Contains reports on four research projects

Adams, N. I., III

Arnold, R. D.

Bitter, F.

Dickinson, W. C.

Feld, B. T.

Gokhale, B. V.

Hillger, R. E.

Ingersoll, J. G.

King, G. W.

Kip, A. F.

Knight, G.

Lawrance, R. B.

Loomis, C. C.

Strandberg, Malcom W. P.

Weiss, M. T.

Wimett, T. F.

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IV. RADIO-FREQUENCY SPECTROSCOPYA. MOLECULAR-BEAM RESEARCHProf. J.R.Zacharias T.M.Hahn,Jr.Prof. B.T.Feld J.LevinsonDr. D.E.Nagle H.LewL.Davis,Jr. C.W.ZabelThe experimental work of the molecular-beam laboratory will be reportedin detail in the next Progress Report.1. Theory of Nuclear Effects in Microwave SpectraA theoretical investigation of the effects of nuclear electric quad-rupole moments on the rotational spectra of slightly asymmetric rotor mole-cules has been completed, and a report is being prepared for publication inthe near future. The results of this investigation have been summarized inprevious progress reports and presented at a meeting of the AmericanPhysical Society(l ). The approximation of asymmetric rotor wave functions,in terms of linear combinations of symmetric rotor wave functions and anasymmetry parameter, leads to an analytical expression for the fine structuresplitting of the spectral lines. Our results are a special case of thegeneral expression given by Bragg , and allow an evaluation of the quad-rupole coupling constants where the molecular constants have previously beendetermined. The results of this investigation should prove particularly use-ful in identifying the microwave spectral lines of molecules for which theapproximations are valid. Prof. B.T.Feld, G.Knight2. Theory of Nuclear Electric Quadrupole Moments(3)Values of nuclear electric quadrupole moments show a very widespread. However, when these values are reinterpreted in terms of the eccen-tricities of the corresponding nuclei, the spread is much smaller.Consider an ellipsoid of revolution about an axis whose length is b,the other two axes having length a. For a uniform charge distribution oftotal charge Z and volume V, the electric quadrupole moment isQ =Z (3Z2 - r2 ) dV = Z(b2 - a 2 ).1. G.Knight and B.T.Feld, Phys. Rev. 74, 354(A)(1948).2. J.K.Bragg, Phys. Rev. 74, 533(1948.3. B.T.Feld, Preliminary Report No. 2, Nuclear Science Series, NationalResearch Council, 1948.-19-(IV. RADIO-FREQUENCY SPECTROSCOPY)We define the ellipticity as:b-aand take the nuclear radius to be (for small ellipticities)R = = 1.5 A/3 x 10- 1 3 cm oThen,_ = _ ZR2 ;= + 1 for a pencil,£ = - 1 for a penny.The deduced values of c are plotted in the accompanying curve, onwhich the circles indicate odd proton and the crosses odd neutron nuclei.An attempt is being made to correlate the observed quadrupole momentwith the orbital angular momentum of the odd particle as indicated by the(1), (2)measured magnetic moment( . The figures besides the points are thenuclear spins.ATOMIC NUMBER IFig. IV-1. Deduced values of c.1. E.P.Wigner and E.Feinberg, Reports on Progress in Physics, A, 274,London(1942).2. T.Schmidt, Zs. f. Phys. 106, 358(1937).-20-I _ 1 -I--4(IV. RADIO-FREQUECY SPECTROSCOPY)B. MAGNETIC NUCLEAR RESONANCEProf. F.Bitter W.DickinsonN.Adams T.Wimett1. Nuclear Resonances at Low TemperaturesApparatus for the observation of nuclear magnetic resonances at liquid-helium temperatures is being designed and assembled. A small magnet weigh-ing about 30 pounds and designed to be operated in liquid air has beencompleted. N.Adams2. Nuclear Resonances at Room TemperaturesThe measurement of new nuclei requires an improvement of the signal-to-noise ratio now available. In addition to the low-temperature experimentsmentioned above, three different ways of making such an improvement arebeing investigated:(a) by increasing the volume of the sample;(b) by increasing the concentration of nuclei being studied;particularly by using fine metal powders, if possible in the moltenstate;(c) by making use of transient signals as previously discussed (1 ).Prof. F.Bitter, W.Dickinson3. Deuteron-Proton Moment RatioThe factors affecting the accuracy with which this important ratio canbe measured have been studied. The limits are primarily determined by theobserved line width, which is set by the homogeneity of the magnetic fieldused, and the symmetry and constancy of the line shape which is determinedby the stability of the r-f bridge circuit used. A Helmholtz coil, designedto produce fields of about 1,200 gauss uniform to a few parts in a millionin a volume of several cubic centimeters, is being built. The circuit-stability problems are being studied. T.Wimett-21-1. R.L.E. Quarterly Progress Report, July 15, 1947, page 26.(IV. RADIO-FREQUENCY SPECTROSCOPY)C. PARAMAGNETIC RELAXATIONProf. A.F.KipR.D.Arnold1. Resonance AbsorptionThe measurement of resonance absorption in CuSO4 *5H20 andCuK2 (S04 )2 6H20 single crystals in a static magnetic field has beencompleted. An RLE technical report (No. 61) is in press and an articlehas been submitted to The Physical Review.2. Absorption in the Absence of a Static Magnetic Field(1)Gorter and coworkers have investigated the absorption in paramag-netic substances at low frequencies and with no applied static magneticfield. Equipment limitations prevented their reaching frequencies of theorder of the reciprocal of the relaxation time. Our next experiment, there-fore, will be to study the behavior of the absorption as a function of fre-quency in this range. The results may shed some light on several as-yet-unanswered questions on the nature of paramagnetic relaxation.Absorption is to be measured by observing the change in Q of a resonantcavity in which a sample is placed. A cavity designed to be adjustable from300 Mc to 600 Mc is under construction, and others for higher frequencieswill follow. Two methods are being considered for separating the magneticand electric losses: (a) to apply a static magnetic field, perpendicular tothe r-f magnetic field, sufficiently large to quench the magnetic absorption,or (b) to move the sample in the cavity from a region of high magnetic fieldto one of high electric field.D. MICROWAVE SPECTROSCOPYProf. G.G.Harvey J.G.IngersollProf. A.G.Hill H.R.JohnsonProf. M.,W.P.Strandberg G.W.KingJ.R.Eshbach R.B.LawranceB.V.Gokhale C.C.LoomisR.E.Hillger M.T.Weiss1. GeneralA good deal of the work of the past three months has been devoted toequipment repair and construction. A high-temperature waveguide system has-22-*1~1 "1-^1 -- ---y ~ ---rs~-~-- r- -^rr^_ ---rrr~-~ --_---ra~-r~..l~---~--r(IV. RADIO-FREQUENCY SPECTROSCOPY)been constructed for use with high-boiling-point molecules, and for enhanc-ing the population of the higher vibrational states. This system, becauseof mechanical limitations, can by used only up to 18000C, and improvementsfor higher-temperature operation are now under way. This system will beused first for further work on OCSe, and for work on the spin of Hg isotopes.M.T.WeissA cavity system is being prepared for use with rare molecules, and forZeeman spectra. Some difficulty has been experienced in introducing mechan-ical modulation. Balanced phase detectors are used to eliminate the ampli-tude modulation so produced. This system will probably be used first withmolecules containing isotopes of Cobalt 59 and 60. C.C.LoomisWork on molecules containing tellurium has been carried on with nopositive results. Search for lines of the (CH3)2 Te molecule in the regionof the spectrum where such lines might be expected to lie has yieldednothing so far. Recent misbehaviors of the spectroscope may be due to break-down of the molecule, and a resulting deposition of Te on the insulators.Other possible molecules, namely OCTe, SCTe and HDTe are probably unstable.R.E.HillgerThe Cs experiment reported in some detail in the October 15 ProgressReport is now completed, and an account is being written for publication.Work on the high-pressure spectrum of 02 has been written up forpublication. Work on the low-pressure and Zeeman spectra is starting.M.W.P.Strandberg, B.V.Gokhale, J.H.Ingersoll2. Spin of BoronSome work on borine carbonyl (H3BCO) is in progress. The J = 1--2transition for this molecule has been reported by Gordy, Ring and Burg(l)They obtain a good fit to the observed quadrupole structure by assuming aspin for boron of 3. There is interest in checking this for the J = 0--)1transition where only a simple triplet structure is to be observed. Thespin in this case is determined from the fine-structure spacing and relativeintensities. We have observed the J = 0-+1 transition for H 3BCO but,as yet,have not resolved any fine structure greater than about 1 Mc/sec. Rota-tional transitions have been observed with the molecule in excited vibra-tional states and the Stark effect and dipole moment determined. The dipolemoment is of the order of 0.5 debye units.1. W.Gordy, H.Ring, A.Burg, Phys. Rev. 74 1191(1948)-23-(IV. RADIO-FREQUENCY SPECTROSCOPY)The detection system used in those experiments is very sensitive, com-pared to published figures for systems elsewhere. The sensitivity is about10- 8 cm 1 with a bandwidth of 1000 cps. We have narrowed this bandwidth toabout 10 cps where the minimum detectable a is 10-  cm- 1 . Figures ofmin = 5 x 10 - 0 m- 1 , for bandwidth of a fraction of a cycle per second,have been reported. The minimum discernible a is proportional to the band-width in our system since phase is preserved in all detection processes.Briefly, the system combines a high-frequency FM modulation which is muchgreater than the line breadth, with a Stark-absorption modulation. The sig-nal is detected in a crystal and amplified at the FM modulation frequencyplus or minus the Stark-modulation frequency. The theory of the system istoo complex to be described here, but it will appear as a technical reportin the near future.It is hoped to finish work on H3BCO in the immediate future and then toproceed to finish up work pending on formaldahyde, ketene, and pyridine.M.W.P.Strandberg3. Hyperfine Structure of Atomic HydrogenAs previously stated, the purpose of this experiment is to study, bymeans of Zeeman effect, the absorption by atomic hydrogen of microwaves inthe region of 1421 Mc. The absorption coefficient has been calculatedtheoretically and comes out to be of the order of (8.6) 10- 9 C cm- 1  or(3.7) 10- 3 c db/km, where e is the fraction of hydrogen atoms which can bemaintained in the cavity. The formula for a completely resolved Zeeman com-ponent is2e2h2 .1j 1 Nv2apeak 12m2 c3 k 0o 21 + 1 TAwhere N is the number of atoms per unit volume, I is the nuclear spin( for hydrogen), Av is the half-breadth at half intensity, and the transi-tion dipole matrix element Pij/po is a numeric, equal to unity for hydrogen.The principal uncertainity in the numerical value of a comes from thekinetic-theory estimate of Av, and a principal experimental difficulty is inkeeping the fraction of atoms large.Comparing this absorption coefficient with that of cesium allows us toestimate that the hydrogen value is about an order of magnitude smaller thanthat for the average well-resolved cesium line. Thus it should be just pos-sible to observe the effect.LII -- L__~..~~^..-l_^_i .._-_1 __^..^ .I(IV. RADIO-FREQUENCY SPECTROSCOPY)The cardinal importance of maintaining a high fraction of atoms hasmade necessary a reinvestigation of agents for poisoning the cavity walls,the electrodes, and the pumping lead. Contrary to expectations, it appearsthat ordinary Lubriseal stopcock grease is quite effective in poisoningeven brass against recombining hydrogen atoms. The apparatus is now beingreassembled.Further study has been made of klystron noise. The low-frequency(26 cps) noise of a klystron is markedly greater on the high-voltage sideof the mode. A method has been devised for measuring the relative amountsof amplitude and frequency fluctuations in the klystron output.R.B.Lawrance4. The Asymmetric Rotora. Extension of the Calculation of Energy LevelsIn a paper submitted to the Journal of Chemical Physics, variousmethods of computing the energy levels of the asymmetric rotor are reviewedfor values of high angular momentum which are required in the interpreta-tion of infrared and microwave spectra. A table of values of J = 11 and12 is given and over the whole range of asymmetry -1) ,< 1. Methods ofusing the Correspondence Principle, Mathier functions, harmonic oscillatormatrices, and power series expansions are discussed, and this survey indi-cates there is always a region of some degree of asymmetry where the exactvalue has to be obtained from the rigorous matrix-mechanical formulation.Methods of obtaining energy values for small intervals in f from exist-ing tables are also discussed.G.W.King-25-

Radio-Frequency Spectroscopy

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Radio-Frequency Spectroscopy

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