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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 Structure—A 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
OCR for page 21
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.
· Clusters—The 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
OCR for page 23
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
OCR for page 24
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 Spectroscopy—to 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 Optics—to 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.
OCR for page 25
A PROGRAM OF RESEARCH INITIATIVES 25
GAS IN (BACKING PRESSURE ~ IOATM)
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JET EXPANSION 100 mm
FOCAL LENGTH
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49.7 nm
7th HARMONIC
35.5 nm
3rd HARMONIC
82.8 nm
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30 40 50 60 70 80 90
7t h HARMON IC
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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 Lasers—New 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 Processes—Coherent 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
OCR for page 27
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
OCR for page 28
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.
Representative terms from entire chapter:
optical physics
