Atomic physics always considers atoms in isolation. Atomic models will consist of a single nucleus that may be surrounded by one or more bound electrons. It is not concerned with the formation of molecules (although much of the physics is identical), nor does it examine atoms in a solid state as condensed matter. It is concerned with processes such as ionization and excitation by photons or collisions with atomic particles.
While modelling atoms in isolation may not seem realistic, if one considers atoms in a gas or plasma then the time-scales for atom-atom interactions are huge in comparison to the atomic processes that are generally considered. This means that the individual atoms can be treated as if each were in isolation, as the vast majority of the time they are. By this consideration atomic physics provides the underlying theory in plasma physics and atmospheric physics, even though both deal with very large numbers of atoms.
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 the electron absorbs a quantity of energy less than the binding energy, it will transition to an excited state. After a statistically sufficient quantity of time, an electron in an excited state will undergo a transition to a lower state. 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 excited atom has been previously ionized, in particular if one of its inner shell electrons has been removed, a phenomenon known as the Auger effect may take place where the quantity of energy is transferred to one of the bound electrons causing it to go into the continuum. This allows one to multiply ionize an atom with a single photon.
There are rather 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.
The majority of fields in physics can be divided between theoretical work and experimental work,
and atomic physics is no exception. It is usually the case, but not always, that progress goes
in alternate cycles from an experimental observation, through to a theoretical explanation
followed by some predictions that may or may not be confirmed by experiment, and so on. Of course, the current state of technology at any given time can put limitations on what can be achieved experimentally and theoretically so it may take considerable time for theory to be refined.
One of the earliest steps towards atomic physics was the recognition that matter was composed
of "atoms", in the modern sense of the basic unit of a chemical element. This theory was developed by the British chemist and physicist 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 properties (in bulk) in a periodic table.
The true beginning of atomic physics is marked by the discovery of spectral lines and attempts to describe the phenomenon, most notably by Joseph von Fraunhofer. The study of these lines led to the Bohr atom model and to the birth of quantum mechanics. In seeking to explain atomic spectra an entirely new mathematical model of matter was revealed. As far as atoms and their electron shells were concerned, not only did this yield a better overall description, i.e. the atomic orbital model, but it also provided a new theoretical basis for chemistry
(quantum chemistry) and spectroscopy.
Since the Second World War, both theoretical and experimental fields have advanced at a rapid pace. This can be attributed to progress in computing technology, which has allowed larger and more sophisticated models of atomic structure and associated collision processes. Similar technological advances in accelerators, detectors, magnetic field generation and lasers have greatly assisted experimental work.
San Mateo, California
San Mateo ( ; Spanish for "Saint Matthew") is a city in San Mateo County, California, United States, in the San Francisco Bay Area. With a population of about 100,000 (97,207 at the 2010 census), it is one of the larger suburbs on the San Francisco Peninsula, located between Burlingame to the north, Foster City to the east, Belmont to the south, and Highlands-Baywood Park and Hillsborough to the west. San Mateo was incorporated in 1864.
Alioramus
Alioramus is a genus of tyrannosaurid dinosaur. It was about 20 feet long. It lived during the Upper Cretaceous period in what is now Mongolia.
The type species, "A. remotus", is known from a partial skull and three foot bones. They were got from Mongolian sediments which were deposited in a humid floodplain about 70 million years ago. A second species, "A. altai", known from a much more complete skeleton, was named and described in 2009. Its relationships to other tyrannosaurid genera are unclear. Some evidence suggests that "Alioramus" is closely related to the contemporary species "Tarbosaurus bataar".
The Maastrichtian stage in Mongolia had a wetter and more humid climate than the previous stages. Sediments show there were floodplains, large river channels and soil deposits, with periodic droughts.
This animal had many teeth, which were smaller than usual in the Tyrannosauridae, and a narrow, lower skull. It may have been a sub-adult (teenager), and must have eaten different prey than "Tyrannosaurus". Since the general area was a riverine delta, the prey may have been fish.
Walking with Dinosaurs
Walking with Dinosaurs is a 1999 nature documentary series produced by the BBC. It shows how Dinosaurs lived. The first episode, "New Blood", follows a "Coelophysis" trying to survive the dry season. The next, "Time of the Titans", shows a baby "Diplodocus" growing up. The third, "A Cruel Sea", follows a flock of "Rhamphorynchus" trying to avoid deadly storms. The fourth, "King of the Skies", follows a sick male "Ornithocheirus" migrating to Spain. The fifth, "Spirits of the Frozen Forest", follows a baby "Leaellynasaura" avoiding a hungry Koolasuchus (a 4 meter salamander). The last one, "Last of the Dino Dynasty", follows a female "Tyrannosaurus" who has already to mated and needs to take care of her babies in a world that is about to end as the Kt extinction nears by day in an already struggling volcanic world. There is also a 2013 film with the same name, a sequel called "Walking with Beasts" and a prequel called "Walking with Monsters".
Ornithocheirus
Ornithocheirus was a crested pterosaur that lived during the late Jurassic period. There was an ornithocheirus in "Walking with Dinosaurs".
Tylosaurus
Tylosaurus was an Upper Cretaceous mosasaur. It was a large, predatory marine lizard closely related to modern monitor lizards and to snakes.
In 1918, Charles H. Sternberg found a "Tylosaurus" with the remains of a plesiosaur in its stomach. The specimen is mounted in the United States National Museum (Smithsonian). The plesiosaur remains are stored in the collections.
Although these important specimens were briefly reported by C.H. Sternberg in 1922, the information was lost to science until 2001. The specimen was rediscovered and described by Everhart. It is the basis for the story line in the new (2007) National Geographic IMAX movie - "Sea Monsters".
Charlotte Gainsbourg
Charlotte Gainsbourg is an Anglo-French singer and movie actress.