Chapter 1
Introduction: The optical
nature of a charged
particle beam
Modern physics teaches that all matter is made of particles which
interact with one another. Every particle is characterized by its
intrinsic charge, mass, and spin. These quantities govern all interactions which a particle can have. For example, an atom consists
of a cloud of negatively charged electrons orbiting a compact, positively charged nucleus. The establishment of this fact in quantitative terms has a fascinating history. It originates with the early
hypothesis of Democritas, proceeds through the origins of quantitative chemistry in the seventeenth century, and culminates with
the elucidation of quantum mechanics in the twentieth century.
Only during the last few decades has it become possible to capture an actual image of a single atom.
Atoms are charge-neutral in their normal state, with the positive
charge of the nucleus precisely offset by negative charge of the orbiting electrons. By bombarding an atom with a beam of light or
charged particles, it is possible to remove one or more electrons
from an atom or molecule. This forms a positively charged ion.
Under special circumstances it is also possible to add electrons to
form a negatively charged ion. Electric and magnetic fields act on
1
Introduction: The optical
nature of a charged
particle beam
Modern physics teaches that all matter is made of particles which
interact with one another. Every particle is characterized by its
intrinsic charge, mass, and spin. These quantities govern all interactions which a particle can have. For example, an atom consists
of a cloud of negatively charged electrons orbiting a compact, positively charged nucleus. The establishment of this fact in quantitative terms has a fascinating history. It originates with the early
hypothesis of Democritas, proceeds through the origins of quantitative chemistry in the seventeenth century, and culminates with
the elucidation of quantum mechanics in the twentieth century.
Only during the last few decades has it become possible to capture an actual image of a single atom.
Atoms are charge-neutral in their normal state, with the positive
charge of the nucleus precisely offset by negative charge of the orbiting electrons. By bombarding an atom with a beam of light or
charged particles, it is possible to remove one or more electrons
from an atom or molecule. This forms a positively charged ion.
Under special circumstances it is also possible to add electrons to
form a negatively charged ion. Electric and magnetic fields act on
1