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Atomic, molecular, and optical physics (AMO) is the study ofmatter–matter andlight–matter interactions, at the scale of one or a fewatoms[1] and energy scales around severalelectron volts.[2]: 1356 [3] The three areas are closely interrelated. AMO theory includesclassical,semi-classical andquantum treatments. Typically, the theory and applications ofemission,absorption,scattering ofelectromagnetic radiation (light) fromexcitedatoms andmolecules, analysis of spectroscopy, generation oflasers andmasers, and the optical properties of matter in general, fall into these categories.
Atomic physics is the subfield of AMO that studies atoms as an isolated system ofelectrons and anatomic nucleus, whilemolecular physics is the study of the physical properties ofmolecules. The termatomic physics is often associated withnuclear power andnuclear bombs, due to thesynonymous use ofatomic andnuclear instandard English. However, physicists distinguish between atomic physics — which deals with the atom as a system consisting of a nucleus and electrons — andnuclear physics, which considersatomic nuclei alone. The important experimental techniques are the various types ofspectroscopy.Molecular physics, while closely related toatomic physics, also overlaps greatly withtheoretical chemistry,physical chemistry andchemical physics.[4]
Both subfields are primarily concerned withelectronic structure and the dynamical processes by which these arrangements change. Generally this work involves using quantum mechanics. For molecular physics, this approach is known asquantum chemistry. One important aspect of molecular physics is that the essentialatomic orbital theory in the field of atomic physics expands to themolecular orbital theory.[5] Molecular physics is concerned with atomic processes in molecules, but it is additionally concerned with effects due to themolecular 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 discreteenergy levels. The smallest energy differences exist between different rotational states, therefore pure rotationalspectra are in the farinfrared region (about 30 - 150μmwavelength) of theelectromagnetic spectrum.Vibrational spectra are in the near infrared (about 1 - 5 μm) and spectra resulting from electronic transitions are mostly in the visible andultraviolet regions. From measuring rotational and vibrational spectra properties of molecules like the distance between the nuclei can be calculated.[6]
As with many scientific fields, strict delineation can be highly contrived and atomic physics is often considered in the wider context ofatomic, molecular, and optical physics. Physics research groups are usually so classified.
Optical physics is the study of the generation ofelectromagnetic radiation, the properties of that radiation, and the interaction of that radiation withmatter,[7] especially its manipulation and control.[8] It differs from generaloptics andoptical 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 forbasic 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 ofelectromagnetically induced transparency byS. E. Harris and ofslow light by Harris andLene Vestergaard Hau.[9][10]
Researchers in optical physics use and develop light sources that span theelectromagnetic spectrum frommicrowaves toX-rays. The field includes the generation and detection of light, linear andnonlinear optical processes, andspectroscopy.Lasers andlaser spectroscopy have transformed optical science. Major study in optical physics is also devoted toquantum optics andcoherence, and tofemtosecond optics.[1] 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.[11]
Other important areas of research include the development of novel optical techniques for nano-optical measurements,diffractive optics,low-coherence interferometry,optical coherence tomography, andnear-field microscopy. Research in optical physics places an emphasis on ultrafast optical science and technology. The applications of optical physics create advancements incommunications,medicine,manufacturing, and evenentertainment.[12]
One of the earliest steps towardsatomic physics was the recognition that matter was composed ofatoms, in modern terms the basic unit of achemical element. This theory was developed byJohn 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 developingperiodic table, byJohn Newlands andDmitri Mendeleyev around the mid to late 19th century.[13]
Later, the connection between atomic physicsand optical physics became apparent, by the discovery ofspectral lines and attempts to describe the phenomenon - notably byJoseph von Fraunhofer,Fresnel, and others in the 19th century.[14]
From that time to the 1920s, physicists were seeking to explainatomic spectra andblackbody radiation. One attempt to explain hydrogen spectral lines was theBohr atom model.[13]
Experiments includingelectromagnetic radiation and matter - such as thephotoelectric effect,Compton effect, and spectra of sunlight the due to the unknown element ofHelium, 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.[15]
Early models to explain the origin of theindex of refraction treated anelectron in an atomic system classically according to the model ofPaul Drude andHendrik Lorentz. The theory was developed to attempt to provide an origin for the wavelength-dependent refractive indexn of a material. In this model, incidentelectromagnetic waves forced an electron bound to an atom tooscillate. Theamplitude of the oscillation would then have a relationship to thefrequency of the incident electromagnetic wave and theresonant frequencies of the oscillator. Thesuperposition of these emitted waves from many oscillators would then lead to a wave which moved more slowly.[16]: 4–8
Max Planck derived a formula to describe theelectromagnetic field inside a box when inthermal equilibrium in 1900.[16]: 8–9 His model consisted of a superposition ofstanding waves. In one dimension, the box has lengthL, and only sinusoidal waves ofwavenumber
can occur in the box, wheren is a positiveinteger (mathematically denoted by). The equation describing these standing waves is given by:
whereE0 is the magnitude of theelectric field amplitude, andE is the magnitude of the electric field at positionx. From this basic,Planck's law was derived.[16]: 4–8, 51–52
In 1911,Ernest Rutherford concluded, based onalpha 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 theRutherford 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.[16]: 9–10
These results, based on adiscrete set of specific standing waves, were inconsistent with thecontinuous classical oscillator model.[16]: 8
Work byAlbert Einstein in 1905 on thephotoelectric effect led to the association of a light wave of frequency with a photon of energy. In 1917 Einstein created an extension to Bohrs model by the introduction of the three processes ofstimulated emission,spontaneous emission andabsorption (electromagnetic radiation).[16]: 11
The largest steps towards the modern treatment was the formulation of quantum mechanics with thematrix mechanics approach byWerner Heisenberg and the discovery of theSchrödinger equation byErwin Schrödinger.[16]: 12
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.[16]: 14 Since the field is treated classically it can not deal withspontaneous emission.[16]: 16 This semi-classical treatment is valid for most systems,[2]: 997 particular those under the action of high intensity laser fields.[2]: 724 The distinction between optical physics and quantum optics is the use of semi-classical and fully quantum treatments respectively.[2]: 997
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.[2]: 556 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.[2]: 754
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.[2]: 871
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 itsmolecular hydrogen ion. It is concerned with processes such asionization,above threshold ionization andexcitation by photons or collisions with atomic particles.
While modelling atoms in isolation may not seem realistic, if one considers molecules in agas orplasma 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 inplasma physics andatmospheric physics even though both deal with huge numbers of molecules.
Electrons form notionalshells around the nucleus. These are naturally in aground 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 abound state. The energy necessary to remove an electron from its shell (taking it to infinity) is called thebinding energy. Any quantity of energy absorbed by the electron in excess of this amount is converted tokinetic energy according to theconservation of energy. The atom is said to have undergone the process ofionization.
In the event that the electron absorbs a quantity of energy less than the binding energy, it may transition to anexcited state or to avirtual state. After a statistically sufficient quantity of time, an electron in an excited state will undergo a transition to a lower state viaspontaneous 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 theAuger 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 strictselection 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.
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