Contains research objectives and reports on five research projects

Bates, V. J.

Brabson, B.

Funkhouser, A. T.

Golub, R.

Johnston, W. D., Jr.

King, John G.

Kukolich, S. G.

Searle, Campbell L.

Stroke, G. W.

Weiss, Rainer

Yaffee, M. A.

Zacharias, J. R.

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IX. MOLECULAR BEAMSProf. J. R. Zacharias R. S. Badessa W. D. Johnston, Jr.Prof. J. G. King V. J. Bates S. G. KukolichProf. C. L. Searle B. Brabson F. J. O'BrienDr. G. W. Stroke R. Golub C. O. Thornburg, Jr.G. L. GuttrickRESEARCH OBJECTIVESThree kinds of research are pursued in the molecular beams group:1. High-precision studies of atomic and molecular radiofrequency spectra, anexample being the study of the rotational spectrum of HF.2. The development and intercomparison of atomic frequency standards. The CSelectric resonance studies and the ammonia molecule decelerator are examples. Workis also being done to determine the system properties of a cesium beam tube, and todevelop complementary electronics to realize its latent frequency stability. These newclocks and others of different types will be intercompared by using the M. I. T. computerfacilities in order to check for possible variations in rate with epoch.3. Experiments that apply parts of these techniques to interesting problems in anyarea of physics, as in the following list:(a) The measurement of the velocity of light in terms of atomic standards,(b) the search for a charge carried by molecules,(c) an experiment on an aspect of continuous creation.These problems are well advanced. In an earlier phase are the following experiments:(d) the velocity distribution of He atoms from liquid He,(e) experiments with slow electrons (10 - 6 ev).J. R. Zacharias, J. G. King, C. L. SearleA. STARK EFFECT AND HYPERFINE STRUCTURE OF HYDROGEN FLUORIDEThe nuclear hyperfine -structure constants and the electric dipole moment of hydro-gen fluoride, HIF 1 9 , in the ground vibration and first excited rotation state have beenmeasured in a molecular beam electric resonance experiment. The hfs constants arecF = 307. 6 ± 1. 5 kcCp = -70. 6 ± 1. 3 kc22 gPgF nm5 ) = 57.6 + .44 kc.h <(rThe apparatus was calibrated by observing Stark transitions in the groundvibration and first excited rotation state of carbonyl sulfide, 016 C12 S 32, and theresults were:HF- -= 2. 554 + .0037 or PHF = 1. 8195 ± .0026 debyeIOCSQPR No. 68(IX. MOLECULAR BEAMS)by using 4OCS = 0. 7124 ± .0002 debye.An absolute measurement of the OCS electric dipole moment gave4OCS = 0. 7120 ± . 003 debye.A digitally computed solution of the Stark effect with magnetic hyperfine structurewas necessary to interpret the data. The theory and experiment are in good agreementover the range of electric field strengths used in the experiment.The hfs constants are in excellent agreement with the averaged absolute values ofthese constants as measured in a molecular beam magnetic resonance experiment.The agreement has significance because of discrepancies between the results of the tworesonance methods, for some other molecules, in previous experiments.R. WeissReferences1. M. R. Baker, M. Nelson, J. A. Leavitt, and N. F. Ramsey, Phys. Rev. 121,807(1961).QPR No. 68(IX. MOLECULAR BEAMS)B. CS MASERThe rotational transition (J = 1-0) of CS should provide a more precise molecularfrequency standard than other molecular resonances currently used. This transitionoccurs near 50 kmc.The principal advantage of using CS, rather than NH 3 or others, is that C12 andS3 2 both have zero nuclear moments so there is no spin-spin or spin-rotational inter-action.The permanent dipole moment of CS is rather large (2 debye), so small rf fieldsare necessary to cause transitions, and small static fields are needed to deflect thebeam. The physical construction of the proposed maser would be similar to the orig-inal "maser" 2 except that a two-pole deflecting field (state selector) and a two-cavityrf system3 will be used.The average lifetime is approximately 30 minutes in an enclosed vessel at roomtemperature, 4 so the CS must be produced as it is used. Sufficient quantities of CSfor a molecular-beam experiment have been produced in our laboratory by dissociationof CS 2 in electrical discharge, and on a hot tungsten wire (2000*C). The resulting vaporis passed through a trap at -100*C, and thus the CS2 is removed, leaving CS.S. G. KukolichReferences1. K. Shimoda, J. Phys. Soc. Japan 12, 1006 (1957).2. J. P. Gordon, H. J. Zeiger, and C. H. Townes, Phys. Rev. 95, 282 (1954).3. S. G. Kukolich, Measurements of the 1-1 Inversion Frequency of NH 3 and Exper-iments with a Two-Cavity Maser, S. B. Thesis, Department of Physics, M. I. T.,May 19, 1962.4. R. C. Mockler and G. R. Bird, Phys. Rev. 98, 1837 (1955).QPR No. 68(IX. MOLECULAR BEAMS)C. AMMONIA DECELERATION EXPERIMENTOne method of increasing the interaction time of free molecular systems andelectromagnetic fields would be to deal with molecules of slow velocity. A lin-ear decelerator designed to provide slow molecules with an intensity that isincreased over that predicted by the Boltzmann distribution has been describedby Zacharias and King. 1The trajectories of ammonia molecules in that device have now been ana-lyzed. We have concluded that the focusing of the last stage was too strong.It is estimated that the transmission of the first 24 stages is greater than33~ (1/25), while the transmission of the last stage is ~ 10-  These estimatesmake no allowance for possible misalignment of the plates.Plates with zero focusing have been designed, and the dynamic character-istics of an array of 15 such stages have been studied by using a computer.The duty cycle and bunching characteristics of these stages are not as satis-factory as the parallel plates, but the over-all transmission of the two systemsis comparable.Work is being done, at present, along three lines:(a) An attempt to improve the characteristics of the nonfocusing plates by consid-ering voltage waveforms other than square waves applied to the plates;(b) Experiment with a hybrid system consisting of 24 parallel-plate stages anda nonfocusing last stage; and(c) Consideration of an experiment to demonstrate the principle of deceleration ina limited number (2 or 3) of stages.R. Golub, B. BrabsonReferences1. J. R. Zacharias and J. G. King, Linear decelerator for molecules, QuarterlyProgress Report, Research Laboratory of Electronics, M. I. T., January 15, 1958,pp. 56-57.QPR No. 68(IX. MOLECULAR BEAMS)D. He 3 AND He 4 BEAMSInitial study has been undertaken on the feasibility of producing and detecting beamsof atoms evaporated from the surface of liquid He 3 and He 4 at low temperatures. It is1proposed that an "Omegatron" type of radiofrequency mass spectometer be employedas a detector, and such a device has been assembled and operated during the pastquarter. A usable sensitivity of ~ 1 ma of ion current per mm Hg for He 4 has been-7obtained, at a background pressure of 10 mm Hg in an oil diffusion-pumped system.It is anticipated that this sensitivity is sufficient to permit analysis of beam velocityspectra, and it is hoped that information regarding phenomena in liquid He 3 and He 4may be gained thereby.W. D. Johnston, Jr.References1. Cf. D. Alpert and R. S. Buritz, J. Appl. Phys. 25, 202 (1954).E. MEASUREMENT OF THE VEOLCITY OF LIGHTDuring the past quarter, all functions of the velocity of light apparatus, electrical,optical, mechanical, for the first time operated simultaneously. The schematic diagramof the apparatus is shown in Fig. IX-1. The figure emphasizes the electronics systemQPR No. 68300KC/SEC ICPSPHASE DETECTORf(units of f)Xf(units of X)D (units of X)r = ROOT OF BESSEL FUNCTION " 3832Fig. IX-1. Schematic diagram of velocity-of-light measurement method. The basic equation for the experiment isc = fXf, where kf is obtained by measurement of the guide wavelength Xg in terms of optical interferencefringes. Diameter D measurements can be eliminated by measurement of guide wavelengths at two fre-quences f, both determined with reference to the atomic clock.(IX. MOLECULAR BEAMS)because we have already given detailed description of the optical system and thehydraulic drive in previous reports. The system measures the relative phase of thecavity in order to achieve as precise a measurement of Xg as possible. The signalsderived from the microwave and optical systems will be recorded on an FM taperecorder to facilitate data reduction.We have previously reported that the cylindrical quartz cavity has been optically5D -6polished so that its diameter has a precision D-of approximately ±3 x 10 Now, fromthe basic equationS= 1/2 (1)+which relates the free-space wavelength Xf to the guide wavelength \g, andfrom the equationc = fkf (2)we find that, for the cavity diameter, D = 161. 1 mm, and the guide wavelength,X =33. 88 mm, used, we haveggc -65 (3)andac 1 D (4)c - 16 D (4)and thereforeAc 5 km/sec-2. 25 (5)AY fringegandAc 3.2 km/secAD 102 fringesIn other words, if no diameter corrections at all are made, then the apparatus shouldyield the value of C within a few parts in 107, provided that Xg can be measured towithin a few parts in 107 or approximately 1/100 fringe.G. W. Stroke, M. A. Yaffee, C. L. Searle,V. J. Bates, A. T. FunkhouserQPR No. 68

Molecular Beams

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Molecular Beams

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