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7
Scientific Interfaces
The boundaries of atomic, molecular, and optical (AMO) physics
penetrate far into the neighboring areas of science. Across its borders
flows a stream of new techniques and vital data. There is hardly an area
of science that has not significantly benefited from these. Geology,
geophysics, and planetary physics, for instance, have all been enriched
by the maps created with optically pumped magnetometers. Laser
surveying, to cite another example, permits monitoring the strains that
lead to earthquakes as well as the motion of the continents as they drift
and of the moon as it wobbles in its orbit. It also makes possible the
high-precision alignment of large particle accelerators and allows
tunnels drilled from opposite sides of a mountain to meet exactly.
There is a second form of commerce between AMO physics and the
neighboring areas: this is the commerce of basic science itself. In this
the boundaries disappear as the unity of science asserts its pre-
eminence. Among the six interface areas that we have chosen to
describe here astrophysics; materials research; surface science; and
plasma, atmospheric, and nuclear physics instances of the underlying
unity constantly occur.
ASTROPHYSICS
Most of what we know about the universe comes from information
brought to us by photons. To decipher their messages, we must
126
OCR for page 127
SCIENTIFIC INTERFACES 127
understand how the photons came into existence and the histories of
their journeys through intergalactic and interstellar space. From this
enterprise we can learn about the early universe and the nature of the
astrophysical entities quasars, galaxies, stars, pulsars, stellar winds,
supernova remnants, nebulae, masers, and molecular clouds. The
events that produce photons and the processes that modify them during
their long journeys lie squarely in the domain of AMO physics. AMO
physics is an essential component of astronomy.
AMO physics ranges broadly in its applications to astronomy. The
physical and chemical processes that created molecular hydrogen in
the early pregalactic universe, which manufactured cyano-octa-
tetrayne in interstellar clouds and propane in the atmosphere of Titan,
which bring molecular clouds to the brink of gravitational collapse and
trigger star formation, which control the abundance of ozone on the
planet Earth, and which determine the radiative losses from stellar
interiors, are all part of the body of AMO physics. The unity of AMO
physics is manifested in the remarkable diversity of environments in
which atomic processes play the crucial role.
Astronomy and AMO physics are mutually dependent. Because the
extraordinary physical environments that arise in astronomical phe-
nomena are often impossible to duplicate in laboratories, astronomy
provides an extended arena for studying atomic processes. For exam-
ple, despite the enormous differences in the scales of laboratory and
astrophysical plasmas, the fundamental processes are identical. The
same mechanism that is believed to be responsible for the lack of
electrons at high altitudes in the atmosphere of Jupiter, as revealed by
data from the Voyager spacecraft, has been proposed for producing the
negative hydrogen ions that are needed to ignite a thermonuclear fusion
plasma. The mechanism involves vibrationally excited molecular hy-
drogen. These molecules are ubiquitous. In the interstellar medium
their Doppler-shifted lines signify the shock waves that accompany the
birth of stars. Provided that the molecular processes are clearly
understood, these lines can provide powerful diagnostic probes of the
earliest stages in the evolution of a star.
Interpreting the abundant data of astrophysics demands a deep
understanding of atomic, molecular, and optical processes. In addition,
it demands a broad data base of atomic and molecular parameters such
as transition energies, oscillator strengths, and photon and particle
collision cross sections. Providing these data is a major challenge for
atomic and molecular physics. Experimental data flow from all
branches of the field, particularly from the discipline that has come to
be called laboratory astrophysics. These experimental data are vital,
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128 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
but more data are required than the experimental community can
possibly provide. Thus, theoretical data are also vital. The need to
generate theoretical data for astrophysics motivates a major portion of
the theoretical effort in the atomic and molecular community.
The required data base is huge. Several million emission lines are
present in the spectra of the Sun and stars. Models of stellar atmo-
spheres have limited success, even in the visible region. Important
questions are unresolved: the discrepancy between the predicted and
measured solar neutrino fluxes may be due in part to errors in the
calculated opacity of the Sun. Stellar explosions, which play a crucial
role in the energetics of the galaxy and the formation of new stars,
provide another example. These explosions leave a remnant in which
elements such as oxygen, sulfur, silicon, iron, and nickel are stripped
of all but one or two electrons. The emission lines of these ions, which
fall in the x-ray region, can yield not only the element abundances but
also the density and temperature of the remnant. Because the relevant
atomic data are not available, a comprehensive description of the
atomic processes occurring in a supernova remnant has yet to be
achieved.
Atomic Processes
Many atomic processes play crucial roles in astrophysics. To cite
one example, in recent years the importance of atomic charge transfer
in cosmic plasmas has become evident. In interstellar gases composed
of elements in their cosmic abundances, ionizing radiation often
produces partly ionized plasmas containing some neutral atomic hy-
orogen. Charge-transfer collisions with the hydrogen drastically mod-
ify the ionization structure of the gas. The emission spectra offer a
highly specific diagnostic probe of the plasma. Provided the charge-
transfer processes are understood, the spectra can serve to establish
the coexistence of multiply ionized and neutral material, provide a
direct measure of the neutral abundances, and give unique information
on the nature of the ionizing source.
Rydberg Atoms
Ionized gases emit photons at all wavelengths. At radio wavelengths,
the photons arise from transitions between highly excited Rydberg
levels with principal quantum numbers that can exceed 300. Rydberg
atoms are large in size and sensitive to disturbances, but the space
between stars is nearly empty, and there is room for the Rydberg atoms
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SCIENTIFIC INTERFACES 129
to survive until they radiate. Because atomic theory can provide a
detailed description of the modes for populating and depopulating
Rydberg levels, these atoms can be a valuable guide to events
occurring in our own galaxy and in external galaxies. They are used to
infer the temperature and densities and the hydrogen-to-helium abun-
dance ratio.
Rydberg atoms can now be generated in the laboratory, and their
study has developed into a lively subfield of atomic physics. (This is
discussed in Chapter 4 in the section on Atomic Structure and in
Chapter 6 in the section on Quantum Optics and Coherence.) The
prominence that Rydberg atoms assumed in the laboratory in the
mid-1970s was stimulated by their discovery in space in the 1960s.
Interstellar Molecules
Transitions between low-lying levels in molecules generate radiation
at radio frequencies. Because the photons suffer little attenuation by
interstellar dust, the radio emission lines can be seen over large
distances. More than 50 interstellar molecules have been discovered.
To cite one consequence, the distribution of matter throughout the
galaxy has been mapped from the emission lines of carbon monoxide.
Radiation from interstellar molecules can extract energy from the
interstellar clouds, cooling them to the brink of gravitational collapse.
Two of the most fundamental astrophysical processes, nucleosynthesis
and the chemical evolution of the galaxy, can be studied by observing
the spatial distribution of isotopic molecules such as '3C~6O and
~2C~8O, though the task requires the mastery of the basic molecular
chemistry. In order to determine reliable isotope ratios, for example,
molecular fractionation must be understood.
Molecular fractionation substantially enhances the abundances of
deuterated molecules- molecules in which a hydrogen atom is re-
placed by a deuterium atom. By joining the theory of ion-molecule
chemistry in interstellar clouds with observations of the abundance
ratio of the deuterated compounds, the electron density in molecular
clouds can be inferred. This density is a critical astrophysical param-
eter. Gravitational collapse and the fragmentation of molecular clouds
to form stars are mediated by free electrons. Despite the deep
significance of molecular fractionation, however, one of the essential
molecular parameters of fractionation theory remains unknown. As a
result, no more than an upper limit can be obtained for the electron
density.
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130 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
Many interstellar molecules are chemically reactive. In the labora-
tory they exist only as short-lived transient species, difficult to study at
high resolution. In some cases butadinyl and cyanoethynyl are ex-
amples- the spectrum can be more accurately measured in space than
in the laboratory. In other cases, thanks to recent developments in
laboratory techniques and laser technology, laboratory measurements
are now superior. Thus, the fine-structure parameters of the reactive
neutral atom, carbon-12, were first determined in the laboratory. The
results pointed the way to the successful detection of atomic carbon in
dense interstellar clouds. A great many other interstellar species with
different isotopic constituents await investigation.
Astrophysical Chemistry
Molecular ions occur at crucial points in the ion-molecular schemes
that attempt to explain the formation of interstellar molecules. The
measured ion abundances provide a sensitive test of the chemical
models. Few of the reaction-rate data are available, still fewer at the
temperatures prevailing in molecular clouds. The most important
reaction pathways may not yet be recognized; the success of the
chemical schemes may be no more than an artifact of unreliable data.
The very first laboratory experiments on molecular reactions at low
temperatures were carried out recently; these may lead to a quantita-
tive description of molecular formation in cold clouds. Molecules have
now been detected not only in interstellar clouds but also in the hostile
environments of stellar atmospheres and circumstellar shells. It seems
likely that they also exist in other astrophysical regimes such as
quasars and that they may someday be useful in detecting of x-ray
sources and supernovas buried inside dense clouds.
Cosmology
Atomic and molecular processes can provide vital clues to the nature
of the cosmos. For example, the distribution of deuterium in the galaxy
provides a direct measure of the matter density in the early universe
and bears directly on the question of whether the universe is closed or
open. The deuterium is detected as a constituent of different molecular
species; the chemistry of deuterated molecules must be understood
before the total deuterium content can be obtained.
The spectrum of the cyanogen molecule also has direct cosmological
significance. From its optical absorption spectrum the relative popula-
tions of the two lowest energy levels can be determined, and from this,
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SCIENTIFIC INTERFACES 131
its temperature. The temperature was found to be 2.8 K. This was the
first measurement of the temperature of the universal blackbody
background radiation left over from the big bang.
Cosmology can provide unique insights into fundamental atomic
principles. From absorption-line measurements toward distant objects
at large red shifts, for example, limits can be set on how the funda-
mental atomic constants can vary in space and time. One result is that
the fine-structure constant cannot vary by more than 1 part in 10-~2 per
year.
Space Physics
Astronomy is primarily driven by remote observations, but one
component, space physics the study of the local solar system is
advanced by local experiments with instruments carried aboard space-
craft. The interplanetary medium undergoes violent upheaval where
the solar wind collides with the ionized gas in the outer regions of the
planets and their satellites, providing a natural laboratory for studying
the effects of electric and magnetic fields on the large-scale motions of
energetic charged particles. Atomic and molecular physics is essential
to understanding the scene. Charged particles are created, scattered,
and lost by atomic collisions. Planetary atmospheres respond to solar
ionizing and dissociating radiation in a complex array of atomic and
molecular processes. The evolutionary paths followed by these atmo-
spheres are affected by escape mechanisms driven by energy transfer
in atomic and molecular collisions. The interpretation can be subtle and
can lead to unexpected conclusions. For example, the Viking lander on
Mars measured a '5N/~4N isotope ratio 60 percent larger than the
terrestrial value, suggesting the operation of a differential escape
mechanism for the two isotopes. On Mars, the process of dissociative
recombination of ions of molecular nitrogen generates nitrogen atoms
with kinetic energies sufficient to escape the gravitational field of the
planet. As the isotopes undergo gravitational separation in the atmo-
sphere, the heavier isotope becomes depleted at the high altitudes
where escape occurs. A careful accounting of the escape efficiency
establishes that Mars once contained a large reservoir of nitrogen gas.
Similar mechanisms occur on Venus with a startling corollary. When
molecular-oxygen ions recombine on Venus, they produce energetic
oxygen atoms that collide with hydrogen atoms and drive the hydrogen
out of the atmosphere. The collisions are too weak to drive out the
heavier deuterium atoms. As a result, the deuterium/hydrogen ratio on
Venus is much larger than anywhere else in the solar system. From the
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132 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
ratio measured by the Pioneer Venus space probe, one can infer that
Venus originally had a large abundance of water.
CONDENSED-MATTER PHYSICS AND MATERIALS SCIENCE
Numerous links join AMO physics with condensed-matter physics
and materials science. The study described in Chapters 4 and 5 of how
x-ray and photoionization spectra of gaseous atoms and molecules
evolve as they assemble into liquids and solids reflects one aspect of
this interface area. AMO physics has generated experimental tech-
niques ranging from molecular-beam epitaxy and clusters to laser
annealing and sputtering. The impact of AMO physics on surface
science one of the liveliest areas in solid-state physics is so large
that it is described separately in the next section. In this section, we
describe three activities: light-scattering spectroscopy, metal clusters,
and the creation of spin-polarized quantum fluids.
Light-Scattering Spectroscopy
The extraordinary spectral purity of gas lasers has made them an
important source of radiation for the observation of thermally excited
fluctuations and of fluid flow in condensed-matter systems. The inter-
action of the laser radiation with spontaneous molecular motion
produces spectral broadening or frequency shifts in the scattered light
that generally range from 1 to 105 Hz. (The frequency of visible light is
about 10~5 Hz.) The accurate resolution of such small-frequency shifts
has become possible using the techniques of optical mixing spectros-
copy. These techniques represent the successful extension of hetero-
dyne and homodyne detection methods, long employed in radio-
frequency and microwave spectroscopy, upward into the optical
frequency domain. As a result of these advances, a new form of
spectroscopy, known variously as quasi-elastic light-scattering spec-
troscopy, photon-correlation spectroscopy, or intensity-fluctuation
spectroscopy, has emerged and been applied to a wide range of
fundamental and applied problems in physics, chemistry, biology,
. . . .
engineering, ant met lclne.
Order-Disorder Transitions: In physics, light-scattering spectros-
copy has provided many of the basic determinations of the critical
exponents for the divergences of the equilibrium and transport coeffi-
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SCIENTIFI C I NTERFA CES 1 3 3
cients of pure fluids and binary mixtures near their order-disorder
phase transitions. The profound nature of the theoretical ideas needed
for the exploration of these and related experiments in magnetic
systems culminated in the creation of the renormalization group
theory, one of the major achievements of modern condensed-matter
theory. Quasi-elastic light scattering has been the principal experimen-
tal tool used to investigate the hydrodynamic modes and phase
transitions of liquid-crystal systems, a field of high current interest.
Light-scattering spectroscopy has been used to study the relaxation of
density fluctuations in gases, the propagation of elementary excitations
in liquid helium, and the development of soft modes in the phonon
spectrum in solids near phase transitions. It is a principal experimental
means of investigating the transition to turbulence (or chaos) in
hydrodynamics and has been used to determine the value of important
universal numbers in the theory of strange attractors.
Applications to Chemistry, Biology, Engineering, and Medi-
cine: In chemistry, the method is widely used to obtain important
microscopic information on the fundamental interactions between
amphiphillic molecules, which self-assemble to produce well-defined
geometrical structures: micelles, microemulsions, vesicles, and bilay-
ers. These structures are fundamental constituents of the living cell and
are of great importance in a wide variety of industrial chemical
processes. Quasi-elastic light-scattering spectroscopy has been used to
discover scaling phenomena in polymer solutions and to examine the
moments of polymer cluster size distributions near the sol-gel transi-
tion. In polymer gels it has been used to discover a rich variety of
hitherto unexpected first-order phase transitions. The latter phenom-
ena are potentially promising for the development of mechano-
chemical, mechano-electrical, and electro-optical devices.
In biology, quasi-elastic light-scattering spectroscopy has been used
to determine quickly and accurately the diffusion coefficients and
hence the size and degree of self-association of a wide variety of
biological macromolecules including proteins, viruses, and antibody-
antigen complexes. These studies have been used to characterize
accurately the precise form of the Coulomb and van der Waals
interactions between polyelectrolytes in ionic solutions. The method is
also used in studies of colloid stability, ordering, and flocculation.
Light-scattering spectroscopy has led to the opening of the broad
field of laser Doppler velocimetry, which permits noninvasive mea-
surements of fluid flow in situations ranging from aircraft wake velocity
fields to the in vivo determination of blood velocity in the human
retinal vasculature.
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134 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
Atoms in Solids: The theoretical insights and techniques developed
to describe atomic and molecular structure are being applied to
problems of condensed-matter structure. Two examples are the de-
scription of electronic states of impurity ions and atoms in crystals,
for instance Ca+ in crystalline LiC1 or H in amorphous silicon, and
the calculation of the band-gap energies of insulators and semicon-
ductors.
The approach is straightforward, at least in principle. In the inde-
pendent-electron approximation, the exchange-correlation interaction
is represented by a one-electron local potential. The variational wave
function is represented as a linear combination of atomic orbitals, just
as in molecular-structure calculations. Carrying out such a calculation
is a formidable task. But by representing the exchange-correlation
interaction with a simple local density-dependent exchange potential
(ignoring correlation entirely), and using a series of Gaussian functions
to describe the atomic orbitals, the theory becomes tractable, even for
disordered systems for which the standard band-structure methods
are not applicable. Density functional theory has established that
the one-electron density uniquely defines the ground-state energy
of a system, but it is surprising how well the local-exchange theory
works.
A serious difficulty is that the theory fails to predict accurately the
band gap, the energy separation between the uppermost valence levels
(top of the valence band) and the lowest conduction levels (bottom of
the conduction band) of insulators and semiconductors. Recently,
dramatic improvement in the theoretical predictions was obtained by
making a simple self-interaction correction to the total energy, thus
bringing the local-exchange theory more in line with true Hartree-Fock
theory, in which the correction is implicit. It has also been shown that
the correction leads to much improved energies for isolated atoms.
While correlation effects may still prove to be important in some
circumstances, the self-interaction correction appears to be a signifi-
cant improvement in the quality of the theory.
Clusters
Chemical and physical processes often occur in a state of aggregation
that lies midway between a dilute gas and condensed matter. The
entities of this state are aggregates of small numbers of atoms or
molecules called clusters. The properties of clusters are intermediate
between those of single atoms or molecules and those of solids or
liquids. Many of the processes that occur in the cluster regime are
OCR for page 135
SCIENTIFIC INTERFACES 135
important to technology and industry and to environmental issues.
These include catalytic reactions; the formation of fog, smog, and
aerosols; and the formation of particulates in combustion reactions.
Clusters play a role in solution chemistry because they can retain their
identity even in the liquid phase.
In contrast to the detailed spectral information that exists for atomic
and molecular dimers, information on the electronic properties of
trimers and heavier clusters is scarce. Recently, the electronic absorp-
tion spectrum of the sodium trimer was determined over the complete
visible region of the spectrum in a two-photon photoionization exper-
iment. The experiment provided the first unambiguous measurement of
the absorption spectrum of a gas-phase triatomic metal cluster. At
present, spectral or structural information about gaseous clusters
beyond the trimer are lacking. These data are critically needed to
provide the link between the dimer and the bulk phase. The one
continuous property known today for heavier clusters, from 2 to 15
atoms, is the photoionization potential. The earliest measurements of
photoionization potentials were on alkali clusters; however, more
recently photoionization thresholds as a function of cluster size have
been reported for other species including rare-gas clusters, metal
clusters, and a few molecular clusters such as (COW, (CS2)n, and
(H2S)n
Laser-induced fluorescence has been used to determine the spectra
of dimers of large organic molecules. These studies provide informa-
tion on the energetics of cluster formation, for instance the bond
dissociation energy, and information on the transfer of energy between
the two moieties of the dimer via the weak van der Waals bond. Other
studies have determined the spectra of an organic molecule bound to an
increasingly large number of rare-gas atoms. Since the rare-gas atom
acts as a weak perturbe-r of the energy levels of the host molecule,
these studies approximate matrix isolation studies, allowing the de-
tailed determination of the ejects of the matrix on the spectra of the
host molecule. One can also approach the cluster region from the solid
state. Here the goal is the size at which the collective properties of the
solid disappear as the particle diameter is reduced. Experimental data
have been reported for melting point, superconductivity, valence-band
narrowing, photoelectric yield, plasmons, Mie optical absorption,
magnetic moments, Compton profile, superparamagnetism, far-
infrared absorption, specific heat, and crystallographic structure.
Clusters can also be used to study surface physics, as described in
the following section on Surface Science.
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136 A TOMIC, MOLECULAR, AND OPTICA ~ PHYSICS
Ultranarrow Optical Transitions
Within the last few years it has been found that certain optical
transitions of impurity ions in solids (the praseodymium ion in
lanthanum trifluoride is one example) display extremely narrow
linewidths, 1 kilohertz or less. These optical transitions, the zero-
phonon lines, are optical analogs of the Mossbauer effect: the optically
excited impurity ion suffers no recoil effect because its momentum is
transferred to the lattice as a whole. Furthermore, at cryogenic
temperatures there is virtually no second-order Doppler broadening.
These systems are prime candidates for studying the interactions that
broaden optical transitions and possibly for establishing secondary
optical-frequency standards. The method has been applied to study the
optical Bloch equations, the starting point for many theories in
quantum optics. It was found that intense laser fields can inhibit the
line-broadening effects of nuclear magnetic interactions. The phenom-
enon has spurred reconsideration of microscopic theories of nuclear
magnetic interactions.
This research has provided the first experimental test of the optical
Bloch equations, the equations of motion that were initially devised by
F. Bloch to describe nuclear magnetic resonance. These equations are
widely applied in quantum optics and laser spectroscopy, particularly
in gases and liquids; they are the starting point for work in these fields.
However, in solids it has been discovered that they fail because the
laser field amplitude increases because of a coherent averaging effect
that reduces the optical linewidth. A microscopic quantum theory, a
modified form of the Bloch equations, has been devised to deal with
this situation.
Spin-Polarized Quantum Fluids
All forms of matter solidify at sufficiently low temperature except for
one class of systems—the quantum fluids which remain in liquid or
gaseous states as the temperature approaches zero. Within the past few
years two new quantum fluids have been created using techniques from
AMO physics: spin-polarized gas 3He and spin-polarized atomic hy-
drogen.
Because 3He has a total spin of one half, the atoms obey the Pauli
principle and there is an effective repulsion between them when their
nuclear spins are parallel. As a result, diffusion, viscosity, and thermal
conductivity of the gas all depend on the nuclear polarization. The
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140 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
1.6
1.4
1.2 1
t.o
0.8
0.6
=. 0.4
~ 0.2
In
- LiF (001)
Bi = 64.2°
k' = 6.061
- Tt =295K
1 1 1
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
Time of flight Emsec]
FIGURE 7.1 Surface Scattering with a Supersonic Helium Beam. An atomic beam of
helium constitutes a powerful probe for studying surfaces. In fact, it is believed that the
role of helium scattering in the study of surfaces may be comparable with the role played
by neutron scattering in the structure of solids. An intense monoenergetic flux of atoms
is required; this is provided using supersonic-beams methods developed from research
i
-
OCR for page 141
SCIENTIFIC INTERFACES 141
the details of such reactions; clusters may provide the key to under-
standing what happens.
Studying Surfaces with Laser Light
Laser light makes it possible not only to study surfaces in ways never
before possible but also to change the surface in new ways. Short
intense laser pulses can reveal dynamical surface phenomena; coherent
UV light can produce new types of nonlinear surface effects when it
strikes adsorbed molecules. Laser light can trigger chemical changes
on surfaces, in the substrate, and in the overlying gas.
How a surface affects a photochemical reaction depends critically on
whether the molecule decomposes immediately in the light or whether
the reaction takes place slowly; with pulsed laser techniques the two
alternatives can be distinguished. Chemical reactions of particles
adsorbed on a semiconductor can be triggered in a controlled fashion-
essentially catalyzed- by using laser light to generate electron-hole
pairs within the material. The holes drift to the surface and trigger the
reaction. If the process can be made to occur at a gas-solid interface it
could provide an immensely useful new catalytic technique. The
surface-sticking coefficient for vapor-phase metal atoms can change by
decomposing a thin adsorbed layer of metal alkyls with laser light,
offering for the first time a precise way of controlling the interchange of
energy between a gas and a surface. Finally, photoreactions on the
surface can trigger the growth of new materials with novel properties.
The technique has important applications to semiconductor electronics
and to electro-optics.
A dramatic discovery from the study of surfaces with laser light is
that Raman spectra on surfaces can be enhanced by a magnification of
the local optical electric field. The enhancement can be enormous as
much as a factor of 106. A typical experiment uses green laser light to
illuminate a silver surface containing microscopic spheres or ellipsoids.
These particles exhibit a plasma resonance that magnifies the electric
fields in their vicinity. The plasmon resonances are of considerable
interest in their own right. The technique provides an extremely
on molecular scattering. The data show clearly resolved structure in the speeds of atoms
scattered from a lithium fluoride crystal. The spacing of the peaks provides detailed
information from which the structure and motions of atoms on the surface can be
determined. (Courtesy of Max-Planck-Institute for Fluid Dynamics, Gottingen, Federal
Republic of Germany.)
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142 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
VAPORI ZATION PROBE
LASER LASER
~ =],= 1
MAIN
L CHAMBER
SUPERSONIC METAL CLUSTER BEAM
APPARATUS
hex
I 1 ~ I I LL Lit
W.
x
NUMBER OF ATOMS IN CWSTER
FIGURE 7.2 Metal Clusters. Clusters are small groups of atoms or molecules in a state
of matter intermediate between a dilute gas and condensed matter. Supersonic beams of
metal clusters are made by vaporizing the metal in an intense pulsed supersonic beam of
helium. Using a high-power pulsed laser it is possible to vaporize even the most
refractory metals. The data show mass spectra for clusters of iron, nickel, tungsten, and
molybdenum. Most of the atoms in the clusters lie on the surface, even for clusters as
large as 100 atoms. Because the physical and chemical properties of the clusters are
dominated by surface phenomena, supersonic metal atom clusters provide an important
new arena for surface science. The technique is particularly valuable for the study of
catalysis, a subject of high scientific interest with potentially important applications in
chemistry, in manufacturing, and in energy programs. (Courtesy of Rice University.)
sensitive tool for studying surfaces because small quantities of adsor-
bates in the vicinity of the metal particles are easily detected.
It has been found that silicon can reproducibly change from a
crystalline solid to the amorphous state and back again to a crystal with
successive picosecond laser pulses. Picosecond-pulse probes have
been used to study the solid-state plasma that is formed in silicon when
it is illuminated by a pulse of light from a short-wavelength laser. Such
studies provide a new and novel way to study the dynamics of crystal
growth and to understand the mechanisms that underlie the many
applications of laser annealing.
These are but a few examples of how laser light can be used to study
surfaces and surface chemistry. The opportunities are great, and the
field is growing rapidly.
The Role of Atomic, Molecular, and Optical Data in Surface
Science
In addition to contributing experimental and theoretical techniques
to surface science, AMO physics provides basic data that are essential
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SCIENTIFIC INTERFACES 143
to the interpretation of much surface research. For instance, Auger
electron spectroscopy and x-ray photoelectron spectroscopy, widely
used techniques for determining surface chemical composition, de-
mand extensive data from AMO physics: ionization cross sections for
electron and photon excitation, electron binding energies, and Auger
transition probabilities. Other data are needed to relate the Auger line-
shapes to the chemical states of atoms in molecules and to interpret the
probabilities of multielectron excitations in core-level spectroscopies.
Laser studies of surfaces require multiphoton ionization probabilities
for atoms and molecules, fluorescence lifetimes and probabilities, and
photoabsorption cross sections. Vibrational and rotational emission
and absorption spectra for hot molecules are also needed.
Stimulated desorption studies and sputtering spectroscopy require
impact cross sections for ionization, branching ratios for dissociative
ionization processes in small molecules, and spectroscopic data on ions
and highly excited neutral species.
PLASMA PHYSICS
Plasmas are systems of ionized gas whose behavior is determined in
large measure by collisions of electrons and ions with each other and
with any neutral material that may be present. Plasmas range in
physical conditions from hot fully ionized magnetohydrodynamic plas-
mas occurring in planetary and interstellar environments to various
experimental devices. As our understanding of the physics of plasmas
has deepened, the importance of atomic processes in plasmas has
become apparent. Atomic processes play a basic role in the creation of
most plasmas. Neutral-particle injection into magnetically confined
plasmas is used to raise the plasma temperature and bring it to ignition.
Atomic processes are crucial diagnostic probes of the physical condi-
tions in a plasma. The temperature, density, and fraction of different
ionization stages can be derived from measurements based on atomic
physics. Atomic processes ultimately destroy the plasma after the
initiating source is terminated. The ionization is lost by atomic recom-
bination, and the gas cools by atomic radiative processes.
There is an enormous variety of plasmas in nature: most of the
universe is one form of plasma or another. A multitude of atomic and
molecular processes occur in plasmas, and there is an expanding need
for reliable data on these processes. National plasma facilities should
be available to AMO physicists so that the relevant atomic experiments
can be carried out. Results of the experiments will be an important
element in the physics of dense plasmas. For example, the effects of
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144 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
high electron densities on atomic and molecular processes are surely
important but largely unknown.
Laboratories with tokamaks, mirror machines, and other devices
intended for the controlled thermonuclear fusion program have become
important centers of atomic-physics research. The plasmas produced
in these machines contain an abundance of radiation, electrons, and
ions in many states of ionization and with energies that are far from
uniform. All of this complexity makes a plasma not only a fertile
ground for applications of diagnostic atomic-physics techniques but
also a valuable source of new information about the interactions of
radiation and particles. In such a complicated environment, it is, of
course, not easy to study specific individual interaction processes, but
observations in plasmas have nevertheless yielded rewarding results
for such areas as the rich spectroscopy of satellite lines of highly
ionized atomic species. Conversely, electron-ion beam collision exper-
iments have recently substantiated the main features of the dielectronic
recombination process that commonly occurs in plasmas. In di-
electronic recombination a free electron is captured by an ion, with the
simultaneous excitation of an atomic electron. Thus it is evident that
atomic physics and plasma physics support each other significantly at
their common frontier.
Weakly ionized plasmas containing molecular gases raise a new set
of questions concerning the influence of atomic and molecular pro-
cesses on the evolution of the plasma. Molecular plasmas, energized by
some external source, can by virtue of internal excitations modify the
course and change the products of molecular reactions.
Fusion research provides a major arena for the interplay of atomic
and plasma physics, as described in Chapter 8 in the section on Fusion.
ATMOSPHERIC PHYSICS
Electrical and chemical processes in the atmosphere effect us vitally:
they govern massive climatic patterns through their influence on the
energy flow from Sun to Earth and from Earth to space; they determine
the ultimate fate of industrial pollutants; they control the quality of
local and worldwide radio communication. Atmospheric physics,
which attempts to understand the complicated web of physical and
chemical processes in the atmosphere, draws heavily on AMO physics
for vital data and for theoretical and experimental guidance.
At its most elementary level, atmospheric physics deals with the
physical processes that occur when our atmosphere is subjected to
radiation, electromagnetic and corpuscular, from the Sun. A complex
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SCIENTIFIC INTERFACES 145
sequence ot atomic and molecular processes determines the distribu-
tion of the absorbed solar energy into ionization, dissociation, lumi-
nosity, and heating; these processes drive dynamical and plasma
interactions and are modified by them. The importance of each
individual process depends on the location in the atmosphere and on
the time: latitudinal, diurnal, seasonal, and solar-cycle variations are
all substantial.
The quality with which radio waves propagate is determined by a
balance between photoionization and recombination in a large electri-
fied region of the atmosphere. Sunlight creates atomic ions; these
recombine after being converted to molecular ions by ion-molecule
reactions. A successful model has been constructed for the chemistry
governing recombination, though important questions remain about the
role of metastable species and vibrationally excited neutral and ionic
molecules. A separate group of physical processes governs the history
of the photoelectrons in the atmosphere. Initially energetic, these lose
their energy first by exciting and ionizing the atmospheric constituents
and finally through elastic collisions with the ambient electron gas. The
ambient gas is preferentially heated, and its temperature rises above
that of the neutral atmosphere. The hot electron gas is cooled by
excitation of the fine-structure levels of atomic oxygen and by excita-
tion of the rotational and vibrational levels of molecular nitrogen and
oxygen.
The processes that lead to the day and night airglow of the atmo-
sphere have been broadly categorized, but they are not understood in
detail. This is also true for the atomic and molecular processes that
follow auroral bombardment at high altitudes. Light from the aurora is
a potentially powerful diagnostic probe of the exciting source and of
the acceleration mechanism that appears to occur. High-latitude
auroral events and polar-cap absorption events produce thermal gra-
dients in the high atmosphere, driving the upper-atmosphere winds.
They modify the composition of the atmosphere, and they may be
related to climatic variations.
The chemistry of the mesosphere and stratosphere has undergone
rapid development, particularly since it was recognized that the release
of fluorocarbons into the atmosphere could attack the ozone layer.
Further studies of the molecular processes are needed to understand
the potential hazards from fluorocarbons and other pollutants. The
penetration of solar radiation is not yet adequately known, nor is the
intensity of ultraviolet radiation at low altitudes.
The terrestrial atmosphere has evolved markedly since the formation
of the planets owing to many influences, including life. Molecular
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146 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
physics is crucially involved in the attempts to reconstruct the history
of the early atmosphere as it responded to changes in solar luminosity
and to understand the interactions today of the physical and biological
processes that are determining the future of the atmosphere. The
effects of an increase in the abundance of carbon dioxide are of crucial
importance to our future.
Basic problems presented by atmospheric science often have imme-
diate consequences. For example, the Space Shuttle was found to glow
in the dark even at altitudes as high as 300 km (200 miles). The origin
of the glow has not been discovered: it may be produced by collisions
of the oxygen atoms of the atmosphere with material on the surface of
the spacecraft. The emitting species appears to be molecular in
character, but its identity is uncertain. A similar glow observed on
orbiting satellites has been attributed to the hydroxyl radical, but the
limited data on the Space Shuttle glow suggest that a different molecule
must be responsible. It is essential to identify the source of the glow,
not least because of its potential impact on the durability and effec-
tiveness of instruments in space such as the Space Telescope.
NUCLEAR PHYSICS
Atomic and nuclear physics are closely related. Here we focus on
three areas of contact. The first is the role of atomic spectroscopy in
measuring the fundamental static characteristics of nuclear states. This
has been an indispensable tool of nuclear physics for decades but now
contributes more information than ever. The second is the use of
atomic techniques to provide polarized nuclei for sources and targets in
nuclear experiments. The third is the study of the dynamical interac-
tions between nuclei and their atomic environments. The physics at
this interface between the two disciplines, still in its infancy, has
already provided nuclear physics with new insights and tantalizing
clues.
Optical Studies of the Nucleus
The mass, size, shape, and internal structure of the nucleus at the
center of each atom slightly alter the positions of the atomic energy
levels. These energy-level shifts can be found from careful determina-
tions of the optical spectral lines of the atoms. The name "hyperfine
structure," given to a large class of these effects, emphasizes their
intrinsic smallness, but atomic spectroscopy stands out in physics
through its extreme precision, and these tiny effects are accessible to
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SCIENTIFIC INTERFACES 147
measurement and analysis. For example, although nuclear diameters
increase with the mass of the nucleus, even the largest nuclei are about
one hundred thousand times smaller than most electron orbits in the
atom. Nevertheless, atomic spectroscopy provides a powerful tool for
accurate measurement of very small changes in the nuclear radius.
The complex nature of the nucleus is revealed to the atomic
electrons that surround it through the electric and magnetic fields that
arise from the nuclear protons, neutrons, and pions and, at a more
fundamental level, the quarks. These electromagnetic properties of the
nucleus can be parameterized in terms of electric and magnetic
moments that describe the size, shape, and charge and current distri-
butions of the nuclear constituents. The spins, the magnetic dipole
moments, and the electric quadrupole moments of a wide variety of
nuclei can be found from measurements of the hyperfine interaction
made by two atomic techniques: high-resolution optical spectroscopy
and atomic-beam magnetic resonance. Such data provide valuable
input to nuclear theorists, who use them to test the still-evolving
theories of nuclear structure.
Hyperfine measurements have been fundamental to our understand-
ing of the most basic properties of the ground states of stable nuclei. It
has now become possible to apply these techniques to a large class of
unstable and short-lived nuclei, and the horizons of the field have
expanded dramatically. In addition to the ground states of vast
numbers of nuclear species, there are many relatively long-lived
excited states whose size, shape, and moments can now be measured.
Every bit of such information adds a further constraint on possible
theories of nuclear structure.
To appreciate the potential of this new development, one only has to
note that while there are about 190 stable or naturally occurring
isotopes, there are close to 1600 known unstable nuclei, and it is
expected that many more will yet be discovered. Lasers, with their
sharp wavelengths and high intensities, have been the principal tool
opening up new vistas in the study of nuclei by atomic spectroscopy.
The field has been enormously enhanced by the installation of lasers
on-line at nuclear reactor and accelerator facilities, where the unstable
nuclear isotopes are produced. (See Figure 7.3.)
Systematic studies of chains of isotopes, especially at the Isotope
Separator On-Line (ISOLDE) facility at the European CERN labora-
tories, are providing vital information for nuclear-structure physicists.
Techniques from atomic physics are extensively employed to study the
sizes and shapes of the nuclei, their spins, and their electric and
magnetic moments. These techniques include atomic-beam magnetic
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148 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
it.
\
\
~ \~.,?
FIGURE 7.3. Atomic Physics at ISOLDE. The earliest evidence that atomic nuclei
possess spin and magnetic moments came from atomic physics, and as new experimental
techniques have been developed, the range and precision of these nuclear studies have
increased steadily. The drawing illustrates a number of atomic experiments on nuclear
properties being carried out at ISOLDE, the on-line isotope separator at CERN
(Geneva). Devoted to research in nuclear physics, ISOLDE is capable of producing
essentially any isotope. The isotopes are formed by bombarding a target with a 600-MeV
beam from a proton synchrocyclotron (right-hand side of drawing). The radioactive ions
are accelerated and mass selected by a bending magnet and then distributed to the
experiments by a "switchyard." Among the equipment shown on the experimental floor
are an atomic-beam magnetic resonance apparatus and a setup for laser spectroscopy
(the laser can be seen two floors above). Optical pumping is also employed. A similar
experimental facility is being developed at the University Isotope Separator (UNISOR)
at Oak Ridge. (Courtesy of CERN, Geneva, Switzerland, and Laboratoire Aime Cotton,
Orsay, France.)
resonance, optical pumping, and laser spectroscopy. Nuclei with
lifetimes as short as 10 milliseconds have been studied. Out of this
research have emerged discoveries such as shape staggering, in which
adjacent isotopes alternate between ablate and prolate forms, and
shape isomerism, in which a nucleus with two nearby excited levels
can assume widely varying shapes. These discoveries have played an
important role in the development of nuclear models.
As laser spectroscopy continues its rapid advances one can expect
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SCIENTIFIC INTERFACES 149
corresponding advances in our ability to obtain spectroscopic informa-
tion about nuclear states. It will become possible to measure the
properties of extremely rare and short-lived nuclear states, including
highly excited collective states with very high spin values. The
remarkable recent progress made by atomic physicists in trapping ions
and atoms for long periods of time is certain to be exploited for further
high-precision measurements of nuclear moments.
Polarized Nuclear Sources
Nuclear physics relies on techniques from atomic physics for pro-
ducing the spin-polarized projectiles and target atoms that are being
used increasingly in nuclear-reaction experiments. Nuclear physicists
require the most intense available beams of nuclei with their spins
oriented in a particular direction in space, rather than being randomly
oriented. A number of different atomic methods are used for producing
polarized nuclei, such as protons, deuterons, 3He nuclei, and lithium
nuclei. The spins of nuclei are aligned through their magnetic moments,
which offer a "handle" for using a magnetic field to rotate and orient
them. However, the nuclear moment is only about one thousandth as
large as the magnetic moment of the electron. Therefore, almost all
nuclear polarization schemes rely on first polarizing one of the atomic
electrons that surround the nucleus. The hyperfine interaction, which
couples the magnetism of the electron to the magnetic moment of the
nucleus, can then be used to align the nuclei. (Two recent advances in
the production of polarized protons and 3He are described earlier in
this chapter in the section on Condensed-Matter Physics and Materials
Science.) In this important area of ion-source technology it is essential
to know the atomic collision cross sections and other atomic parame-
ters that determine the efficacy of proposed new polarization mecha-
nisms.
Dynamics at the Atom-Nuclear Frontier
Atomic and nuclear physics have a common frontier that has
recently become the site for research into questions that previously
could never be addressed but that now, thanks to experimental and
theoretical advances, we can hope to answer. What happens to the
electrons in an atom when a nuclear particle, such as a proton or a
heavy ion, penetrates through the atomic electron shells on its way into
or out of the nucleus, where it initiates a nuclear reaction? How does
the course of the nuclear reaction and in particular its duration—
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150 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
affect the atomic electrons? Can lessons for nuclear physics be learned
by studying these atomic effects? Or, conversely, can useful atomic
information be gained? In a number of accelerator laboratories that use
all types of accelerators from tandem Van de Graaffs to the most
advanced heavy-ion linear colliders, the effects of nuclear reactions at
millielectron volt to gigaelectron volt energies on the participating
atoms have come under study. The research has begun to yield
valuable information on nuclear reactions and atomic-collision pro-
cesses.
One of the most direct measurements at the atomic-nuclear frontier
is the determination of the lifetime of a compound nucleus, before it
comes apart again with re-emission of a proton. On its way into or out
of the nucleus, the projectile may knock an inner-shell electron out of
its orbit, leaving a vacancy, which eventually leads to the emission of
an x-ray photon. From such x-ray measurements, and with the aid of
results from calculations in atomic-collision theory, it is possible, from
purely atomic observations, to determine the lifetime of the compound
nucleus, using the atom as a clock. Lifetimes in the range from 10-~6 to
10-'8 second have been measured by such atomic techniques. Life-
times of even much shorter-lived nuclear states (10-'8 and even 10-2°
second) can now be determined by crystal blocking techniques that
exemplify the overlap of nuclear physics with both condensed-matter
and atomic physics.
Finally, we note here a new development: an experiment that is
being carried forward at a laboratory of particle physics but that
represents a confluence of particle, nuclear, and atomic physics. This
is the attempt to make protonium, an atom composed of a proton and
an antiproton. The antiproton storage ring at CERN produces enough
of these particles to provide the hope of making protonium, using
hydrogen negative ions as the source of protons. Observation of this
atom would provide an important new avenue for the study of quantum
electrodynamics and quantum chromodynamics.
Representative terms from entire chapter:
scientific interfaces
