for use in the Versitron study. For this purpose, equip-ment has been built to measure the relaxation times (T 1 's) of paramagnetic salts.These are observed simply as the exponential decay after a saturation (magnetron)pulse. Although this technique is applicable only to materials with T 1 that is longcompared with a magnetron pulse, they are the ones required for Versitron operation.Data and oscilloscope pictures were taken on dilute ammonium chrome alum atliquid helium temperature. Based on the resonance recovery to thermal equilibrium,the relaxation time is roughly 4 msec.Two electromagnets are being designed. These will have pole diameters of 10. 5inches and operating fields as high as 13 kilogauss. The field coils will be separateand removable to facilitate adjustment and repair. Models have been built and tested.C. F. Davis, Jr.D. INTERACTIONS BETWEEN TORSIONAL AND OVER-ALL ROTATIONS INSYMMETRIC-TOP MOLECULESThe experimental investigation of the effects of hindered internal torsion on thespectrum of symmetric-top molecules was resumed and completed (1, 2). Measure-ments of the J = 1 - 2 rotational transitions were made on a conventional Stark spec-troscope by using a 1N26 crystal driven as a harmonic generator by a 2K33 klystron.The satellite lines resulting from the n = 0, 1, 2 internal torsional states were observedand were found to be in good agreement with Kivelson's theoretical treatment (3). Therequired empirical constants were determined from J = 0 - 1 measurements. Thevariation in dipole moment with torsional state, which was indicated by the second-order Stark effect measurements on the J = 0 - 1 spectrum, was confirmed by thefirst-order Stark effect observations on the J = 1 - 2 spectrum. S. KrongelbReferences1. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., July 15, 1955, p. 19.2. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., Oct. 15, 1955, p. 23.3. D. Kivelson, J. Chem. Phys. 22, 1733 (1955).(IV. MICROWAVE SPECTROSCOPY)E. PARAMAGNETIC RESONANCEWork on irradiated alkali halides has been extended to very lightly irradiatedsamples. This is possible because of the high sensitivity realized in the spectrometer.The advantages of this approach to the problem are that light irradiation minimizesthe extent of crystal damage and that possible interactions between the paramagneticcenters are minimized by the increased average separation of centers. The first ofthese, decreased crystal damage, means that the spectrum of each absorbing centerwill add more nearly coherently (in the magnetic field domain) and thus the complicatedstructure of the resonance can be resolved.Measurements were made on lightly irradiated lithium fluoride and sodium fluoride.The lithium fluoride spectrum can be fitted quite well to the case in which 19 reso-nances are superposed, spaced by an amount comparable to the individual linewidth,and in which the relative magnitude of the lines can be determined from quantumstatistics. Such a spectrum arises from quantum mechanical considerations if weinclude only nearest nuclear neighbor interactions and assume a contact type of hyper-fine interaction AI - J. A perturbation treatment of this problem was carried out tosecond order in the energy.Greater resolution was achieved with sodium fluoride, for which a very extensiveand complicated spectrum was observed. A previous report (Quarterly ProgressReport, Jan. 15, 1957, p. 25) gave results for heavily irradiated samples of sodiumfluoride. With the very lightly irradiated samples the original spectrum of 17 or 19lines splits into hundreds of lines which are grouped in a regular manner, and extendsover a wider range of magnetic field with the spectrum center corresponding verynearly to g = 2. In addition, a definite angular dependence of the spectrum wasobserved. The angle being varied, 0, is between the crystalline electric field for thecubic crystal and the dc magnetic field. The spectrum is invariant to rotations ofo = rr/2. Further experimental work is in progress. A theoretical understanding ofthese phenomena is being sought from crystalline-field effects which would involvespin-orbit coupling, higher-order tensor interactions of the usual magnetic types, andan exact treatment of the perturbation Hamiltonian for the hyperfine interaction.The operating range of the. spectrometer has been extended to low temperatures,in that high-frequency modulation at 6 kc can be used from room temperature down toliquid helium temperature. A solid quartz cavity coated with silver and fed from adielectric loaded waveguide has enabled the reduction of physical dimensions suf-ficiently to allow double dewar operation. Experiments were successfully run at liquidhelium temperature.G. Wolga(IV. MICROWAVE SPECTROSCOPY)F. ATOMIC RECOMBINATIONSA paramagnetic resonance system similar to one already in use (Quarterly ProgressReport, April 15, 1956, p. 20), but using a permanent magnet to establish the field, isbeing set up. It will be used to study the recombination of oxygen atoms by observingthe atomic oxygen resonance as a function of time. By this means, both volume andsurface effects in the recombination process will be studied.S. KrongelbG. MICROMODULATORIt was pointed out (Quarterly Progress Report, Jan. 15, 1957, p. 26) that g-factoranisotropy causes inhomogeneous broadening in polycrystalline samples of diphenyltrinitrophenyl hydrazyl and that this yields a linewidth (> 3 gauss or 9000 mc) that istoo great for oscilloscope display in a conventional Stark modulation microwave spec-troscope. A method of circumventing this difficulty has been developed. It consists oflinearly sweeping the dc magnetic field in synchronism with the klystron sweep. Sincethe line-shape factor is a function of H(t) - 1/,e v(t), where H(t) is the magnetic field,'e is the electron gyromagnetic ratio, and v(t) is the klystron frequency, the normal-ized line-shape factor, g(H - v/ye), can be arbitrarily compressed in the time domainfor display purposes.It can be shown that the absolute intensity of a Stark-modulated absorption can bedetermined if the following product is known:C = C XC XC . XC XE 2display demodulator amplifier crystal 0where the various C's describe the transfer functions of the spectroscope components,and E is the amplitude of the electric vector emitted by the klystron. C can bedetermined from measurements with the micromodulator, and by using the followingequation (which is derived from first-order theory):EH V N Vs(t) dtm  r o 2 1 (c2 awhere= magnetic field sweep rate (gauss/sec)s = klystron sweep rate (cps/sec)-e = electron gyromagnetic ratioE = transfer constant of integrator inserted between demodulator and RC filterH m = amplitude of modulating magnetic field(IV. MICROWAVE SPECTROSCOPY)V r = amplitude of demodulator reference voltagek = Boltzmann constantfi = Planck's constant/2-T = temperature (°K)N o = total number of hydrazyl moleculesa, b = waveguide dimensions in centimetersv = klystron frequencyVs(t) = hydrazyl resonance display in oscilloscope unitsNote that since the only line-shape information entering the derivation of this equa-tion is the normalization of the shape factor to unity, broadening caused by field inhomo-geneity and the polycrystalline nature of the sample have no effect on the result.Remaining aspects of the investigation include consideration of the effects of reflec-tion in the guide, attenuation in the Stark cell and phase shift in the hydrazyl, perfectionof the electronic integrator, construction of a prototype micromodulator that will includea new magnet, and testing the device on several known resonances in ammonia.R. D. MattuckH. TRAVELING-WAVE CAVITYUse of a traveling-wave cavity results in an electromagnetic energy density step-upproportional to the Q of the cavity and makes it possible to utilize the points of purecircular polarization found in a traveling wave. The present form of the cavity is con-structed of rectangular waveguide bent in the H-plane into a circular ring. Coupling tothe main waveguide is achieved through Bethe coupling; ideally, it should yield aninfinite directivity.In order to set up a traveling wave in the cavity, the two degenerate modes thatexist, because of the cylindrical symmetry of the cavity, must remain exactly degen-erate. This is achieved if the directivity of the coupling hole is perfect. For this pur-pose, several tuning screws are introduced into the cavity to compensate for anyimperfection in coupling which might destroy the exact degeneracy of the two modes.The cavity has been analyzed theoretically and experimentally and will be utilizedin resonance studies of paramagnetic materials in the solid state.J. J. McNichollI. PERMANENT MAGNETSThree permanent magnets are now in service. One magnet, with a gap of 1. 5 inches,has been aligned for optimum homogeneity by making a point-by-point plot of the mag-netic field with a proton probe. An extremely accurate plot was made possible byusing a positioner, constructed for the purpose, to hold the proton probe as it was(IV. MICROWAVE SPECTROSCOPY)Fig. IV-3. Proton resonance.moved through the field.Inhomogeneity estimated to be 10 milligauss over a water sample 1/4 inch in diam-eter and 3/8 inch long was obtained in a region 1. 5 cm in diameter at the center ofthe gap. An inhomogeneity of 0. 2 gauss was obtained in a region 3 cm in diameter atthe center of the gap.Figure IV-3 shows a proton resonance obtained in a field of 4290 gauss by using atransitron oscillator (1). Sinusoidal field modulation at 50 cps and peak amplitude ofapproximately 5 gauss was employed. The number of "wiggles" observed is indicativeof the homogeneity of the field.B. Josephson, Jr.References1. W. F. Knoebel and E. L. Hahn, Rev. Sci. Instr. 22, 904 (1951).2. N. Bloembergen,E. M. Purcell, and R. V. Pound, Phys. Rev. 73, 679 (1948)..~-C- ____--~IIII 1 ~------- - -- -- ~L~--=-

Microwave Spectroscopy

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Microwave Spectroscopy

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