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1 A Program of Research Initiatives . . THE NATURE OF THE FIELD The central goals of atomic, molecular, and optical physics (AMO physics) are to elucidate the fundamental laws of physics, to under- stand how matter is composed and how it evolves at the atomic and molecular level, to understand the interactions of light with matter, and to create new techniques and devices. AMO physics is part of a scientific bridge that links physics with astronomy, chemistry, aeron- omy, and biophysics. The experimental and theoretical techniques generated by research on atoms, molecules, and light are often taken up by other areas of physics nuclear, plasma, atmospheric, condensed matter, surface, and high-energy as well as by other sciences. The data generated by AMO physics, including the precisely determined fundamental con- stants of nature, are an essential part of the base of knowledge on which all natural science rests. AMO research also extends into wide areas of applied science; for example, it contributes to the national security system and to the nation's energy programs. AMO laborato- ries have created advanced technologies that have led to the develop- ment of new industries. Such industries are vital for assuring that the United States will retain industrial leadership in the face of the increasing international challenge. AMO physics plays an important role in the education of scientists in 7

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8 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS the United States, both at the undergraduate and graduate levels. AMO laboratories, most of which are located on the campuses of colleges and universities, train many of the physicists in our national and industrial laboratories. Approximately 140 Ph.D. degrees are awarded each year in AMO physics. Many of these scientists help to carry forward the nation's energy, military, and environmental programs. AMO physics is a tremendously diverse field, and this diversity is an essential source of its intellectual vitality. The interplay between various streams of research within AMO physics and neighboring fields of science is demonstrated frequently throughout this volume: an experiment on the quantum electrodynamics of electrons and positrons spurs a new technique for making atomic clocks and optical frequency standards; a close connection is discovered between inner-shell pro- cesses in energetic atomic collisions and in elementary chemical reactions; laboratory experiments with low-energy ions cause a re- thinking of a basic astrophysical process. The unity of science is manifest throughout the field of AMO physics. The influence of AMO physics extends into other areas of science and engineering. To cite one example, many outstanding young scien- tists in this field now work in Departments of Chemistry. This exten- sion is a sign of the widening realization of the power of the methods of AMO physics for understanding broad classes of natural phenom- ena. Although chemists may discover fresh viewpoints on chemical reactions from the theory of atomic collisions or from experiments with colliding beams of atoms, there remains a vital core to the subject whose approach is that of the physicist: the search for unity and generality in the natural world. The chemist and biologist need the in- sights of the atomic physicist to elucidate the diversity of the properties of matter and of organisms. This means that a healthy component of physics will remain necessary no matter how widely the achieve- ments of this field are taken up by other sciences and engineering. This report attempts to provide a balanced description of AMO physics; to portray its role in the nation's programs in basic science, applied science, and technology; to indicate likely areas for scientific advance; and to describe the steps needed to pursue these opportuni- ties. ORGANIZATION OF THE REPORT The remainder of this chapter describes the Program of Research Initiatives that is intended to assure that AMO physics will continue to advance as a science and to meet vital national needs. Research

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A PROGRAM OF RESEARCH INITIATIVES 9 initiatives in AMO physics are described in the last three sections of this chapter. Chapter 2 discusses the role of AMO physics in the United States; Chapter 3 presents recommendations for assuring the continued productivity of the field and for implementing the Program of Research Initiatives. The main body of the report summarizes recent scientific activities in atomic, molecular, and optical physics, in Chap- ters 4, 5, and 6, respectively, and describes the scientific interfaces and applications of AMO physics, in Chapters 7 and 8, respectively. INTRODUCTION TO THE RESEARCH INITIATIVES The field of AMO physics contains diverse scientific opportunities. The vitality of the field stems from the pursuit of a wide range of these opportunities. We describe here a Program of Research Initiatives intended to support scientific innovation and to provide an environ- ment for rapid scientific advance. The goals of the Program are To advance our understanding of the laws of physics, the structure and behavior of matter, and the interaction of matter with light; ~ To assure continued U.S. leadership in the development of new instruments and techniques; To provide experimental and theoretical techniques and physical data that are vital to other areas of science, to industry, and to national programs; To attract able young men and women to the frontiers of atomic, molecular, and optical physics; To train physicists for careers in universities, in national labora- tories, and in industrial laboratories. In preparing the Program of Research Initiatives we are aware that attempts to predict the most promising avenues of scientific advances are likely to miss the most important developments. For instance, if we had met 10 years ago we would have failed to mention, or would have seriously underestimated, many areas of major progress in the past decade: laser cooling of atoms and ions, low-energy highly charged ions, transient molecular states, Rydberg atoms, molecular clusters, four-wave mixing, phase conjugation, and ultrasensitive detection, for example. The list, which could easily be extended, illustrates the point that 10 years ago AMO physics was developing too rapidly to permit a knowledgeable forecast. Today the field appears to be moving even more rapidly. Nevertheless, we believe that the Program of Research Initiatives represents a realistic basis for scientific advance in the near future.

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10 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS Because AMO physics is such a diverse field, and because so many different areas have a high potential for scientific reward, the Program is necessarily broad. Fortunately, basic research in AMO physics is generally carried out by small groups rather than by massive research teams. Forefront research in a wide range of activities can be carried out by individuals working in programs whose cost is, by the standards of contemporary physics, small. INITIATIVE IN ATOMIC PHYSICS Rapid experimental and theoretical advances have opened new frontiers in many areas of atomic physics. We have selected three areas of basic scientific inquiry in which to exploit these new opportunities: Fundamental Tests and High-Precision Techniques to use the atom as a laboratory for the study of basic properties of space and time, to test the elementary interactions and symmetries in nature, and to create new techniques for precision measurements. The Many-Electron Atom to obtain physical understanding and quantitative descriptions of many-body systems generally through the elucidation of how electron motions are correlated and to extend these concepts to help interpret the dynamical behavior of atoms within molecules and other, more complex, systems. Transient States of Atomic Systems to describe qualitatively and quantitatively the physical nature of intermediate, nonstationary states; the exchange of energy, angular momentum, and particles during atomic collisions; and to understand the role of highly correlated intermediate states. Fundamental Tests and High-Precision Techniques The styles of physics the theoretical and experimental techniques of the various fields of physics differ dramatically, but central to all of physics is the study of the elementary laws of nature. These studies play a conspicuous role in atomic physics, and activity is at a high level. During the past decade, for instance, time-reversal invariance has been tested at new levels of sensitivity through searches for the dipole moment of the neutron; the isotropy of space with respect to the speed of light has been confirmed by laser interferometry to a few parts in 10~5; parity violation by the electroweak interaction has been observed in atoms, and the effect of the Earth's gravity on time has been measured using a rocketborne clock with a stability greater than 1 part in 10~4. (See Figure 1.1.) These experiments, which involve

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A PROGRAM OF RESEARCH INITIATIVES 11 measurements of extraordinary sensitivity, often generate techniques that find useful applications in other areas of science and industry. For example, the atomic clock, which was developed to test the eject of gravity on time, now plays a key role in radio astronomy with very-long-baseline interferometers. Atomic clocks have also made possible a new type of navigational system, which can determine one's position anywhere on Earth with an accuracy of about 10 m. The high-precision optical interferometry developed in conjunction with the test of the isotropy of space has applications ranging from gravity-wave detection to seismic monitoring. Sensitive testing of the limits of quantum electrodynamics (QED) is one of the most important tasks in this area of AMO physics. The confrontation between theory and experiment has moved to a level of precision that is unique in the world of physics. The anomalous magnetic moment of the electron, which has been evaluated in one of the most ambitious calculations of theoretical physics, has now been measured to an accuracy of 40 parts in 109 in an experiment that employs a single electron or positron confined in an electromagnetic trap. Together with a new measurement of the radiative-energy-level shift (the Lamb shift) to an accuracy of 9 parts in 106, this constitutes one of the most demanding experimental tests of theory ever carried out. The results check the convergence of QED in areas not previously examined; this is a crucial test, considering that QED is the prototype for all gauge theories, including the electroweak theory and quantum chromodynamics. Tests of QED have come from the recent observa- tion of the Lamb shift in the electron-muon atom (muonium) and laser spectroscopy of the electron-positron atom (positronium). Further discussion of activities in this area can be found in Chapter 4 in the section on Elementary Atomic Physics. Research opportunities include the following: Elementary StructureA hundredfold improvement in the mea- surement of the electron magnetic moment appears to be feasible using new types of single-particle traps. Provided that the theoretical calcu- lations undergo similar progress, this would test QED and charge- parity-time (CPT) invariance with a precision of 1 part in 10~3. There are new opportunities to search for the breakdown of time-reversal symmetry with a hundredfold increase in the sensitivity of the search for the neutron's electric dipole moment, to study particle-antiparticle symmetries, and to study electroweak interactions in atoms. QED in Highly Charged Systems Highly charged ions with one or a few electrons can now be produced in fast ion beams. Hydrogen- like uranium (uranium with 91 of its 92 electrons removed) has recently

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12 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS :\~ a) c) b) d) 1 FIGURE 1.1 Four\Tests of Fundamental Principles. a) Gravity and Time. According to general relativity, time is slowed by gravity. The effect due to the Earth's gravity is tiny: a clock on the Earth's surface runs slow by only 7 parts in 10' compared with an identical clock in free space. Nevertheless, this effect has been measured to high accuracy by comparing an atomic clock carried by a rocket in a high trajectory orbit with a similar clock on Earth. (Atomic clocks use the natural frequencies of atoms to control the rate of an oscillator.) During the experiment the two clocks maintained a precision of a few parts in 10'5, approximately one second in ten million years. Atomic clocks are essential elements of modern navigational and global positioning systems, and they play a vital role in very-long-baseline radioastronomy. b) How Constant is the Speed of Light? Einstein's assertion that the speed of light in empty space is a universal constant, unaffected by any motions of the light's source or by the observer, is accepted as a fundamental law of physics. Like all such laws, however, its validity rests on careful observation. The constancy of the speed of light in different directions has been verified to a few parts in 10'5 by comparing signals from highly stabilized lasers while their relative directions were changed. Laser stabilization techniques similar to those used in this experiment have applications that range from ultraprecise spectroscopy to new types of metrology and communications. c) Parity Violations in Atomic Physics. The electroweak theory, which unifies the previously separate descriptions of electromagnetism and the weak interaction, is a milestone of modern physics. According to this theory, free atoms can display an intrinsic preference between right- and left-handedness due to what are known as parity-violating interactions. Without the electroweak interactions, parity violations in free atoms would be strictly forbidden. Parity violation effects in atoms have been measured by laboratories in France, Great Britain, the United States, and the Soviet Union. In the experiment illustrated, the parity-violating interactions cause the light

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A PROGRAM OF RESEARCH INI TIA TI VES 13 been produced. A new area of QED phenomena where both radiative and relativistic effects are large is ripe for study, including research on bound states in strongly relativistic electromagnetic fields, a problem of deep interest in QED and relativistic quantum mechanics. Laser Spectroscopy of Exotic Atoms With the coming of age of precision spectroscopy in positronium, accurate measurements of the Lamb shift and elusive relativistic many-body effects in this elementary two-body system have become possible. Muonium has recently been obtained in vacuum, opening the way to spectroscopy of the excited states of muonium. Intense sources of muons and pions are now available, offering the possibility of developing intense pulsed sources of muonium and pionium that are matched to the cycle factor of pulsed lasers. Laser spectroscopy of exotic atoms provides a new arena for the study of quantum electrodynamics in pure leptonic systems. Trapped Electrons, ions, and Atoms Techniques for high- resolution spectroscopy of trapped particles are rapidly advancing. The g factors of electron and positron have been compared to a few parts in 10~; new tests of general and special relativity are under way; and it should be possible to obtain an improved value for the ratio of the electron mass to the proton mass. With trapped-ion techniques it may be possible to compare masses of nuclei to an unprecedented precision, possibly providing a new way to measure the neutrino rest mass. Trapped ion spectroscopy can provide precise tests of theory in fields radiated by laser-excited atoms to become circularly polarized. The experiment has a plane of symmetry, but the circular polarization of the radiated light breaks the symmetry. Although the effects are small most of the radiated light is linearly polarized and the circular polarization is typically only one part in a million they have been measured with such precision that the atomic data complement and augment the measurements made by particle physicists using accelerators. d) Time-Reversal Symmetry and the Electric Dipole Moment of the Neutron. The basic laws of physics are ordinarily not sensitive to the direction in which time flows; in general, if time were reversed, the motions would be exactly reversed. In all of physics, only one exception to this time-reversal symmetry has so far been observed; it occurs in the decay of the neutral K meson. Theories of K meson decay predict other breakdowns of time-reversal symmetry. One of the most sensitive tests of this symmetry has been the search for the existence of an electric dipole moment of the neutron. Very slow neutrons from a neutron reactor are trapped in a cell by a valve, and their intrinsic magnetic moment is measured by the technique of radio-frequency magnetic resonance. A strong electric field is applied; an electric dipole moment would reveal itself by a change in the resonance frequency when the electric field is reversed. The experiment illustrated is carried out by a team of physicists using an internationally operated neutron reactor in France. A related experiment is under way in the Soviet Union. The limit on the possible size of the neutron dipole moment has been systematically reduced, placing important constraints on possible theories. Other applications of AMO physics to basic tests are discussed in Chapter 4 in the section on Elementary Atomic Physics.

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14 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS ranging from QED and relativistic quantum mechanics in few-electron systems, to electron correlations in multielectron systems. The recent demonstrations of laser techniques for slowing, cooling, and stopping neutral atoms suggest the possibility of also trapping and studying atoms. Trapped particles can be cooled to temperatures in the millikelvin range by a variety of spectroscopic techniques, opening the possibility of observing unusual states of matter, including Bose condensation for atoms and strongly coupled plasmas for ions. The methods open the way to new types of atomic clocks and optical frequency standards. Many-Electron Dynamics The many-electron atom poses a major intellectual challenge for physics. The independent particle model was formulated 50 years ago, and with the power of fast computers it has now been fully realized numerically. However, comparisons with experiments have revealed a rich array of phenomena that cannot be explained by the independent particle model. These result from many-body electron-electron inter- act~ons, which give rise to correlated motion in which the electrons drastically affect each other's behavior. The recognition of the role of dynamical symmetries in correlated systems, and the discovery of correspondences between correlated motion and single-electron motion in strong applied fields, mark recent theoretical advances. There have been numerous experimental ad- vances. Two-electron systems the "hydrogen atoms" of many- electron systems can now be studied in new types of high-resolution electron-scattering experiments. By applying laser spectroscopy to relativistic beams of the negative hydrogen ion, the motion of corre- lated electrons has been observed with a clarity never before possible. Doubly excited states have been discovered in two-electron multiphoton ionization experiments. Using multiple-laser techniques it is possible to create "planetary" atoms atoms with two very highly excited electrons that can display new types of electronic motion. The spectra of highly excited atoms in strong fields have revealed unex- pected systematics that may hold important clues to the behavior of correlated electron systems. The many experimental and theoretical advances in the study of correlated electron motions make the many-electron atom problem ripe for attack. Such an achievement would have ramifications for our understanding of the structure of matter and would have numerous

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A PROGRAM OF RESEARCH INITIATIVES 15 applications in chemistry and materials science. Furthermore, the study of correlated motions in a relativistic framework is expected to bear on problems in nuclear and particle science. Further discussions can be found in Chapter 4 in the sections on Atomic Structure, Atomic Dynamics, and Accelerator-Based Atomic Physics. Research opportunities include the following: Inner-Shell Spectroscopy The structure of an atom having an electron missing from an inner shell differs radically from that of a normal atom. In certain cases, correlation ejects are enormously magnified by the effects of double-well potentials. Because the energies of inner-shell electron states can be hundreds or thousands of times larger than the energies for the outer electrons, these states are also ideally suited for studies of relativistic and QED effects in heavy atoms and of the basic process of formation and decay of unstable states. Recent developments of high-intensity synchrotron light sources, hard ultraviolet lasers, and charged-particle beams open the way to major advances in this area. Spectroscopy of Highly Charged Ions Studies of sequences of ions having the same number of electrons but ever-increasing nuclear charge provide a unique opportunity to investigate the dependence of relativistic and QED effects on nuclear charge. Such investigations yield a better understanding of basic physical theories and an improved knowledge of the properties of the highly charged ions. These ions commonly occur as impurities in hot fusion plasmas, and often they reveal important plasma diagnostic information. In addition, most plausible schemes for short-wavelength lasers involve radiative transi- tions in highly stripped ions. The recent discovery of methods for creating and trapping highly charged ions provides a major opportunity for high-precision studies of this important class of atomic systems. Multiphoton Processes In the intense electromagnetic field of focused laser light, multiphoton processes occur in which an atom or molecule absorbs several light quanta simultaneously. It has been discovered that intense infrared laser pulses can produce doubly charged rare-gas ions in a many-photon absorption process. Two electrons are simultaneously excited in highly correlated intermediate states. Other experiments using ultraviolet radiation have revealed selective multiphoton ionization of inner-shell electrons. The energy exchanged between optical and electronic modes in these processes is greater than any known chemical reaction. Theory is so far lacking, but

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16 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS it appears that multiphoton processes are providing a new avenue of approach to correlated electron dynamics. Highly Excited Atoms New techniques for preparing and study- ing Rydberg atoms (atoms with one electron in a highly excited state) allow one to study for the first time regimes in which applied electro- magnetic fields are as strong as the electromagnetic fields of the atom. These strong fields have permitted physicists to begin to search for new dynamical symmetries of the atom-field system. Planetary atoms (atoms with two electrons in highly excited states) provide the oppor- tunity to control and study electron correlation phenomena in ways never before possible. The excited electron can serve as a very sensitive probe of the ionic core of the atom and of the quantum- mechanical theory of the coupling between discrete and continuum states. Transient States of Atomic Systems Major innovations in the generation, control, and detection of charged- and neutral-particle beams, the creation of new light sources, and new analytical and numerical methods make possible a wide range of precise studies of unstable transient states of atomic systems. Such states are interesting because they are intrinsically nonstationary; the challenge is to describe the time-dependent many-electron system and the special time-dependent characteristics of electron correlation and electron-nuclear exchange of energy and momentum. The traditionally sharp distinction between bound and continuum electronic states has largely disappeared as more is learned about the rich structure in the continuum (the sea of unbound states) of atoms, negative and positive ions, and the transient quasi-molecular systems of colliding atomic species. This structure manifests itself in such processes as autoioniza- tion and dielectronic recombination, in electron capture and loss to the continuum, and in associative ionization during collisions of excited and ground-state atoms. Study of the transient electronic states that occur during violent collisions between ions and other ions or atoms has led to the discovery of approximate conservation laws, such as the electron promotion model that was created to explain the unexpected x-ray emission during ion-atom collisions. Other approximate conservation laws gov- ern the evolution of highly excited atoms in electric fields, and a wide range of other atomic-physics phenomena. The new experimental and theoretical techniques of atomic-collision physics have for the first time permitted the study of the adiabatic limit

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A PROGRAM OF RESEARCH INI TIA TI VES 17 of ultraslow collisions. These studies are leading to major advances in long-pursued areas such as the threshold law in ionization of atoms by electron impact, quantum effects in cold ion-molecule collisions, and the role of the transient dipole in threshold photodetachment of electrons from polar negative molecular ions. The ability to carry out scattering experiments in which every important variable is measured (often called a complete scattering experiment), complemented by the development of quantitative theo- ries of complex collision phenomena, provides an unprecedented opportunity to the transient states of atomic systems. The goal is to seek hidden symmetries that will help to organize and simplify the description of the dynamics of a large class of complicated multi- electron systems. Further discussion is presented in Chapter 4 in the sections on Atomic Dynamics and Accelerator-Based Atomic Physics. Research opportunities include the following: Particle-Beam Collision Measurements Advances in the design of ion sources, the cooling and control of ion and neutral beams, ion traps, merged beams, and position-sensitive detectors open the way to definitive studies of excitation, ionization, charge-transfer, and reac- tive scattering. The results will help to elucidate a variety of processes ranging from chemical rearrangement at ultralow temperatures to the evolution of transient quasi-molecular orbitals in relativistic collisions of stripped ions. Accurate measurements of ionization and recombina- tion rates are needed to understand the radiation from cosmic and laboratory plasmas and charge-transfer reactions, which play impor- tant roles in the heating of magnetic fusion plasmas by neutral atomic beams. . . New techniques have opened the way to detailed studies of the interactions of atoms and ions with surfaces and solids. The experi- ments can reveal the dynamics of electron pickup and loss in individual quantum states and open the way to the study of energy transfer under controlled conditions. Collisions in Laser Light As tunable lasers are developed over a wider spectral range, and femtosecond pulse techniques become widely available, new opportunities will be created for probing the elementary encounter event in a collision, a process heretofore inac- cessible. Such experiments can lead to a new and far deeper under- standing of inelastic collision processes and the nature of chemical reactivity. In addition, collisions in laser light can induce atomic

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18 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS transitions that would not otherwise occur and also reveal new photochemical pathways. ~ Quantitative Collision Theory Accurate calculation of collision properties is an essential complement to experimental studies of collision phenomena. Indeed, there are a number of systems for which calculations are more reliable and less expensive than the correspond- ing experiments, for example, the excitation of ions by electron impact. Major advances in computational methods and machine capacity and capability offer the opportunity for more-ambitious quantitative theo- retical studies to deepen our understanding of dynamical processes and to provide essential data for astrophysics, plasma physics, and other applications. INITIATIVE IN MOLECULAR PHYSICS Molecular physics is undergoing a renaissance. Triggered by laser methods, advances in molecular beams, and a host of other experi- mental techniques and motivated by new theoretical developments, molecular physics holds the promise of achieving a deep understanding of fundamental molecular behavior. (See Figure 1.2.) Opportunities abound for major advances, calling for a timely initiative, which we present in terms of two broad areas: the physics of isolated molecules and the physics of molecular collisions. The Physics of Isolated Molecules to understand the fundamen- tal bonding and electronic properties of simple molecules; to under- stand the joint motion of electrons and nuclei in molecular fields; to elucidate the formation, evolution, and decay of excited molecular states; and to investigate novel molecular species. The Physics of Molecular Collisions to study the correlated motions of electrons during collisions between atoms and molecules. By using new laser- and molecular-beam methods, one can observe collisions on a quantum state-to-state basis, including the coupling between electric and nuclear motion and the evolution of energy during collisions involving atoms and molecules. The Physics of Isolated Molecules Laser methods, synchrotron light sources, modern molecular-beam techniques, and other experimental advances provide the opportunity for a major advance in basic molecular physics and in the host of disciplines that hinge on molecular behavior. It is now possible to

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A PROGRAM OF RESEARCH INI TIA TI VES 19 prepare virtually any simple molecule in any desired quantum state and to study its structure with clarity. Furthermore, it is possible to study the physics that underlies this structure: the dynamics of electrons moving in molecular fields. There are two major directions in the research. The first centers on the spectra and structure of simple molecular systems. The study of unusual molecular species, for instance molecular ions, van der Waals and Rydberg molecules, metal cluster molecules and metastable spe- cies present new opportunities for understanding molecular structure. One can hope to understand how the electronic properties of molecules evolve into those of bulk material as one progresses from isolated atoms, through dimers and trimers, and to high states of aggregation; to understand what determines the geometric structure of a molecule whose constituents are held together by the weak van der Waals bond, and how the geometry of van der Waals molecules relates to crystal structure; to understand how the level structure of molecular Rydberg states resects the interactions within the molecular ion core. The second stream of research emphasizes the dynamical behavior of electrons and nuclei in each others' fields. The electron cloud in an isolated molecule continuously distorts in response to the slow vibra- tional motion of the nuclei, whereas the nuclei travel on a potential energy surface determined by the electrons' motions: the isolated molecule is a microcosm of elementary chemical behavior. We can now study the motions of electrons in molecules with unprecedented clarity. Picosecond and femtosecond laser experiments can reveal how energy flows from one part of a molecule to another, the transition to chaotic vibrational motion, and the rates and mechanisms that deter- mine the system's choice of a particular decay mode. Lasers provide highly selective excitation and interrogation schemes that reveal mo- lecular processes at the quantum-state-specific level. The possibilities are enormous; multilaser methods for studying photodissociation, double-resonance spectroscopy, and photoelectron spectroscopy are but a few of the techniques recently demonstrated. Synchrotron-radiation sources extend and complement the opportu- nities provided by lasers. These sources give unique access to intense, tunable radiation from the ultraviolet to the hard x-ray range, permit- ting excitation of virtually any molecular orbital of any stable molecule. Resonances can be observed shape resonances and autoionizing resonances in which the molecule is temporarily stalled in a quasi- bound state where the subtle effects of the internal dynamics are enormously amplified.

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20 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS (a) ~ b C d - ~1 - I I ~ I I I I I I . \ ~ 420 GHz X10O - X20 x2000 ~ n ,~1 1 i _ 3850 MHz 185 MH z 1_ 83 kHz FIGURE 1.2 A Revolution in Spectroscopy. The resolution of molecular spectroscopy has Increased by more than one million within a 12-year period. These four spectra of the sulfur hexafluoride molecule show the wealth of new details revealed every time the

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A PROGRAM OF RESEARCH INITIATIVES 21 The power of electron-scattering spectroscopy has been greatly increased by experimental advances such as spin analysis, coincidence techniques, and position-sensitive detectors. Theoretical progress in understanding the electronic continuum of molecules promises to provide new insight into vibrational and electronic excitation by electron impact, molecular photoionization, and other processes in- volving highly excited molecular systems. Further discussion can be found in Chapter 5 in the sections on The New Spectroscopy and Molecular Photoionization and Electron- Molecule Scattering. Research opportunities include the following: Transient Molecular Species Short-lived highly reactive species, including ions, free radicals, and metastable molecules, can now be produced in supersonic beams and studied in detail. These species offer new insights into chemical dynamics and methods for controlling chemical processes. ClustersThe transition region between a few atoms and mole- cules and the condensed phase solid or liquid is now open to study. Clusters provide a powerful new probe for unfolding the behavior of matter as it evolves from individual atoms to the liquid state, and for studying catalysis. The research bears on our basic understanding of materials and on industrial, technological, and environmental pro- cesses. Chaos Small molecular systems at low levels of excitation dis- play highly organized internal motions; at higher levels the energy often appears to be distributed randomly. Understanding the transition from orderly to chaotic motion would be an important advance in molecular dynamics and in the quest for state-specific chemical reactions. resolution is improved. The first spectrum, (a), is a conventional infrared absorption spectrum in the region of 10 m. The next spectrum, (b), obtained with diode lasers, shows a blown-up section of a small portion of (a). A rich new structure is revealed. The resolution is limited by Doppler broadening because of the motion of the molecules. In (c) the spectrum was taken by saturation spectroscopy, which avoids Doppler broaden- ing. The resolution of this spectrum is limited by the frequency jitter of the laser. This blowup of a small portion of (b) reveals yet more structure. Finally, in (d) the laser jitter has been reduced by electronic control and the resolution achieves the maximum value allowed by the uncertainty principle. The resolution is limited only by the finite observation time of the molecules. A single sharp line of (c) is revealed to consist of a complex of lines. The data represent work carried out at laboratories in France and in the United States. Further discussion is in Chapter 5 in the section on The New Spectroscopy. (Courtesy of University of Paris-North, Villetaneuse, France.)

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22 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS Reactive Plasmas The physics and chemistry of even simple low-density plasmas is not fully understood. Recently developed laser techniques such as velocity-modulation absorption spectroscopy and laser-frequency-modulated spectroscopy over powerful new ways to probe the dynamics of such plasmas. One can now measure the quantum-state dependencies of transport properties of ions in plasmas; one can investigate dissociative recombination; one can observe clearly the interaction of excited atoms, molecules, and ions with surfaces. Laser techniques offer an important opportunity to study low-density plasmas, a poorly understood state of matter, and to develop further the rapidly growing industrial use of plasmas. Excited-State Dynamics Individual quantum states are now ac- cessible by laser excitation, and their dynamics can be studied in detail. Femtosecond spectroscopy can freeze a molecule as it vibrates, permitting the study of how energy flows within a molecule. By using two or more independently tunable lasers and advanced charged- particle and photon detectors, excited-state photoionization dynamics can be studied with quantum-state specificity at each stage of the process. VUV and X-Ray Photoionization Advances in synchrotron radi- ation have opened the way for studying the fundamental dynamical parameters underlying molecular photoionization, including branching ratios, angular distributions, and spin states. The studies can now be extended to core levels with binding energies in the x-ray range. The energy resolution can be high enough to distinguish between different vibrational modes in polyatomic molecules. By combining lasers with synchrotron radiation sources, a new arena is provided the study of molecular electron dynamics in core-excited molecules. The Physics of Molecular Collisions Experimental and theoretical advances now make it possible to study molecular collisions with the quantum states fully resolved. Moreover, it is now possible to produce molecular beams of a wide variety of unusual molecules including highly vibrationally excited molecules and molecular ions, radicals, and van der Waals molecules- for use in novel collision studies. Modern lasers can dissociate mole- cules so as to create a "half-collision'' in which dynamical interactions occur only during separation of the products. Such studies now make it possible to study one of the most fundamental aspects of molecular reactions: how the available energy and angular momentum are shared

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A PROGRAM OF RESEARCH INI TIA TI VES 23 by the reactants. These developments presage a new and deep physical understanding of simple chemical reactions. In Chapter 5, the sections on Molecular Dynamics and Some Novel Molecular Species contain further discussions. Research opportunities include the following: State-to-State Chemistry With VUV lasers it is now possible to study the chemistry of hydrogen on a state-to-state basis and to compare the results with fully quantum-mechanical calculations of the dynamics on ab initio potential surfaces. New state-to-state experi- ments on heavier systems should open the way to understanding the interplay of translational, vibrational, and rotational energy in simple molecular collisions. Collisions in Laser Light- The absorption of laser light and the emission of radiation during a collision can drastically alter the course of the collision. This provides a new opportunity for studying the detailed evolution of chemical reactions and may open the way for using lasers to control reaction products. "Half-Collisions" In the last decade lasers have been developed that are capable of breaking apart polyatomic molecules with ultravi- olet light and multiphoton absorption of infrared visible light. Because the particles fly apart just as if they had initially collided, photodis- sociation can be viewed as a half-collision, but one in which the reaction complex has precisely known energy and angular momentum. The dynamics of the separating fragments can be studied by modern collision techniques. This research presents a new opportunity to study collision dynamics and to study the evolution of.free radicals. The results bear on problems ranging from combustion to atmospheric chemistry. Special Molecules Laser techniques now make it possible to produce molecular beams of radicals, of vibrationally hot polyatomics, and of small clusters of most atoms. Many of these are highly reactive. By causing two such beams to cross, the collision dynamics of the two species can now be studied. INITIATIVE IN OPTICAL PHYSICS The invention of new lasers and other novel light sources, the development of new methods of spectroscopy and nonlinear optics, and the continued discovery of scientific and practical applications for these new technologies have combined to promote optics and optical physics to a forefront area of contemporary physics. The research has

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24 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS had a strong impact on broad areas of science, on industry, and on many of our national programs. To assure the continued productivity and growth in this area, we propose the following initiatives: New Light Sources to develop techniques for producing electro- magnetic radiation from the far infrared to the x-ray region, including new lasers and coherent frequency multiplication techniques and new methods for producing short-pulse, short-wavelength radiation. Advanced Spectroscopyto develop new methods of ultraprecise spectroscopy, ultrafast spectroscopy, and ultrasensitive detection for the study of atomic and molecular structure and for application to chemistry, materials research, and surface science. Quantum Opticsto investigate new coherent states of the elec- tromagnetic field, to study nonlinear optical phenomena including the interaction of matter with light at extreme intensities, to exploit new opportunities to study radiative processes of atoms in altered states of the vacuum. Nonlinear optics, which has been a major arena for scientific and technical advances in optical physics, is not identified in a separate initiative area since it plays a role in nearly every one of the optics initiatives, as well as in the atomic and molecular initiatives. New Light Sources During the last decade a host of new light sources has become available for research and for industrial applications and for use in national programs: semiconductor diode lasers whose applications range from high-resolution infrared spectroscopy to fiber-optic com- munication; the tunable dye laser, which has revolutionized spectros- copy by providing a thousandfold increase in resolution and by opening the way to the preparation and study of atoms, ions, and molecules in states never before achieved; excimer lasers for applications in photo- chemical processing; the free-electron laser, which holds the promise of providing intense coherent radiation from the infrared through the ultraviolet regions; neodymium/glass lasers that are powerful enough to ignite thermonuclear fusion reactions; and laser-based ultraviolet and x-ray sources. The light sources have opened new areas in the study of the structure of atoms and molecules and in materials science and have applications ranging from ultrasensitive detection of pollut- ants to metalworking and other manufacturing processes.

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A PROGRAM OF RESEARCH INITIATIVES 25 GAS IN (BACKING PRESSURE ~ IOATM) ~: :~ -.: ~ - ~ : :::::: ::~ ::: ::~ ::: SOLENOID COIL :: ::~:INPUT ~ ::: uv 3 :~: LASER ~ _ I: :~ JET EXPANSION 100 mm FOCAL LENGTH LENS PROBE Mar _ LASE R 5th HARMONIC 49.7 nm 7th HARMONIC 35.5 nm 3rd HARMONIC 82.8 nm _ - - 11 J ~1In ~ ~ A . . . . . 30 40 50 60 70 80 90 7t h HARMON IC (SECOND ORDER) WAVELENGTH (nary) TANTALUM TARGET r \ . N /- ~ t 1 1 1 1 1 1 1 1 1 1 1 1 HIGH POWER INFRARED LASER SOFT X-RAY / RAD I ATI ON i-! '. ; -. -I// - .- ~ _ :/.---.~:- I \ LITH I UM VAPOR ~ FIGURE 1.3 Short-Wavelength Light Sources. Surface science, chemistry, biology, materials processing, and holography are a few of the many scientific and technical applications that await the development of intense sources of coherent light in the extreme-ultraviolet and soft x-ray regions of the spectrum. The upper left drawing shows a recently developed method for generating extreme-ultraviolet light by harmonic generation of near-ultraviolet laser light in a pulsed supersonic jet of helium. In the spectrum of light from the jet, upper right, signals at the third, fifth, and seventh harmonics of the laser light are clearly visible. The lower drawing illustrates a method for generating a continuous spectrum of radiation in the extreme-ultraviolet and soft x-ray regions. An infrared laser is focused onto a metal target, creating a tiny hot sphere of plasmas, which radiates soft x rays with an efficiency that is often as high as 30 percent. In the setup shown, the x rays are used to photoeject an inner-shell electron from lithium, creating population inversion in the vapor, which may be used to create a short- wavelength laser. (Courtesy of AT&T Bell Laboratories and Stanford University.) Figure 1.3 illustrates two novel techniques for generating short- wavelength light. Further discussion is presented in Chapter 6 in the section titled Lasers The Revolution Continues. Research opportunities include the following: Short-Wavelength LasersNew techniques including multipho- ton excitation and inner-shell excitation of atoms and molecules offer

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26 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS possibilities for producing laser radiation in the far ultraviolet and in the x-ray regions. Intense beams of pulsed heavy ions can excite a gaseous target and may lead to intense pulsed soft x-ray lasers. Such coherent sources would have an important impact on atomic and molecular spectroscopy, on materials research, on holography, and in many other areas. Nonlinear ProcessesCoherent radiation in the far ultraviolet region can be generated by harmonic multiplication of optical and near-ultraviolet lasers in pulsed gas jets, by four-wave mixing in gases, and by other nonlinear techniques. Such sources are complementary to synchrotron light sources, providing extremely bright light in certain spectral regions with relatively small-scale equipment. New methods for image formation and optical processing based on phase conjugation and other nonlinear processes can have important applications in optical communication, in astronomy, and in the manufacture of integrated circuits. Short Pulses 19-femtosecond (19 x 1o-~5 second) light pulses have been generated. As the technology for producing femtosecond pulses improves, increasing numbers of applications can be expected in molecular physics and materials research. Femtosecond pulses make it possible to study the time dependence of basic molecular, solid state, and biological processes. Femtosecond optics provides a new technol- ogy for the development of very fast circuits. New Lasers New types of lasers are needed, including efficient and powerful optical lasers; simple tunable lasers throughout the infrared, visible, and ultraviolet regions; and ultrastable lasers. Appli- cations for these lasers include communications, trace-element detec- tion and environmental monitoring, chemical and plasma diagnostics, medicine, and manufacturing. / Advanced Spectroscopy Spurred by the development of new lasers and light sources during the past decade, there has been a revolutionary advance in the spectroscopy of atoms, molecules, and materials. The major source of line broadening in conventional spectroscopy Doppler broadening- has been effectively eliminated by techniques such as saturation spectroscopy and two-photon Doppler-free spectroscopy. By employ- ing stabilized tunable lasers with those techniques, spectroscopic resolution has been improved by a factor ranging between 103 and 106. (See Figure 1.21. The methods have been applied to problems ranging from the ultraprecise spectroscopy of exotic atoms to determining the

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A PROGRAM OF RESEARCH INITIATIVES 27 barriers to internal rearrangement in phosphorous compounds that provide prototypes for the basic energy reactions in living organisms. Optical-frequency-counting methods have achieved such high preci- sion and range that the definition of the meter has been fundamentally altered: previously the meter was defined in terms of the wavelength of a spectral line of krypton; now it is defined as the distance light travels in a given interval of time. Light-scattering spectroscopy has found numerous applications in condensed-matter physics, particularly in the study of critical phenomena, and in chemistry, biology, engineering, and medicine. New methods of ultrasensitive detection have made it possible to study molecular ions and free radicals, which are important in many chemical reactions; these methods are being applied to industrial processing and remote sensing. Techniques based on coher- ent Raman processes have made it possible to study chemical species in hostile environments, for instance in the combustion chamber of an engine. Multiphoton spectroscopy has made it possible to study new classes of atomic and molecular states; while techniques of labeling spectroscopy have vastly simplified the interpretation of complex molecular spectra and have made it possible to prepare molecules in selected quantum states. Further discussion can be found in Chapter 6 in the section on Laser Spectroscopy. Research opportunities include the following: Ultraprecise Spectroscopy Several different techniques are com- ing together to permit major advances in ultraprecise spectroscopy. These include highly stabilized tunable lasers, methods for trapping ions and atoms, laser cooling of atomic beams and trapped particles, coherent spectroscopic techniques, and optical-frequency-counting methods. In addition to the application to high-precision spectroscopy, including the study of slow dynamical processes in molecules, these advances create opportunities for new types of optical-frequency standards and atomic clocks, with applications in high-precision mea- surements and optical communications. Doppler-Free Spectroscopy Methods for studying optical transi- tions with a resolution at the limit of the natural line width offer the possibility of major advances in the study of elementary systems such as hydrogen, positronium, and muonium. The techniques are applica- ble to large classes of atoms and simple molecules. Ultrasensitive Detection Intracavity absorption spectroscopy, laser magnetic resonance, photoacoustic detection, resonant multi- photon ionization, and other techniques permit the study of unstable

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28 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS molecular species. In addition, they offer the possibility of new types of trace analysis for scientific and industrial applications. Ultrasensi- tive optical detection provides a new method for measuring the solar neutrino flux by detecting nuclei produced by solar neutrinos. Quantum Optics Understanding the statistical properties of light and the electrody- namics of matter and light is central to understanding the generation and propagation of light, the transmission of information, and many other physical processes. Further discussion can be found in Chapter 6 in the section on Quantum Optics and Coherence. Research opportunities in quantum optics include the following: Preparation of Light in Novel Statistical States- It may be possible to generate new types of light by learning to control the light's statistical properties. Such a development, particularly the creation of light in squeezed states, has potential applications to quantum metrol- ogy and to communication. Optical Bistability Optical bistable devices provide new oppor- tunities to study the transition from uniform to chaotic motion. By permitting controlled experiments at a very rapid rate, they may provide a useful tool for studying large-scale chaotic motion, such as the onset of turbulence. Optical bistability has important potential applications to optical processing, including new types of optical logic elements. Electrodynamics at Long Wavelengths New types of radiative processes can be seen at microwave or millimeter-wave frequencies using highly excited atoms. The evolution from irreversible to revers- ible motion can be observed, and the basic source of noise in nature- spontaneous emission can be modified. New types of electrodynamic effects can be observed, and coherence in small atomic systems can be studied.