Contains research objectives and reports on five research projects

Peter, M.

Strandberg, Malcom W. P.

Wolga, G. J.

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V. MICROWAVE SPECTROSCOPYProf. M. W. P. Strandberg B. Josephson, Jr. P. W. SieckDr. M. Peter J. D. Kierstead H. G. R. VenkatesJ. G. Ingersoll S. Krongelb G. J. WolgaR. D. MattuckRESEARCH OBJECTIVESThe work of this group embraces the study of matter and the interaction of its con-stituent parts as it is exposed by the effect of matter on microwave radiation.Many of the problems in the structure of molecules that may be studied in this wayhave been examined in the past. Work on the analysis of internal torsional states willsoon be terminated. This analysis of torsional states appears to be a great forwardstep, and it brings a completeness to our present picture of molecular rotational state.Investigation of vibration-rotation interaction is continuing in the examination ofthe spectrum of tritiated water. This observed spectrum is being compared with thespectrum predicted by theory from measurements on ordinary and deuterated water.This kind of test is the crucial one by which an analysis should be judged; it should notonly fit observations but it should also lead to satisfactory prediction.Work is being started on the rotational relaxation problem. This is the study ofthe time-dependent interaction between molecules that is caused by translational colli-sions. The effect of these collisions on the observed rotational spectrum will receivespecial attention. The immediate objective is, of course, to observe pressure broad-ening, a type of problem that is persistent in all fields of physics. Nuclear physicistshave discussed it in terms of cross sections for scattering; more recently, for electricexcitation. Gaseous-discharge analysts speak of Stark broadening or electron scatteringproblems. Acoustic physicists study the anomalous dispersion of sound waves attri-butable to the collision relaxation of molecular vibrational states. Chemists interestedin kinetics study the more involved problem of recombination. It is hoped that the appli-cation of the existing general formulation of this collision problem, together with preciseavailable microwave data, will allow general advances to be made in this universallyimportant field.The work on paramagnetic resonances continues to give evidence that investigationsin this field will have greater interest and application in the future.We are or will be studying the paramagnetic spectrum of materials of evanescentexistence-free radicals and atoms. This is the logical development of our very suc-cessful investigation, in the past, of the paramagnetic spectrum of the oxygen molecule.The actual source of the interaction that gives rise to the oxygen triplet structure wasfinally determined by accurately observing the paramagnetic spectrum of 02 and byinterpreting the observational data by means of the refined theory, developed in thislaboratory, of the ground electronic state of the oxygen molecule. It is this tripletstructure that gives rise to the 5-mm atmospheric absorption band. The present workin progress, beside allowing a study of otherwise unobservable systems (for example,the methyl radical), will provide data on atomic recombination rates.Research on molecular-beam microwave spectroscopy, always a specialty of thislaboratory, continues. The present system will soon be operating with alkali halidemolecules, providing us with tremendously accurate and bountiful data with which tocontinue our work on the precise description of molecular energy levels. Here again,molecular electronic wave functions should soon be available to aid in the interpretationof these results.Finally, a conscious effort is always being made to add to, or to improve, our labo-ratory apparatus. It is certainly an important function of laboratory management toactively sponsor the continual upgrading of laboratory equipment. Such an effort pre-vents the occurrence of complete obsolescence at some inconvenient time in the future.In the past year we have developed a greatly improved Stark waveguide cell for gaseousmicrowave spectroscopy. Improvements in the sensitivity of our paramagnetic reso-nance apparatus have also been made. The art of stabilization and control of microwaveoscillators has been advanced by our new phase-locking and frequency-locking systems.(V. MICROWAVE SPECTROSCOPY)We shall make special effort to devise better and simpler microwave-frequency meas-uring apparatus. Studies will be undertaken on quartz secondary-frequency standards,the operation of harmonic converters in the microwave region, and the development ofsimple multiplier chains and reliable high-speed counters.Research of this kind is never started until we are certain that definite improvementcan be made. For example, the laboratory has long needed an absolute absorption meterto measure absolute gaseous absorption. We now have a promising device (alreadynamed the "micromodulator") which is being built to fill this need.M. %%. P. StrandbergA. MOLECULAR MOTION IN ELECTROSTATIC FIELDSThe energy of an electrical dipole of moment [. in a field E is W = 4E cos 0. If[ is induced by the field, i = aE cos 0, a being the polarizability. A polarizable2 2particle then experiences a force, F = W = a cos 0 VE . The paths of polarizableparticles traveling through various electrostatic-field configurations have been studied.The fields were assumed to be generated by rods between infinite parallel plates.In the case shown in Fig. V-lb, for instance, the force is determined by a potential12 2 22 cosh x • sinh ] +[cos -y sin -yU a2 cos 2dipole L 3cosh ) - sin y)where the electric potentialsinh x cos y4 dipole = 2 2cosh Lx - cos LIn Fig. V-la, the deflection angle imposed on a trajectory parallel to the groundedplates, if the particle travels from x = -c to x = +o, is (for small angles)2coso2  L y cos- y- sinL-yy) 2 2 2 L L2 2 Lo sin Lwherecosh x + cos ycosh - x - cos- yand W is the kinetic energy of the particle.The NH 3 molecule furnishes an example of polarizable particles. The value of(V. MICROWAVE SPECTROSCOPY)00(a) (b)Fig. V-i. Electrostatic focusing fields.a cos2 0 depends on the various internal degrees of freedom (1) but it is found that thedescribed configurations and combinations thereof make effective cylindrical lenses,particularly if it is taken into account that for stronger fields a itself depends on E.Mi. PeterReferences1. J. M. Jauch, Phys. Rev. 72, 715 (1947).14B. WEAK-FIELD ZEEMAN EFFECT IN N H 3A comprehensive treatment of the zero-field spectrum of N14H3 has been given byGunther-Mohr, Townes, and Van Vleck (1) and by Gordon (2). Their calculations, basedon a coupling scheme corresponding to successive additions of rotational momentum J,nitrogen spin I n , and spin of the three protons I, yield J + In = Fl; FI + I = F.If a field H z is applied that is weak compared to the magnetic coupling of the protonsin N H 3 , the degeneracy with respect to the magnetic quantum number M F is removed,and each level splits into 2F + 1 sublevels W + AW.For J = 3, and K = 3, the pertinent constants have these values: gN = 0. 40, gM0.48, g, = 5.4, I 1 = 1, I = 3/2.For a--components the selection rules are: AM F = +1; AF = +1, 0; AF = +10;AI = 0; A J = 0, +1, giving rise to 84 energy levels.The intensities of the transitions are known from the intensities of the unsplit linesand the known MF dependence of the dipole elements. The splitting caused by thehydrogen magnetic interaction with molecular rotation measured by Gordon is 60 kc/sec.The effective g-factor is Igeff I< 4.6 kc/sec/gauss. Hence we find that for fields ofapproximately 10 gauss and splittings of approximately 60 kc/sec, the hydrogen inter-action will be quenched and so, with spectroscopes used in the past, this Zeeman effectwas not observable. Measurements of this effect are planned with the molecular-beammicrowave spectroscope.The value of geff indicates a possible 1 kc/sec splitting of the N14H 3 3-3 line in theearth's magnetic field.M. Peter(V. MICROWAVE SPECTROSCOPY)References1. G. R. Gunther-Mohr, C. H. Townes, and J. H. Van Vleck, Hyperfine structure inthe spectrum of N14H 3 . II. Theoretical discussion, Phys. Rev. 94, 1191 (1954).2. J. P. Gordon, Hyperfine structure in the inversion spectrum of N 14H 3 by a newhigh-resolution microwave spectrometer, Phys. Rev. 99, 1253 (1955).C. A SIMPLE FREQUENCY DISCRIMINATORThe circuit shown in Fig. V-2 makes a very simple discriminator of essentiallyconstant peak-to-peak width and easily adjustable center frequency.In a simple analysis, the detector crystals X l , X 2 may be replaced by Ceq. Theinput is connected to a dc grounded source whose impedance is high compared tol/wC eq. For example, the input might be connected to the output of a double-tunedfilter. The capacitors C 3 , C 4 , and C 5 are large rf admittances. The input is thendivided between two impedances.1Z I1 iwC eq 1= + i2 iC2C 2 2eq C( 1 2If w = l,' then Z2 = oo, and the current i flows entirely in X 1 , creating the positive peakshown in Fig. V-3. The negative peak appears when i is flowing entirely through X 2 ;that is, when Z 2( w2 ) = 0; from which we find11 1/2(LC)INPUT2 eqw 2CCeqUTPUTIICeSICeqFig. V-2. Discriminator.T 1Z'(V. MICROWAVE SPECTROSCOPY)0OUT I+2-+1-- 2Fig. V-3. Discriminator characteristic.If necessary, C e might be changed by adding a parallel capacitor. This discrimi-nator, working at wj1 2 = 20 Mc/sec has been used successfully in an afc system thatwe designed recently for the stabilization of klystron sources.M. PeterD. TRAVELING-WAVE CAVITYMicrowave spectroscopy can be carried on by observing the absorption of a wavetraveling through a waveguide, or by observing a standing wave in a cavity. Bothmethods have advantages.In a cavity, a large amount of electromagnetic energy interacts with the substanceunder observation even if little input power is available, because of an energy step-upproportional to the Q-factor of the cavity. In addition, cavities are of smaller sizethan waveguides.On the other hand, the use of waveguides concedes the application of homogeneouselectric fields for measurements of the Stark effect. Also, points of pure circularpolarization are found in a traveling wave. It is therefore of interest to analyze acircuit that permits the step-up of the power level of a wave traveling through a wave-guide. A circuit of this kind is shown in Fig. V-4.Through an x-db directional coupler, a fraction P /Q = 10-10x P of the input poweris coupled into the circuit with the absorption cell. The phase-shifter and attenuatorare then adjusted in such a way that power in the circuit builds up to a level Q Po. Afraction Po will then be coupled back through the directional coupler and will interferedestructively with the input signal so that zero output, is produced, thus presenting uswith a bridge spectroscope.An analysis of this circuit based on an analog circuit for a quarter-wave branched-guide directional coupler (1) shows that the signal power from the Stark modulated bridgeV(V. MICROWAVE SPECTROSCOPY)MICROWAVEABSORPTIONOUTPUTBRANCHED GUIDE COUPLERFig. V-4. Traveling-wave cavity.equals the power that we would get from a conventional Stark modulated cell of the samedimensions, operated with the same energy in the cell. The advantages of the new cir-cuit are the low input power required and the fact that the unused power is bridged outfrom the output.M. PeterReferencesi. R. J. Harrison, Design considerations for directional couplers, Report 724,Radiation Laboratory, M.I.T., Dec. 31, 1945.E. PARAMAGNETIC RESONANCEWork is nearly completed on converting the paramagnetic resonance spectrographfor high-frequency modulation.The revised system will be used to investigate paramagnetic substances in the solidand gaseous states.G. J. WolgaINPUT

Microwave Spectroscopy

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Microwave Spectroscopy

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