Th desorption of surface substances by ultraviolet radiation.
The process by which atomic nuclei are broken apart into their constituent protons and neutrons by the impact of high energy gamma photons. Photodisintegration takes place during the core collapse phase of a → Type II supernova explosion.
photodissociation region (PDR)
nâhiye-ye šid-vâhazeš, ~ nur-vâhazeš
Fr.: région de photodissociation
A neutral region at the boundary of a → molecular cloud created by the penetration of → far ultraviolet (FUV) radiation from associated stars. The FUV radiation (6 eV ≤ hν ≤ 13.6 eV) dissociates the molecules and heats the gas and dust. A warm, atomic → H I region is thus created and the chemistry and thermal balance of the region are determined by the penetrating FUV photons. The progressive absorption of FUV photons leads to the occurrence of transitions between atomic and molecular phases, such as H I/H2 and C II/C I/CO transitions. By extension, any neutral region where the physics is controlled by FUV photons can be called a PDR, as it is the case for → diffuse interstellar clouds or the edge of → circumstellar disks. The PDR concept was first studied by A. G. G. M. Tielens and D. Hollenbach (1985, ApJ 291, 722).
Pertaining to electronic or other electrical effects that are due to the action of electromagnetic radiation, especially visible light.
Fr.: courant photoélectrique
oskar-e šid-barqi, ~ nur-barqi
Fr.: effet photoélectrique
The process of release of electrically charged particles (usually → electrons) as a result of irradiation of matter by light or other → electromagnetic radiation. The classical electromagnetic theory was unable to account for the following characteristics of the phenomenon. Light below a certain threshold frequency, no matter how intense, will not cause any electrons to be emitted. Light above that frequency, even if it is not very intense, will always cause electrons to be ejected. The electrons are ejected after some nanoseconds, independently of the light intensity. The maximum kinetic energy of the emitted electrons is a function of the frequency and does not dependent on the intensity of the incident light. The classical theory could not explain how a train of light waves spread out over a large number of atoms could, in a very short time interval, concentrate enough energy to knock a single electron out of the metal. In 1905, based on Planck's idea of → quanta, Einstein proposed that light consisted of quanta (later called → photons); that a given source could emit and absorb radiant energy only in units which are all exactly equal to the radiation frequency multiplied by a constant (→ Planck's constant); and that a photon with a frequency over a certain threshold would have sufficient energy to eject a single electron. His photoelectric equation is descibed as (1/2)mu2 = hν - A, where m is the electron mass, u is the electron velocity, h is Planck's constant, ν is the frequency, and A the → work function, which represents the amount of work needed by electrons to get free of the surface. See also → photoelectron, → photoelectric current, → external photoelectric effect, → internal photoelectric effect.
Fr.: chauffage photoélectrique
A heating process occurring in → diffuse molecular clouds which is believed to be the main heating mechanism in cool → H I regions. Far-ultraviolet (FUV) photons, in the energy range 6 eV <hν < 13.6 eV, expel electrons from → interstellar dust grains and the excess → kinetic energy of the electrons is converted into gas → thermal energy through → collisions. The high energy limit corresponds to the cut-off in the → far-ultraviolet (FUV) radiation field caused by the hydrogen absorption (hν = 13.6 eV), while the low energy limit corresponds to the energy needed to free electrons from the grains (hν ~ 6 eV). In the cold neutral medium (Tkin≥ 200 K) photoelectric heating accounts for most of the heating, the → X-ray and → cosmic ray heating rates (→ cosmic-ray ionization) being more than an order of magnitude smaller. In a relatively dense neutral medium (nH≥ 100 cm-3), where a significant fraction of carbon is in the neutral form, carbon ionization becomes an important heating source, but it is still not comparable to the photoelectric effect. The heating rate cannot be directly measured, but it can be estimated through observations of the [C II] line emission, since this is believed to be the main → coolant in regions where the photoelectric heating is dominant (See, e.g., Juvela et al., 2003, arXiv:astro-ph/0302365).
borz-e šidsanjik, ~ nursanjik
Fr.: magnitude photoélectrique
The magnitude of an object as measured with a photoelectric photometer.
Fr.: photométrie photoélectrique
A photometry in which the magnitudes are obtained using a photoelectric photometer.
An electron emitted from an atom or molecule by an incident photon in the → photoelectric effect.
Fr.: effet photoémissif
A process going on in a molecular cloud surface whereby the surface material ionized by ultraviolet photons of neighboring stars is dissipated.
The mechanism of raising an electron to higher energies by photon absorption, when the energy of the photon is too low to cause photoionization.
aks, šidnegâr, nurnegâr
A picture produced by photography. → picture.
Fr.: magnitude photographique
The apparent magnitude of a star as determined by measuring its brightness on a photographic plate. The photographic magnitude scale is now considered obsolete.
Fr.: relevé photographique
Recording a large area of the night sky by photographic techniques, as practiced in the past before the advent of electronic detectors.
aksbardâri, šidnegâri, nurnegâri
The process of recording and producing images by exposing light-sensitive detectors to light or other forms of radiation.
Aksbardâri, literally "taking photograph," from aks,
+ bardâri verbal noun of bardâštan "to take," composite verb from
bar- "on; up; upon; in; into; at; forth; with; near; before;
according to" (Mid.Pers. abar; O.Pers.
upariy "above; over, upon, according to;" Av. upairi "above, over,"
upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;"
L. super-; O.H.G. ubir "over;" PIE base *uper "over") +
dâštan "to have, to possess" (Mid.Pers. dâštan;
O.Pers./Av. root dar- "to hold, keep back, maintain, keep in mind;" cf.
Skt. dhr-, dharma- "law;"
Gk. thronos "elevated seat, throne;"
L. firmus "firm, stable;" Lith. daryti
"to make;" PIE *dher- "to hold, support").
The physical process in which an incident high-energy photon ejects one or more electrons from an atom, ion, or molecule.