Atomic, molecular, and optical physics (AMO)
is the study of matter-matter and light-matter
interactions; at the scale of one or a few
atoms and energy scales around several electron
volts. The three areas are closely interrelated.
AMO theory includes classical, semi-classical
and quantum treatments. Typically, the theory
and applications of emission, absorption,
scattering of electromagnetic radiation (light)
from excited atoms and molecules, analysis
of spectroscopy, generation of lasers and
masers, and the optical properties of matter
in general, fall into these categories.
== Atomic and molecular physics ==
Atomic physics is the subfield of AMO that
studies atoms as an isolated system of electrons
and an atomic nucleus, while molecular physics
is the study of the physical properties of
molecules. The term atomic physics is often
associated with nuclear power and nuclear
bombs, due to the synonymous use of atomic
and nuclear in standard English. However,
physicists distinguish between atomic physics
— which deals with the atom as a system
consisting of a nucleus and electrons — and
nuclear physics, which considers atomic nuclei
alone. The important experimental techniques
are the various types of spectroscopy. Molecular
physics, while closely related to atomic physics,
also overlaps greatly with theoretical chemistry,
physical chemistry and chemical physics.Both
subfields are primarily concerned with electronic
structure and the dynamical processes by which
these arrangements change. Generally this
work involves using quantum mechanics. For
molecular physics, this approach is known
as quantum chemistry. One important aspect
of molecular physics is that the essential
atomic orbital theory in the field of atomic
physics expands to the molecular orbital theory.
Molecular physics is concerned with atomic
processes in molecules, but it is additionally
concerned with effects due to the molecular
structure. Additionally to the electronic
excitation states which are known from atoms,
molecules are able to rotate and to vibrate.
These rotations and vibrations are quantized;
there are discrete energy levels. The smallest
energy differences exist between different
rotational states, therefore pure rotational
spectra are in the far infrared region (about
30 - 150 µm wavelength) of the electromagnetic
spectrum. Vibrational spectra are in the near
infrared (about 1 - 5 µm) and spectra resulting
from electronic transitions are mostly in
the visible and ultraviolet regions. From
measuring rotational and vibrational spectra
properties of molecules like the distance
between the nuclei can be calculated.As with
many scientific fields, strict delineation
can be highly contrived and atomic physics
is often considered in the wider context of
atomic, molecular, and optical physics. Physics
research groups are usually so classified.
== Optical physics ==
Optical physics is the study of the generation
of electromagnetic radiation, the properties
of that radiation, and the interaction of
that radiation with matter, especially its
manipulation and control. It differs from
general optics and optical engineering in
that it is focused on the discovery and application
of new phenomena. There is no strong distinction,
however, between optical physics, applied
optics, and optical engineering, since the
devices of optical engineering and the applications
of applied optics are necessary for basic
research in optical physics, and that research
leads to the development of new devices and
applications. Often the same people are involved
in both the basic research and the applied
technology development, for example the experimental
demonstration of electromagnetically induced
transparency by S. E. Harris and of slow light
by Harris and Lene Vestergaard Hau.Researchers
in optical physics use and develop light sources
that span the electromagnetic spectrum from
microwaves to X-rays. The field includes the
generation and detection of light, linear
and nonlinear optical processes, and spectroscopy.
Lasers and laser spectroscopy have transformed
optical science. Major study in optical physics
is also devoted to quantum optics and coherence,
and to femtosecond optics. In optical physics,
support is also provided in areas such as
the nonlinear response of isolated atoms to
intense, ultra-short electromagnetic fields,
the atom-cavity interaction at high fields,
and quantum properties of the electromagnetic
field.Other important areas of research include
the development of novel optical techniques
for nano-optical measurements, diffractive
optics, low-coherence interferometry, optical
coherence tomography, and near-field microscopy.
Research in optical physics places an emphasis
on ultrafast optical science and technology.
The applications of optical physics create
advancements in communications, medicine,
manufacturing, and even entertainment.
== History ==
One of the earliest steps towards atomic physics
was the recognition that matter was composed
of atoms, in modern terms the basic unit of
a chemical element. This theory was developed
by John Dalton in the 18th century. At this
stage, it wasn't clear what atoms were - although
they could be described and classified by
their observable properties in bulk; summarized
by the developing periodic table, by John
Newlands and Dmitri Mendeleyev around the
mid to late 19th century.Later, the connection
between atomic physics and optical physics
became apparent, by the discovery of spectral
lines and attempts to describe the phenomenon
- notably by Joseph von Fraunhofer, Fresnel,
and others in the 19th century.From that time
to the 1920s, physicists were seeking to explain
atomic spectra and blackbody radiation. One
attempt to explain hydrogen spectral lines
was the Bohr atom model.Experiments including
electromagnetic radiation and matter - such
as the photoelectric effect, Compton effect,
and spectra of sunlight the due to the unknown
element of Helium, the limitation of the Bohr
model to Hydrogen, and numerous other reasons,
lead to an entirely new mathematical model
of matter and light: quantum mechanics.
=== Classical oscillator model of matter ===
Early models to explain the origin of the
index of refraction treated an electron in
an atomic system classically according to
the model of Paul Drude and Hendrik Lorentz.
The theory was developed to attempt to provide
an origin for the wavelength-dependent refractive
index n of a material. In this model, incident
electromagnetic waves forced an electron bound
to an atom to oscillate. The amplitude of
the oscillation would then have a relationship
to the frequency of the incident electromagnetic
wave and the resonant frequencies of the oscillator.
The superposition of these emitted waves from
many oscillators would then lead to a wave
which moved more slowly.
=== Early quantum model of matter and light
===
Max Planck derived a formula to describe the
electromagnetic field inside a box when in
thermal equilibrium in 1900.
His model consisted of a superposition of
standing waves. In one dimension, the box
has length L, and only sinusoidal waves of
wavenumber
k
=
n
π
L
{\displaystyle k={\frac {n\pi }{L}}}
can occur in the box, where n is a positive
integer (mathematically denoted by
n
∈
N
1
{\displaystyle \scriptstyle n\in \mathbb {N}
_{1}}
). The equation describing these standing
waves is given by:
E
=
E
0
sin
⁡
(
n
π
L
x
)
{\displaystyle E=E_{0}\sin \left({\frac {n\pi
}{L}}x\right)\,\!}
.where E0 is the magnitude of the electric
field amplitude, and E is the magnitude of
the electric field at position x. From this
basic, Planck's law was derived.In 1911, Ernest
Rutherford concluded, based on alpha particle
scattering, that an atom has a central pointlike
proton. He also thought that an electron would
be still attracted to the proton by Coulomb's
law, which he had verified still held at small
scales. As a result, he believed that electrons
revolved around the proton. Niels Bohr, in
1913, combined the Rutherford model of the
atom with the quantisation ideas of Planck.
Only specific and well-defined orbits of the
electron could exist, which also do not radiate
light. In jumping orbit the electron would
emit or absorb light corresponding to the
difference in energy of the orbits. His prediction
of the energy levels was then consistent with
observation.These results, based on a discrete
set of specific standing waves, were inconsistent
with the continuous classical oscillator model.Work
by Albert Einstein in 1905 on the photoelectric
effect led to the association of a light wave
of frequency
ν
{\displaystyle \nu }
with a photon of energy
h
ν
{\displaystyle h\nu }
. In 1917 Einstein created an extension to
Bohrs model by the introduction of the three
processes of stimulated emission, spontaneous
emission and absorption (electromagnetic radiation).
== Modern treatments ==
The largest steps towards the modern treatment
was the formulation of quantum mechanics with
the matrix mechanics approach by Werner Heisenberg
and the discovery of the Schrödinger equation
by Erwin Schrödinger.There are a variety
of semi-classical treatments within AMO. Which
aspects of the problem are treated quantum
mechanically and which are treated classically
is dependent on the specific problem at hand.
The semi-classical approach is ubiquitous
in computational work within AMO, largely
due to the large decrease in computational
cost and complexity associated with it.
For matter under the action of a laser, a
fully quantum mechanical treatment of the
atomic or molecular system is combined with
the system being under the action of a classical
electromagnetic field. Since the field is
treated classically it can not deal with spontaneous
emission. This semi-classical treatment is
valid for most systems, particular those under
the action of high intensity laser fields.
The distinction between optical physics and
quantum optics is the use of semi-classical
and fully quantum treatments respectively.Within
collision dynamics and using the semi-classical
treatment, the internal degrees of freedom
may be treated quantum mechanically, whilst
the relative motion of the quantum systems
under consideration are treated classically.
When considering medium to high speed collisions,
the nuclei can be treated classically while
the electron is treated quantum mechanically.
In low speed collisions the approximation
fails.Classical Monte-Carlo methods for the
dynamics of electrons can be described as
semi-classical in that the initial conditions
are calculated using a fully quantum treatment,
but all further treatment is classical.
== Isolated atoms and molecules ==
Atomic, Molecular and Optical physics frequently
considers atoms and molecules in isolation.
Atomic models will consist of a single nucleus
that may be surrounded by one or more bound
electrons, whilst molecular models are typically
concerned with molecular hydrogen and its
molecular hydrogen ion. It is concerned with
processes such as ionization, above threshold
ionization and excitation by photons or collisions
with atomic particles.
While modelling atoms in isolation may not
seem realistic, if one considers molecules
in a gas or plasma then the time-scales for
molecule-molecule interactions are huge in
comparison to the atomic and molecular processes
that we are concerned with. This means that
the individual molecules can be treated as
if each were in isolation for the vast majority
of the time. By this consideration atomic
and molecular physics provides the underlying
theory in plasma physics and atmospheric physics
even though both deal with huge numbers of
molecules.
== Electronic configuration ==
Electrons form notional shells around the
nucleus. These are naturally in a ground state
but can be excited by the absorption of energy
from light (photons), magnetic fields, or
interaction with a colliding particle (typically
other electrons).
Electrons that populate a shell are said to
be in a bound state. The energy necessary
to remove an electron from its shell (taking
it to infinity) is called the binding energy.
Any quantity of energy absorbed by the electron
in excess of this amount is converted to kinetic
energy according to the conservation of energy.
The atom is said to have undergone the process
of ionization.
In the event that the electron absorbs a quantity
of energy less than the binding energy, it
may transition to an excited state or to a
virtual state. After a statistically sufficient
quantity of time, an electron in an excited
state will undergo a transition to a lower
state via spontaneous emission. The change
in energy between the two energy levels must
be accounted for (conservation of energy).
In a neutral atom, the system will emit a
photon of the difference in energy. However,
if the lower state is in an inner shell, a
phenomenon known as the Auger effect may take
place where the energy is transferred to another
bound electrons causing it to go into the
continuum. This allows one to multiply ionize
an atom with a single photon.
There are strict selection rules as to the
electronic configurations that can be reached
by excitation by light—however there are
no such rules for excitation by collision
processes.
== See also ==
== Notes
