Atomic, molecular, and optical physics

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. While nuclear physicists concern themselves primarily with techniques to investigate atomic nuclei like particle accelerators, AMO physicists tend to be more interested in techniques in spectroscopy. Within AMO physics, many are interested in the study of molecular physics. Although this field is related to atomic physics, there are also additional degrees of freedom that create more complicated Hamiltonians with interesting underlying physics. Molecular physics also has siginificant overlap with the fields of theoretical chemistry, physical chemistry and chemical physics.

Both subfields are primarily concerned with electronic structure and the dynamical processes by which these arrangements change. The way that this is studied is by characterizing the system's Hamiltonian and understanding the energy levels of the system. Understanding the dynamics of these systems, particularly for molecules, is known as quantum chemistry. To probe the structure of these atoms and molecules, physicists use atomic orbital and molecular orbital theory to understand the electronic structure. Notably in molecular physics, understanding the electronic properities of your respective molecule will tell you the leading order effect in the energy contribution in the Born-Oppenheimer approximation of a molecules Hamiltonian.

Molecular physics is concerned with atomic processes in molecules, but it is also concerned with effects due to the molecular structure. In addition 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.

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

thumb|right|200px|The [[Bohr model of the Hydrogen atom]]

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.

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.

<!-- Integrate these into the history section, remove as they are mentioned

; Pre quantum mechanics

Joseph von Fraunhofer
Johannes Rydberg
J.J. Thomson

; Post quantum mechanics

Alexander Dalgarno
David Bates
Max Born
Clinton Joseph Davisson
Enrico Fermi
Charlotte Froese Fischer
Vladimir Fockii
Douglas Hartree
Harrie S. Massey
Nevill Mott
Mike Seaton
John C. Slater
George Paget Thomson
Ernest M. Henley
Peter Zoller
Fano
Peter Lambropoulos

-->

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.