Contains reports on eight research projects.National Science Foundation (Grant PHY83-06273)National Bureau of Standards (Grant NB83-NAHA-4058)National Science Foundation (Grant PHY84-11483)Joint Services Electronics Program (Contract DAAG29-83-K-0003)U.S. Navy - Office of Naval Research (Contract NO0014-79-C-0183)U.S. Navy - Office of Naval Research (Contract N00014-83-K-0695)National Science Foundation (Grant PHY83-07172-A01

Ahmad-Bitar, Riyad N.

Bagnato, Vanderlei S.

Brewer, Lawrence R.

Chan, So-Kuen

Flanagan, Robert W., Jr.

Gentile, Thomas R.

Gould, Phillip L.

Hilfer, Eric

Hughey, Barbara J.

Hulet, Randall G.

Kash, Michael M.

Kelleher, Daniel

Kinsey, James L.

Kleppner, Daniel

Lafyatis, Gregory P.

Ligare, Martin

Magill, Peter D.

Martin, Peter J.

Moskowitz, Philip E.

Moskowitz, Warren E.

Nir, Ishai

Pritchard, David E.

Raab, Eric L.

Scott, Thomas P.

Stewart, Brian A.

Weisskoff, Robert M.

Welch, George R.

DSpace@MIT

Atomic Resonance and Scattering21. Atomic Resonance and ScatteringAcademic and Research StaffProf. D. Kleppner, Prof. D.E. Pritchard, Dr. R. Ahmad-Bitar2 5 , Dr. D. Derouard2 5Dr. T. Ducas 25, Dr. R.G. Hulet, Dr. G. Lafyatis, Dr. P. Moskowitz, Dr. J-L. Picque 2 5Graduate StudentsV. Bagnato, P.P. Chang, R. Fianagan, T. Gentile, P.L. Gould, E. Hilfer, B.J.Hughey, M.M. Kash, P.D. Magill, P. Martin, A.L. Migdall, W.P. Moskowitz,S. Paine, E. Raab, T.P. Scott, B.A. Stewart, R. Stoner, R. Weisskoff, G.R. WelchUndergraduate StudentsS K. Chan, M. Lee, S. LePage, I. Nir, S. Osofsky21.1 Resonant Multiphoton Ionization of HydrogenNational Science Foundation (Grant PHY83-06273)National Bureau of Standards (Grant NB83 -NAHA-4058)Lawrence R. Brewer, Martin Ligare, Daniel Kelleher, Daniel KleppnerMultiphoton processes are playing an increasing role in atomic, molecular, and optical physics.Hydrogen, because of its simplicity, lends itself to rigorous theoretical analysis and provides anatural benchmark system. We have completed a study of 4-photon ionization of hydrogen in thevicinity of 365 nm. Three photons are resonant with the ls-2p transition: the fourth ionizes the 2pstate at threshold.The major experimental challenge was to generate 365 nm light and to understand its spectraland temporal properties in detail. The generation system was based on a pulsed Quantal 581Nd:YAG laser at 1.06 t and a pulsed dye laser and oscillator at 554 nm. The 1.06 t and 554 nmlight were combined in a KDP crystal to generate light pulses at 565 nm with the followingproperties: energy - 10 mJ; pulse length, approx 8 ns; spectral width, 0.25 cm - 1 .The special distribution of the laser light was monitored with a Reticon diode array. Thetemporal behavior was studied using a high speed oscilloscope. The properties of the laser wereanalyzed in terms of a model for multimore operation of the Nd:YAG light source and found to bein good agreement with the model. The power was measured on a shot-by-shot basis using acarefully calibrated Scientech 362 power meter.2 5 Visiting ScientistRLE P.R. No. 127129Atomic Resonance and ScatteringMeasurements were made in a simple atomic beam apparatus in which hydrogen could beproduced at temperatures as low as 5 K. The experiments were carried out at 78 K. The photoionswere swept out by a small field and detected with an EMI D233 electron multiplier.The ion yield was measured as a function of frequency for a variety of pulse energies andspectral profiles. The results were compared to the prediction of the theory of Holt, Raymer, andReinhardt.' This theory is nonperturbative, and include processes such as stimulatedrecombination, which are often negligible. Fig. 21-1 compares theory and experiment for onevalue of pulse energy.10 0 i I I I i,J9 0 -3LASER DETUNING (cm -')Figure 21-1: Comparison of theory1 and experiment on the ionizationyield for 4-photon resonant ionization in hydrogen. Thepulse energy is 7.1 mJ. Zero on the laser detuning scale isthe ionization threshold.This work confirms the validity of the theory of Holt, Raymer, and Reinhardt, and demonstratesthat multiphoton processes in hydrogen can be understood in detail for powers up to 3x10 10 wattscm-2References1. C.R. Holt, M.G. Raymer, and W.R. Reinhardt, "Phys. Rev. A 27, 2971 (1983).PublicationsKelleher, D.E., M. Ligare, and L.R. Brewer, "Resonant Four-Photon Ionization of AtomicHydrogen," Phys. Rev. A, accepted for publication.Brewer, L.R., "Resonant Multiphoton Excitation of Atomic Hydrogen," Ph.D. Thesis, PhysicsDepartment, M.I.T., October 1984.21.2 Atoms in Strong Magnetic FieldsNational Science Foundation (PI Y83-06273)Michael M. Kash, George R. Welch, Ishai Nir, Daniel KleppnerUnderstanding the diamnagnetic spectrum of an atomfn with a single valence electron in a strongRLE P.R. No. 127 130Atomic Resonance and Scatteringmagnetic field presents a formidable challenge to theory and experiment.' The classical andquantum mechanical equations of motion are easily constructed, but in spite of the fundamentalsimplicity of the system the solutions are elusive and our understanding is far from complete.Numerical techniques have been applied to both the classical2 and quantum 3 dynamics, but theresults possess features which are difficult to interpret. Construction of an approximate constantof motion is a promising method for predicting the spectrum of a hydrogen atom in a uniformmagnetic field.4We have produced and detected Rydberg atoms in a lithium atomic beam using cw dye lasers.This represents substantial progress toward our research goal of studying highly excited atoms ina strong magnetic field by a cw laser spectroscopy. Our primary task is to measure the levelanticrossing sizes and linewidths in the diamagnetic spectrum of lithium.The excitation of lithium Rydberg atoms is performed with a two-step, three-photon process.The first step is a two-photon transition from the 2s to the 3s state. This process is detected byobserving the cascade fluorescence (3s- 2p,2p-+ 2s) through a fiber optic bundle with asensitive photomultiplier tube. The transition is excited by 735 nm light from a Coherent ring dyelaser. The 2/ - 2s fluorescence as a function of ring laser frequency is shown in Fig. 21-2.12000 I I I I I ICf)LIOU)F-0I GHz FULL SCALEFigure 21-2: Cascade (2p - 2.s) Fluorescence as a function of a laserfrequency. The two peaks reflect the hyperfine structureof the 32S1/2 and 2 s2 /2 states of lithium 7.The ring laser is stabilized against slow drift by locking it to the signal. The second step is aone-photon transition from the 3s state to the np state. This has been observed by observing adecrease in the cascade fluorescence as a Coherent linear dye laser is scanned through theresonance. Digital lock-in techniques are used to enhance the signal-to-noise ratio. Thedecrease in the cascade fluorescence as a function of linear laser frequency for the 15 p state isshown in Fig. 21-3. To minimize the electric field in the atom's rest frame, the atomic beam isparallel to the axis of a solenoid superconducting magnet. The interaction region consists of analuminum cylinder whose axis is parallel to the atomic beam. The cylinder also contains prisms toRLE P.R. No. 127Atomic Resonance and Scattering4500 I 1I0O:-z0740 MHz FULL SCALEFigure 21-3: Decrease in cascade fluorescence as a function of laserfrequency at the 15p state. The measurement time was 6seconds and the chopping frequency was 665 Hz.deflect the lasers so that the laser and atomic beams intersect at right angles, to minimize Dopplerbroadening. The interaction region combines efficient collection of cascade fluorescence withgood geometric rejection of scattered laser light. Also, the entire cylinder may be floated at -20kV to cause field ionization of Rydberg atoms just outside the interaction region. Using amagnetic field-insensitive surface barrier diode detector for the Rydberg electrons, fieldionization is expected to provide a substantially higher signal-to-noise ratio detection techniquethan the decreased cascade fluorescence. Preliminary observation of field ionization confirmsthis.We expect to start taking data in the near future.References1. J.C. Gay, "High-Magnetic-Field Atomic Physics," in H.J. Beyer and H. Kleinpoppen (Eds.),Progress in Atomic Spectroscopy (Plenum 1984).2. J. Delos, S.K. Knudson, and D.W. Noid, "Trajectories of an Atomic Electron in a MagneticField," Phys. Rev. A 30, 1208 (1984).3. J.C. Castro, M.L. Zimmerman, R.G. Hulet, D. Kleppner, and R.R. Freeman, "Origin andStructure of the Quasi-Landau Resonances," Phys. Rev. Lett. 45, 1780 (1980).4. D.R. Herrick, "Symmetry of the Quadratic Zeeman Effect for Hydrogen," Phys. Rev. A 26, 323(1982).5. M.L. Zimmerman, M.M. Kash, and G.R. Welch, J. de Phys. Col. C2, 43, 113 (1982).21.3 Cavity Quantum ElectrodynamicsNational Science Foundation (Grant PHY84- 11483)Joint Services Electronics Program (Contract DAAG29-83-K-0003)U.S. Navy - Office of Naval Research (Contract NO00 14-79-C-0 183)Thomas R. Gentile, Eric Hiller, Barbara J. Hughey, Daniel Kleppner, Randall G. HuletRLE P.R. No. 127 132Atomic Resonance and Scatteringa) Spontaneous Emission in a Tuned CavityDuring the past year we have continued work on an apparatus to study spontaneous emission ofa single Rydberg atom in superconducting cavity. The goal is to study a single excited atom in aneffective two-level configuration interacting with the single mode of cavity in its photon groundstate. At sufficiently high Q the atom-cavity system behaves like a pair of coupled oscillators, oneman-made, the other atomic. The experiment will be carried out with calcium at a frequency ofabout 35 GHz.b) Inhibited Spontaneous EmissionInhibited spontaneous emission is being investigated using circular states of cesium and atime-of-flight apparatus to measure the spontaneous lifetime on the transition n = 22 -* 21.Spontaneous emission will be inhibited by two conducting plates separated by X /2 = 0.25 mm.Apparatus has been built and is being tested.21.4 Trapping of Neutral AtomsU.S. Navy - Office of Naval Research (Contract N00014-83-K-0695)Joint Services Electronics Program (Contract DAAG29-83-K-0003)Vanderlei Bagnato, Riyad Ahmad-Bitar, Giregory P. Lafyatis, Philip E. Moskowitz, DavidE. Pritchard, Eric RaabAn experimental apparatus has been set up to slow and trap neutral atoms. We havesuccessfully slowed a Na-beam using the technique of Phillips and Metcalf.1 Atoms in a specificmagnetic sublevel are slowed by radiation pressure; a specially tailored magnetic field produces aZeeman shift which compensates the changing doppler shift of the slowing atoms and keepsthem resonant with the laser. The light scattered from the Na beam is observed by a photodiodearray. Presently, we are modeling the dynamics of the deceleration to fit our observations ofslowing as a function of laser frequency, laser intensity, polarization, and magnetic field strength.Our present apparatus has a 30 mK deep magnetic trap that should be able to confine some of thedecelerated (paramagnetic) Na atoms.In addition, we have designed and are constructing a second slowing and trapping apparatuswhich will use superconducting magnets. It will function in a fashion similar to the 1st apparatus,but will have a much deeper well - -1K for Na atoms - allowing for much greater densities oftrapped atoms.Once we have trapped atoms, we will cool them. Proposed schemes2 should produce coolingto 10- 7 K. The trapped cooled atomns should be an excellent system for studying collectivephenomena (superradiance, Bose condensation, and phase transitions), studying collisions suchas Na--Na and Na-Na*, and performing ultra-high resolution spectroscopy.RLE P.R. No. 127133Atomic Resonance and ScatteringReferences1. W.D. Phillips and H. Metcalf, Phys. Rev. Lett. - , 596 (1982).2. D.E. Pritchard, Phys. Rev. Lett. 51, 1336 (1983).21.5 High Precision Mass Measurement on Single Ions UsingCyclotron ResonanceNational Science Foundation (Grant PHY83-071 72-A01)Joint Services Electronics Program (Contract DAAG29-83-K-0003)Robert W. Flanagan, Jr., Phillip L. Gould, Gregory P. Lafyatis, Peter J. Martin, David E. Pritchard,Robert M. WeisskoffIn the past few years techniques for capturing, manipulating, and storing single electrons havebeen developed.' We intend to extend these techniques to permit the capture and storage ofsingle ions in a Penning trap, which should permit measurement of (relative) masses to one part in1011. Such high precision will enable us to weigh chemical bonds, measure energy differences inisomer nuclei, provide more precise determination of physical constants,2 and give a clearerunderstanding of many-body interactions.The signal emitted by one single oscillating ion is expected to be quite low; consequently, weplan to conduct these measurements in a cryogenic environment using a SQUID detector. TheSQUID is an extremely sensitive device with virtually no thermal noise limitation. Anotheradvantage of low temperature, apart from noise considerations, is that it enables us to maintainan extremely high vacuum (less than one particle/cc),3 permitting long trapping times.A custom, high-homogeneity NMR magnet has been designed and is nearing completion. ASQUID has been obtained and tile development of associated superconducting circuitry iscurrently being undertaken. A Penning trap has been designed and is expected to bemanufactured using computer-aided machining in the immediate future. The cryogenic envelopefor the trap has been designed and plans for its construction finalized. Acoustic and RF isolationas well as the difficulties associatcd \with operating a SQUID in proximity to a high field magnet arecurrently being considered.References1. R.S. Van Dyck, Jr, , u B. Schwinberg, and Hans . ehme!t, in, Daniel leppnJoir and FrancisPipkin (Eds.), Atomic Physics. Vol. 17. (Plenum Press, 1981), p. 340; see also R.S. VanDyck, Jr., and P.B. Schwinberg, Phys. Rev. Lett. 47, 395 (1981).2. johnson, private communication.3. W. Thompson and S. Hanrahan, J. Vac. Sci. Technol. 14, 643 (1977).RLE P.R. No. 127 134Atomic Resonance and Scattering21.6 Diffraction of an Atomic Beam by Transverse StandingWave Laser RadiationNational Science Foundation (Grant PHY83-07172-A01)Phillip L. Gould, So-Kuen Chan, David E. PritchardWe are studying the momentum transfer to an atomic beam by a transverse standing wave laserradiation in an effort to gain a better understanding of the atom-field interaction. The force felt byan atom (and the resultant deflection) is a direct result of interactions of the induced electricdipole moment of the atom with gradients of the applied optical field. The effects of spontaneousemission depend on the length of time the atom spends in the field. For short times (relative to thenatural lifetime), atomic "diffraction" is predicted1 and has been observed by us.2 For long times,momentum "diffusion" is expected.3 We are currently studying the transition regime betweendiffraction and diffusion. A preliminary result for an interaction time of 5 natural lifetimes is shownin Fig. 21-4. Diffraction is seen to persist, in contrast to existing theories.3z_ I_ 1_1___ . 1 1 1 1 1_!1 1 1 f L A 1-16 -12 -8 4 0 4 8 12 16Ap! kFigure 21-4:The experiment was performed by crossing a sodium beam with a standing wave laser field andobserving the momentum transfer downstream from the interaction region. The atomic beam iswell collimated (2x10 - 5 rad) and has a narrow velocity distribution (10% FWHM). The atoms areoptically pumped upstream from the interaction region with a second laser in order to prepareeach atom as a two-level system. Our resolution is sufficient to detect the momentum transferredby a single photon (but note that in Fig. 21-4 we see momentum transferred in units of twice thephoton momentum) - the Kapitsa-Dirac effect.4References1. R.J. Cook and A.F. Bernhardt, Phys. Rev. A 18, 2533 (1978).RLE P.R. No. 127135Atomic Resonance and Scattering2. P.E. Moskowitz, Phillip L. Gould, Susan R. Atlas, and David E. Pritchard, Phys. Rev. Lett. 51,370 (1983).3. C. Tanguy, S. Reynaud, C. Cohen-Tannoudji, J. Phys. B 17, 4623 (1984).4. P.L. Kapitza, P.A.M. Dirac, Proc. Camb. Phil. Soc. 29, 297 (1983).21.7 Vibrational Dependence of Rotational RainbowStructure in Na2-ArNational Science Foundation (Grant PHY83-07172-A01)Warren E. Moskowitz, Brian A. Stewart, James L. Kinsey, David E. PritchardWe have measured rotationally inelastic differential cross sections for collisions of atoms withhighly vibrationally excited molecules. Specifically: Na2(j i = 7&9, vi = 31) + Ar -- Na2(j,r vf = 31)+ Ar at a center of mass energy of 0.3 eV. The Na2 is in its ground electronic state but hasvibrational energy equal to about 50% of dissociation. The measurements were made in crossedmolecular beams. The initial Na2 level was populated by optical pumping, and the final level wasdetected by laser induced fluorescence. The angular distribution was determined by our provenDoppler technique, called ADDS, in which a single mode, c.w. dye laser beam intersects thecollision region along the relative velocity vector, and excites molecules which have beenscattered into the final state with the correct Doppler-selected velocity component in thedirection of the laser. The laser is tuned, and the resulting fluorescence signal yields theangle-differential cross section (Fig 21-5).6(e)cos(e) -4 .IFigure 21-5: Scattering signal for jf = 15.RLE P.R. No. 127 136Atomic Resonance and Scattering21.8 Vibrationally Inelastic CollisionsNational Science Foundation (Grant PHY83-07172-AO1)Thomas P. Scott, Brain A. Stewart, Peter D. Magill, David E. PritchardWe have thoroughly investigated the phenomenon of quasi-resonant vibration -+ rotationenergy transfer discovered in our laboratory' in the system,i2 *(AY +,v j 1)+ X , Li(2 *(vi A v) + Xwith target gases X = He, Ne, Ar, Xe. We have studied the dependence of this phenomenon on vi,ji, target gas, and collision velocity, measuring both thermally averaged rate constants andvelocity dependent cross sections. Noteworthy features of the rate constant data include:The quasi-resonant narrowing of the final rotational state distribution with increasing ji isobserved for each of the target gases; moreover, in the case of Ne, Ar, and Xe, the shape of thisdistribution is identical within experimental error (see Fig. 21-6). Increasing ji not only inducesthe resonant peaking, but also increases the rate constants dramatically enough that therotationally summed vibrationally inelastic rate constants increase with ji despite the narrowing ofthe j, distribution.Increasing vi also enhances both the peak rate constant and the rotationally summed rateconstant which is about 50% larger for v. = 9 than for v. = 4.Our velocity dependent rate cross section data indicate that:The total vibrationally inelastic cross section is very large in this system, especially for lowvelocity, where for A v = - 1 alone it is 30A 2. This implicates moderately long range collisions inthe vibrationally inelastic collision process.For A v < 0 the position of the peak of the quasi-resonant distribution depends little on velocityor target gas, being given by jf = ji - 4A v; this indicates that roughly 70% of the vibrationalenergy is exchanged with rotational energy. The vibrationally exoergic cross section decreasesas 1/vrel for all jIThe width of the quasi-resonant peak decreases as vrel decreases; this increase in specificity ismost pronounced in Li2* - Ne collisions.Because of this enhancement in specificity with decreasing collision velocity, we expect to seeevidence of quasi-resonant transfer at low velocity for reduced ji. Experiments are underway totest this hypothesis.RLE P.R. No. 127137Atomic Resonance and Scattering28 58Figure 21-6:1. K.L. Saenger, N. Smith,4076 (1983).Rate constants k V ,ji=42 --* v.+ Av, j versus f forLi2 *-He, Ne, Ar and XeA v= -1 collisions. For Lt * - Ie, adashed line connects the data points. ForLi 2* - Ne, Ar, andXe solid curves demonstrate the identicalshape of the distributions for these systems. The solidcurves are the average of the latter three systems, scaledfor best fit to each system.ReferencesS.L. Dexheimer, C. Engelke, and D.E. Pritchard, J. Chem. Phys. 79,RLE P.R. No. 127 138

Atomic Resonance and Scattering

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Atomic Resonance and Scattering

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