An attosecond (abbreviated as as) is a unit of time in the International System of Units (SI) equal to 10 or ⁄1 000 000 000 000 000 000 (one quintillionth) of a second.
An attosecond is to a second, as a second is to approximately 31.69 billion years.
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word laser originated as an acronym for light amplification by stimulated emission of radiation. The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories, based on theoretical work by Charles H. Townes and Arthur Leonard Schawlow and the optical amplifier patented by Gordon Gould.
A laser differs from other sources of light in that it emits light that is coherent. Spatial coherence allows a laser to be focused to a tight spot, enabling uses such as optical communication, laser cutting, and lithography. It also allows a laser beam to stay narrow over great distances (collimation), used in laser pointers, lidar, and free-space optical communication. Lasers can also have high temporal coherence, which permits them to emit light with a very narrow frequency spectrum. Temporal coherence can also be used to produce ultrashort pulses of light with a broad spectrum but durations measured in attoseconds.
In particle physics, a pion (/ˈpaɪ.ɒn/, PIE-on) or pi meson, denoted with the Greek letter pi (π), is any of three subatomic particles: π
, π
, and π
. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more generally, the lightest hadrons. They are unstable, with the charged pions π
and π
decaying after a mean lifetime of 26.033 nanoseconds (2.6033×10 seconds), and the neutral pion π
decaying after a much shorter lifetime of 85 attoseconds (8.5×10 seconds). Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays.
The exchange of virtual pions, along with vector, rho and omega mesons, provides an explanation for the residual strong force between nucleons. Pions are not produced in radioactive decay, but commonly are in high-energy collisions between hadrons. Pions also result from some matter–antimatter annihilation events. All types of pions are also produced in natural processes when high-energy cosmic-ray protons and other hadronic cosmic-ray components interact with matter in Earth's atmosphere. In 2013, the detection of characteristic gamma rays originating from the decay of neutral pions in two supernova remnants has shown that pions are produced copiously after supernovas, most probably in conjunction with production of high-energy protons that are detected on Earth as cosmic rays.
In atomic physics and chemistry, an atomic electron transition (also called an atomic transition, quantum jump, or quantum leap) is an electron changing from one energy level to another within an atom or artificial atom. The time scale of a quantum jump has not been measured experimentally. However, the Franck–Condon principle binds the upper limit of this parameter to the order of attoseconds.
Electrons can relax into states of lower energy by emitting electromagnetic radiation in the form of a photon. Electrons can also absorb passing photons, which excites the electron into a state of higher energy. The larger the energy separation between the electron's initial and final state, the shorter the photons' wavelength.