Contains research objectives and reports on two research projects.Joint Services Electronics Programs (U.S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E

Ezekiel, Shaoul

Weiss, Rainer

DSpace@MIT

VII. GRAVITATION RESEARCHAcademic and Research StaffProf. R. WeissGraduate StudentsG. D. BlumT. R. BrownS. EzekielRESEARCH OBJECTIVESResearch in this group is concerned with an experimental investigation of thegravitational interaction. At present, we are engaged in an experiment to meas-ure periodic, as well as possibly secular, variations in G, the Newtonian gravi-tational constant, as predicted by a group of scalar gravitational field theories.The approach is to measure g, the Earth's gravitational attraction at the surface,with a stable molecular beam gravimeter that was described in Quarterly ProgressReport No. 77 .(pages 60-61). The experiment is in no sense a "clean" one, sincethe local value of g is dependent upon several geophysical factors that must becrosscorrelated with the gravimeter, in particular with radial earth distortionsand tilt. Development of a stable tilt meter and a laser strain seismometer I isin progress.An experiment to test the Freundlich cosmological red-shift hypothesis is alsounder consideration. The hypothesis states that photon-photon scattering of a var-iety not predicted by quantum electrodynamics is responsible for the cosmologicalred shift, rather than the recessional Doppler shift of an expanding universe. Anexperiment to determine whether 14-key Mossbauer effect y rays lose energy intraversing an X-band radiation field has been performed previously. 2 The experi-ment gave a null result; however, it demanded an extrapolation of the red-shifthypothesis from the optical region to 14-key photons. Our present experiment willtest the hypothesis for the interaction of the 6328 A line of the He-Ne laser with anX-band radiation field. Impetus for the experiment stems from the recent obser-vation by Penzias and Wilson 3 of an isotropic radiation background alleged to bedue to a cosmic black-body temperature of 3. 5K. Although it is probably for-tuitous, the Freundlich hypothesis predicts just such a temperature.R. WeissReferences1. R. A. Sramek, "An Interferometric Linear Strain Seismometer," S.B. Thesis,Department of Physics, M. I. T., 1965.2. R. Weiss and L. Grodzins, Phys. Letters 1, 342 (1962).3. A. A. Penzias and R. W. Wilson, Astrophys. J. 142, 419 (1965).*This work is supported by the Joint Services Electronics Programs (U.S. Army,U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E).QPR No. 80(VII. GRAVITATION RESEARCH)A. LONG-TERM FREQUENCY STABILIZATION OF A LASER EMPLOYING AMOLECULAR OR ATOMIC BEAM AS PRIMARY FREQUENCY REFERENCEA laser strain seismometer to measure long-period Earth distortions requires thedevelopment of a long-term laser frequency stabilization technique. Although existinggas lasers exhibit excellent short-term stability (Af/f ~ 10-11 for a few minutes), theyare still not suitable for measuring Earth distortions of Af/ ~ 0-1  per year. Variousfeedback stabilization schemes have been proposed and some have been attempted:1. Stabilization with reference to the Lamb dip at the natural atomic resonance in theDoppler-broadened laser gain curve.1, ' 22. Stabilization relative to an absorption resonance in an external gas-discharge3cell.3. Stabilization relative to an external high-Q cavity 4 or interferometer. 5None of these methods is entirely satisfactory. Scheme 1 suffers from pressureshifts, Scheme 2 from pressure shifts, as well as Doppler broadening, and inScheme 3 the reference elements are subject to temperature dependence and materialcreep.We propose to employ an absorption resonance as observed in an atomic or molecularbeam as the reference element in a feedback stabilization scheme. The method hasproved its effectiveness in the Cesium beam atomic clock. The beam offers the distinctadvantages that (a) there is no interaction between the beam particles and consequentlyno shift of the natural resonance, and (b) since the beam is directed, the resonance doesnot suffer first-order Doppler broadening.The difficulty with the beam is the following. Existing lasers rely on radiative decayto depopulate the lower of the two states between which the laser oscillation takes place.In order to maintain a population inversion, the lifetime of the lower level must beshorter than the upper level; typical lifetimes are of the order of 10 - 8 second. It doesnot appear feasible to form a beam of these excited atoms directly. We are consideringthree approaches:1. Use of an atom or molecule in the beam which is different from the one in thelaser and has an optical transition from the ground state or a metastable state thatmatches the laser line. The matching condition is stringent; a frequency match of betterthan 1 part in 105 is necessary.2. Once the beam is formed, the excited states are generated in the interactionregion with the laser light. The excitation mechanism should not introduce Doppler orStark shifts.3. Design a laser that employs an atom or molecule for which the lower levelof the oscillating transition is long-lived in the isolated conditions of the beam butcan be made to have a short lifetime in the atmosphere of the laser, for example,QPR No. 80(VII. GRAVITATION RESEARCH)by collision quenching or field quenching.Another consideration is whether to detect the absorption directly by measuring thechange in intensity of the laser light as it passes through the beam or to measure theoccurrence of an absorption by its effect on the beam. The second method gives promiseof a better signal-to-noise ratio.Thus far, Scheme 1 has not offered any possibilities. A comparison of published cwgas laser lines with optical transitions coupling the ground and metastable states ofatoms from which beams can be produced conveniently has not uncovered any sufficientlyclose matches. The search for these is now being automated and extended to diatomicmolecules.COLLIMATING SLIT CORNER REFLECTORMETASTABLE DETECTORMETASTABLE UV DETECTING SURFACE (ELECTRON EJECTION)G A S ---. . . . . . . . . . . . . . . . . . .- ,S ELECTROMETE CMICROWAVE RETARDING ECTO TEDISCHARGE GRID ELECTRONL - MULTIPLIERLASERFig. VII-1. Experimental arrangement.We are now engaged in the construction of a beam apparatus that can be used for anyof the schemes (see Fig. VII-1). If we assume that a laser that has a metastable levelas its terminal state will be successful, the detection of an absorption by the beam willbe straightforward. The laser light raises the metastable beam atoms to a level that cancouple to the ground state, and the absorption is measured as a reduction in the meta-stable beam. Another method that we shall try is to stabilize the 6328 A line of theHe-Ne laser with a metastable Neon beam by a double-resonance method. The beam1in the S 5 metastable state is irradiated both by the 5944 A Ne line from a reso-nance lamp and by the laser. The double absorption can be detected either by theUV radiation from the 3S decay of the beam atom or the additional reduction inthe metastable beam.S. Ezekiel, R. WeissReferences1. Spectra-Physics Laser Model 119.2. K. Shimoda and A. Javan, J. Appl. Phys. 36, 718-726 (1965).3. A. D. White, E. I. Gordon, and E. F. Labuda, Appl. Phys. Letters 5, 97-98(September 1964).4. A. D. White, Journal of Quantum Electronics (to be published in November 1965).5. E. A. Ballik, Phys. Letters 4, 173 (April 1, 1964).QPR No. 80(VII. GRAVITATION RESEARCH)B. COLLISION QUENCHING OF METASTABLE KRYPTON BY XENONThe collision quenching of the IS 5 and 1S3 metastable levels of the rare gases isbeing studied for their eventual application as the lower level of a laser particularlysuited to the atomic beam stabilization method described in Section VII-A. The experi-mental arrangement is shown in Fig. VII-2. Light from a DC discharge resonance lampis passed through a pulsed-discharge absorption cell containing the rare gas and a knownand adjustable amount of quenching gas. The resonance radiation absorbed by metastableSAWTOOTHTO SCANDELAY0 secTEKTRONIX OSCILLOSCOPEFig. VII-2. Experimental arrangement.atoms as a function of time after extinction of the discharge is recorded by a samplingtechnique. The following process for a possible metastable laser has been investigated:Ar( 1s 5 ) + Kr(1 ) - - A r ( 1P) + Kr( 2 P 6)KrP 6) -Kr(1S 5) + 7601. 5 A (Photon)Kr (S 5 ) + Xe -- Kr(1Po) + Xe (Excited)Excitation mechanismLaser transitionQuenching mechanismThe excitation mechanism appears to work. If 0. 5 mm Hg of A is added to0. 1 mm Hg of Kr, the 7601. 5 A line is enhanced by a factor of 3. The quenching cross1 -16 2 1section of the S5 state by the Kr is Q ~ 2 x 10 cm . The quenching of the Kr S5 )-15 2by Xe exhibits a cross section of Q ~ 3 X 10 cm . This would demand a high Xe pres-sure of 30 mm Hg to reduce the lifetime of the Kr( 1S5) state to 10- 8 sec. The lifetimeof the Kr(ZP6) state is calculated to be 2.6 x 10- 8 sec, with the Coulomb approximationmatrix elements of Bates and Damgaard I used. It appears, however, that the Xe alsoQPR No. 80(VII. GRAVITATION RESEARCH)Fig. VII-3. Absorption vs time curve.quenches the A (S 5 ) state, although not as effectively as it does the Kr. Anotherquenching scheme that we shall try isKr (S 5 ) + Hg(S o ) -- Kr (P o ) + Hg(8 3 D 2 ).Figure VII-3 is a typical absorption versus time curve, in this case, the time-dependent absorption of the 7601 A Kr line by Kr S 5 metastable atoms in the pulsed-1discharge. The Kr pressure is 1. 4 x 101 mm Hg, while the Xenon pressure is first8 X 10-3 mm Hg, and then 1. 6 X 10 - 2 mm Hg. Each large horizontal division is60 t sec.R. WeissReferences1. D. R. Bates and A. Damgaard, Trans. Roy. Soc. (London) 242, 101 (1949).QPR No. 80

Gravitation Research

http://dspace.mit.edu/bitstream/1721.1/54167/1/RLE_QPR_080_VII.pdf

Gravitation Research

Abstract

Similar works

Full text

Available Versions

DSpace@MIT