Contains research objectives and summary of research.U. S. Air Force-Office of Scientific Research (Contract F44620-72-C-0057)National Science Foundation (Grant GP-39061X)Joint Services Electronics Program (Contract DAAB07-71-C-0300

Kleppner, Daniel

Pritchard, David E.

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IV. ATOMIC RESONANCE AND SCATTERINGAcademic and Research StaffProf. D. KleppnerProf. D. E. PritchardDr. T. W. DucasDr. R. R. FreemanGraduate StudentsJ. Apt III W. E. Cooke E. M. MattisonG. M. Carter M. G. Littman W. D. PhillipsRESEARCH OBJECTIVES AND SUMMARY OF RESEARCHOur interests center on the structure and interactions of atoms and simple molecules,and on their interactions with radiation at both optical and radio frequencies. Experi-mental methods involve colliding beam scattering, atomic and molecular beam resonancespectroscopy, high-precision maser spectroscopy on stored atoms, and optical fluores-cence with tunable lasers.1. Studies of Superradiance and CoherenceU. S. Air Force -Office of Scientific Research (Contract F44620-72-C-0057)D. KleppnerThe objectives of this work are to study superradiance and subradiance in small sys-tems, and to investigate the dynamics of free-to-bound and bound-to-free radiative pro-cesses in diatomic systems.We have undertaken a study of superradiance in small systems. In order for super-radiance to occur in a two-particle system, the radiators must remain within a wave-length during their radiative lifetime. We have met this problem by making use of thefirst-order dipole energy in a coherently excited two-atom system to form a quasi-stationary two-atom superradiant complex. The lifetime of the complex is approximatelyone-half the lifetime of a single radiator. The complexes are also capable of existingin subradiant states, in which the two dipoles oscillate out of phase and dipole radiationis forbidden.In addition to revealing the effects of coherence in small systems, the excited com-plexes yield a detailed picture of free-to-bound and bound-to-free transitions in simplemolecules. Such processes can play an important role in gas discharges and stellaratmospheres, but heretofore they have never been observable at low densities where thecomplexes evolve in essential isolation.2. Van der Waals MoleculesNational Science Foundation (Grant GP-39061X)D. Kleppner, D. E. PritchardThe objective of this work is to study magnetic interactions in weakly bound para-magnetic systems. The binding energy of a van der Waals molecule is so small, typi-cally a few percent of that of a chemical bond, that the atoms are relatively unperturbed.The molecular interactions can thus be approached from an atomic point of view, and theQPR No. 112(IV. ATOMIC RESONANCE AND SCATTERING)systems offer a unique testing ground for atomic and molecular theory.We have undertaken a series of experiments on paramagnetic van der Waals mole-cules, with our initial interest centered on alkali-rare gas molecules. The molecules arecreated by expanding a mixture of alkali vapor and rare gas into a vacuum through asmall orifice, and their properties are observed by molecular-beam techniques. Wehave succeeded in creating a beam of potassium-argon, and recently we observed thespin rotation interaction. The alkali hyperfine frequency is expected to be decreasedbecause of the polarization effects associated with van der Waals binding. Work on thisis in progress.3. Alkali Atom InteractionsJoint Services Electronics Program (Contract DAAB07-71-C-0300)D. E. PritchardIn this work our objectives are as follows.1. To study mechanisms of atomic collisions involving alkali atoms in specific states(e. g., spin-polarized or optically excited).2. To interpret these results in terms of models that have application to widerclasses of collisions (such as nuclear collisions or atomic collisions in different energydomains and environments).3. To improve the technology for producing high-intensity beams of state-selectedatoms.The approach that we have adopted is to observe the effects of an isolated collisionbetween a state-selected alkali atom and a target molecule (or atom). High-intensityjet sources are typically used for both primary atoms and target molecules; the stateof the primary atoms is determined with a magnet or a laser. Use of a velocity selectorand a small detector permits the determination of atomic scattering cross sections withgood angular and energy resolution. (Another state selector is optionally used prior tothe detector to analyze the state of the alkali atoms after the collision.)In 1973, our main efforts were directed toward building and testing a new high-intensity source of polarized alkali atoms, installation of a new velocity selector (per-mitting study of the energy lost by the atoms as a result of collisions), final developmentand testing of a dissociating source for iodine atoms, and writing and debugging computerprograms to analyze data taken with these new improvements. In addition, we designedand built (but have yet to test) a system for using a tunable dye laser to produce a beamof excited-state atoms for use in these experiments.Measurements were made on collisions between Na and 12 and Na and I. In the formercase we hope to analyze the results in terms of the "optical model" which has been suc-cessfully applied to nuclear collisions. In the latter, we are trying to isolate the effectsof "curve crossing" collisions, a type of collision that might be responsible for anoma-lously large cross sections for inelastic processes such as energy and excitation trans-fer.In 1974, we plan to concentrate heavily on studies involving sodium atoms in anexcited state. A beam containing a large fraction of excited-state atoms will be producedby directing our continuous-wave tunable dye laser at the atomic beam in the collisionregion. By changing the frequency and polarization of the laser, we shall be able toselect the desired excited state and also control its polarization.This technique should be capable of producing an excited-state atomic beam whoseintensity is comparable to the ground-state atomic beams that have revolutionized thefield of atomic scattering in the last decade. Techniques have been developed to interpretQPR No. 112(IV. ATOMIC RESONANCE AND SCATTERING)the scattering of ground-state atoms. We shall now be in an excellent position to applythese techniques to the understanding of the interactions of excited-state atoms. Ourinitial application of the excited-state beam will be to the determination of excited-stateinteratomic potentials for alkali-rare gas systems. One application of these potentialsresults from the fact that they determine the operating characteristics of excimer lasers.This type of laser (in a system with a repulsive ground state) offers the prospect of tun-ability and high power, attributes that indicate wide potential application.QPR No. 112

Atomic Resonance and Scattering

Contains reports on two research projects.Joint Services Electronics Program (Contract DAAB07-71-C-0300

IV. ATOMIC RESONANCE AND SCATTERINGAcademic and Research StaffProf. Daniel Kleppner Dr. Theodore W. Ducas Dr. Richard R. FreemanProf. David E. Pritchard Dr. Edward M. MattisonGraduate StudentsJerome Apt III William E. Cooke William D. PhillipsGary M. Carter Michael G. Littman Francine WrightA. OPTICAL FREQUENCY STANDARD JSJoint Services Electronics Program (Contract DAAB07-71-C-0300)David E. PritchardWe have developed a method for using the intracavity etalon in our single-modecontinuous-wave dye laser as an interferometric wavelength standard. The key to ourmethod was the determination of the free spectral range of this etalon with an accuracyexceeding one part in 104, using two photon transitions in sodium as a reference. Thispermits rapid adjustment of the laser wavelength with an accuracy of 0. 1 A.By using a series of external reference etalons in addition to the intracavity etalon,we plan to get our accuracy down to the range 10- 3 A to 10 A. The systemwill also be able to provide a series of marker pulses during a frequency sweep withcomparable precision. We shall use the system to take precise molecular spectra; thiswill be helpful in understanding molecular structure and in providing convenient ref-erence frequencies for other workers.B. STUDIES IN OPTICAL PHYSICS AND EXCITED-STATEINTERACTIONSJoint Services Electronics Program (Contract DAAB07-71-C-0300)Daniel KleppnerDuring the past year we have performed experiments measuring the Stark mixing oftwo excited states. We have extended our investigation of bound-to-free transitions inexcited alkali dimers, and are constructing an apparatus for observing excited-state col-lision processes.We now propose to study optical excitation processes in atoms that are part of a jetbeam. These beams allow much higher intensity and much lower internal temperaturesthan conventional beams, and should make Doppler broadening negligible while still pro-viding substantial signal strength.Cross-section data and knowledge of atomic potentials are necessary in many areas JSQPR No. 114(IV. ATOMIC RESONANCE AND SCATTERING)JS of physics, atmospheric and ionospheric studies, plasma research, and so forth. Mostaccurate data come from scattering experiments in the ground state. Methods that arenow available for making such measurements in the excited state give promise of a vastimprovement in our knowledge of excited-state interactions. We propose to carry outsuch studies, using a combination of atomic-beam scattering and laser excitation tech-niques. The first studies will involve the alkalis.By coherent population of atomic excited states, information on the level structurecan be recovered through detection of "quantum beats" in the emitted photons. We planJS to apply this technique to a previously unobserved hyperfine interaction in potassium.QPR No. 114

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Atomic Resonance and Scattering

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