This review discusses how low-energy, valence excitations created by swift electrons can render information on the optical response of structured materials with unmatched spatial resolution. Electron microscopes are capable of focusing electron beams on sub-nanometer spots and probing the target response either by analyzing electron energy losses or by detecting emitted radiation. Theoretical frameworks suited to calculate the probability of energy loss and light emission (cathodoluminescence) are revisited and compared with experimental results. More precisely, a quantum-mechanical description of the interaction between the electrons and the sample is discussed, followed by a powerful classical dielectric approach that can be in practice applied to more complex systems. We assess the conditions under which classical and quantum-mechanical formulations are equivalent. The excitation of collective modes such as plasmons is studied in bulk materials, planar surfaces, and nanoparticles. Light emission induced by the electrons is shown to constitute an excellent probe of plasmons, combining sub-nanometer resolution in the position of the electron beam with nanometer resolution in the emitted wavelength. Both electron energy-loss and cathodoluminescence spectroscopies performed in a scanning mode of operation yield snap shots of plasmon modes in nanostructures with fine spatial detail as compared to other existing imaging techniques, thus providing an ideal tool for nanophotonics studies.

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Optical excitations in electron microscopy

Lunar initial 143Nd/144Nd: Differential evolution of the lunar crust and mantle

Following the invention of electron optics during the 1930s, lens aberrations have limited the achievable spatial resolution to about 50 times the wavelength of the imaging electrons. This situation is similar to that faced by Leeuwenhoek in the seventeenth century, whose work to improve the quality of glass lenses led directly to his discovery of the ubiquitous "animalcules" in canal water, the first hints of the cellular basis of life. The electron optical aberration problem was well understood from the start, but more than 60 years elapsed before a practical correction scheme for electron microscopy was demonstrated, and even then the remaining chromatic aberrations still limited the resolution. We report here the implementation of a computer-controlled aberration correction system in a scanning transmission electron microscope, which is less sensitive to chromatic aberration. Using this approach, we achieve an electron probe smaller than 1 A. This performance, about 20 times the electron wavelength at 120 keV energy, allows dynamic imaging of single atoms, clusters of a few atoms, and single atomic layer 'rafts' of atoms coexisting with Au islands on a carbon substrate. This technique should also allow atomic column imaging of semiconductors, for detection of single dopant atoms, using an electron beam with energy below the damage threshold for silicon.

Sub-ångstrom resolution using aberration corrected electron optics

The current state of understanding of the lunar interior is the sum of nearly four decades of work and a range of exploration programs spanning that same time period. Missions of the 1960s including the Rangers, Surveyors, and Lunar Orbiters, as well as Earth-based telescopic studies, laid the groundwork for the Apollo program and provided a basic understanding of the surface, its stratigraphy, and chronology. Through a combination of remote sensing, surface exploration, and sample return, the Apollo missions provided a general picture of the lunar interior and spawned the concept of the lunar magma ocean. In particular, the discovery of anorthite clasts in the returned samples led to the view that a large portion of the Moon was initially molten, and that crystallization of this magma ocean gave rise to mafic cumulates that make up the mantle, and plagioclase flotation cumulates that make up the crust (Smith et al. 1970; Wood et al. 1970). This model is now generally accepted and is the framework that unifies our knowledge of the structure and composition of the Moon. The intention of this chapter is to review the major advances that have been made over the past decade regarding the constitution of the Moon’s interior. Much of this new knowledge is a direct result of data acquired from the successful Clementine and Lunar Prospector missions, as well as the analysis of new lunar meteorites. As will be seen, results from these studies have led to many fundamental amendments to the magma ocean model.

Much of what we know from sample analyses has been previously summarized elsewhere, and only their most important aspects will be discussed in this chapter. The reader is referred to the relevant chapters in the books Basaltic Volcanism on the Terrestrial Planets (Basaltic Volcanism Study Project 1981), The …

The Constitution and Structure of the Lunar Interior

Theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early-accreting planetesimals. The apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet. The processes of magma ocean formation and solidification, therefore, strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets, and they form the starting point for cooling to clement, habitable conditions and for the onset of thermally driven mantle convection and plate tectonics. This review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean, distribution and degassing of volatiles, and cooling.

Magma oceans in the inner solar system

The mineralogy and petrology of the Apollo 11 rocks are consistent with impact melting
of ilmenite and pyroxene crystals plus liquid derived from fractional crystallization of basaltic magama.
The complementary plagioclase accumulate should exist in the highlands. The ferrobasaltic magma is
derived from a differentiated hot moon of modified chondritic composition. The one-sided distribution
of large irregular seas is explained by tidal attraction of the last fraction of liquid to the
near side of the moon and release of liquid by meteorite impact.
The vesicular ferrobasalt lavas contain ilmenite, clinopyroxene and plagioclase and have textures
similar to some terrestrial basalts. Delayed appearance of plagioclase indicates an unusual source of
the magma. The iron-enrichment of the coarser microgabbros is extreme at the end of crystallization
resulting in a new iron metasilicate, pyroxferroite.
The oxygen fugacity at 1050°C of these rocks indicated by the composition of opaque oxides and
troilite-iron intergrowths is 10^(-14)-10^(-16) compared to 10^(- 11) for terrestrial basalts and about 10^(16) 
for ordinary chondrites. Absence of hydrous minerals indicates loss of volatiles at some stage in
the moon's history.
The breccias and soil have the same mineralogy modified by shock. Plagioclase vitrophyres are
probably melted cumulates and may derive from the highlands. Lithification of the breccia results
principally from welding of debris discharged from a hot gas cloud created by meteorite impact.
Small glass spheres have surface features consistent with a fiery rain of boiling silicate liquid rounded
by surface tension and later impacted by high-velocity micrometeorites.
Melting experiments of synthetic material of mean Apollo 11 rock composition revealed low
liquidus temperature, delayed appearance of plagioclase and narrow crystal-liquid interval. These
support the concept of advanced fractional crystallization of a basaltic liquid under reducing conditions leading to high iron enrichment. Flotation of plagioclase and sin king of ilmenite and pyroxene
should occur in the liquid basalt.
We propose that meteorite impact blasted away a plagioclase-rich crust, melting a mixture of
fractionated basalt liquid and ilmenite plus pyroxene crystals. The new liquid formed the Sea of
Tranquillity and yielded the near-surface lava flows represented by the ferrobasalts and microgabbros.
This new liquid could yield plagioclase only after the extra ilmenite and pyroxene had
crystallized.
We propose that the original basalt liquid was derived by fractional crystallization of a molten
moon of modified chondritic composition yielding a metallic core surrounded by pressure-stable
Mg-rich o li vine and pyroxene. The fractionated liquid became Fe-rich resulting in an inverse density
stratification. Primitive ultra basic crust should occur in the highlands, along with dominant plagio clase-
rich cumulates. The temperature-time relations of the model are discussed qualitatively. Key factors are early
removal of radioactive material from the center by fractional crystallization and possible enhancement
of radiative heat transfer in volatile-free silicates. The moon rocks should be more refractory
and rigid because of the low volatile content.

Petrologic history of the moon inferred from petrography, mineralogy, and petrogenesis of Apollo 11 rocks

A simple scanning microscope has been built which uses a field emission electron gun alone, without the aid of auxiliary lenses. The design and operation of the microscope are described and the calculated performance is compared with experiment. Resolution of 100 A has been obtained and is shown in transmission electron micrographs. The probe current is of the order of 10−10 to 10−11 A, a value which is high enough to allow micrographs to be taken with scan times of 10 sec.

/pdf/a-simple-scanning-electron-microscope-550j6t8lgq.pdf

A Simple Scanning Electron Microscope

Radiation damage of various biological molecules is measured in an ultra-high vacuum scanning microscope. The damage measurements are made using radiation-induced changes in electron energy-loss spectra and in electron diffraction patterns. Damage cross sections are compared with previously measured, inelastic electron-scattering cross sections. Using the measured damage doses, comparisons are made with the doses necessary for electron microscopy of biological specimens.

Electron beam excitation and damage of biological molecules; its implications for specimen damage in electron microscopy.

The design and construction of a double focusing uniform field magnetic electron spectrometer are described. The pole pieces of the magnet are curved in an effort to correct all median plane aberrations and thereby increase the collecting power. The spectrometer has been installed on a scanning microscope which uses a field emission gun to produce a 100 A probe. Energy analysis of the transmitted electrons as well as filtered diffraction patterns can be obtained from areas as small as 100 A in diameter. In addition, the area under study can be observed directly in the scanning transmission mode, using electrons of any selected energy loss. A resolution of the spectrometer of 2 ppm at an electron energy of 20 keV is shown and the measured second order aberration coefficient is <5% of that of an equivalent straight edge sector magnet. Also, the solid angle for collection of electrons is about an order of magnitude greater than that of the equivalent straight edge magnet at any given resolution. Experimental results in all modes of operation are presented.

A high resolution electron spectrometer for use in transmission scanning electron microscopy.

As part of a programme to investigate the interactions of fast electrons with important biological molecules, we have measured the energy loss spectra of ∼ 20 keV electrons transmitted through thin films of five nucleic acid bases. We measured these spectra in order to determine whether the energy loss information could be used as a contrast mechanism in high resolution scanning transmission electron microscopy1. But the spectra can also provide information about atomic, molecular, and solid state transitions in the range ∼ 1 eV to 103 eV. Furthermore, changes in the spectra as a function of irradiation can provide a measure of the radiation damage sustained by the molecules due to the action of the incident electron beam.

Electron energy loss spectra of the nucleic acid bases.

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