GLOSSARY     SPECTROSCOPY Angstrom
Unit of length equal to E-10 metre. Often used to denote wavelengths.
absorption line
Atom absorbs photon whose energy corresponds to the energy difference between two bound levels. Continuum seen through cool gas. Bound-bound absorption.
atom*
Used in this glossary only to mean "atom, ion or molecule".
atomic transition
Usually refers to bound-bound transitions. Atom gains/loses energy by inelastic/superelastic collision with another atom ion or electron or by absorption/emission of a photon.
Balmer lines
Emission or absorption lines that arise from transitions between the n=2 (first excited) level and higher levels of hydrogen.
bands
Series of closely spaced emission or absorption lines that arise in molecular spectra.
Bohr atom
A model of the atom developed by Neils Bohr in which electrons orbit the nucleus with quantized orbital angular momenta. Historical interest only.
bound-bound
Atomic* transition between energy levels. Line spectra.
bound-free
Atomic* transition between energy level and continuum. Continuum spectra.
Bowen mechanism
Optical pumping of O III by the "Lyman alpha" resonance line of He II results in abnormally strong fluorescence from the 2p3d³P₂ level. Also, pumping of N III by the 37.4436 nm line of O III. Amazing coincidences of nature.
bremsstrahlung
Free-free transitions. Continuous emission or absorption. Brems=brakeing, strahlung=radiation, radiation due to unbound electron passing and being (de)accelerated by an ion. Do not confuse with cyclotron or synchrotron radiation.
coherent radiation
A (light beam) that maintains a steady phase correlation [like E=cos(wt)] with time and over space. "Laser light". Associated with stimulated emission. "Waves in phase".
cyclotron radiation
Nonthermal radiation from charged particles in magnetic fields. A low energy (non relativistic) particle deflected by a magnetic field (such as happens in a cyclotron particle accelerator) will obey: ACCELERATED CHARGES EMIT ELECTROMAGNETIC RADIATION. Relativistic charges emit synchrotron radiation.
electron
One of the four fundamental particles of nature. Charge -1.6021917E-19 Coulomb, mass 9.109534E-31 kg, spin one-half.
electron volt
A unit of energy often used for convenience in spectroscopy equal to the kinetic energy gained by an electron falling through a potential difference of one volt, (charge on electron times one) 1.6021917E-19 Joule.
emission line
A bright spectral line. Bound-bound transition. Stimulated or spontaneous de-excitation of an atom. Transition from upper to lower state.
emission nebula
Interstellar gas fluorescing in the ultravioler light of a young star(s), or supernova shock, or energetic processes involving neutron stars or black holes.
energy level
(Atomic and molecular) An allowed energy or state of an atom ion or molecule. Most have lifetimes on the order of nanoseconds. See ground state, excited state, metastable state.
excitation
A collisional or radiative process whereby an atom* is raised from a lower to an upper state.
extinction
The attenuation of light by absorption or scattering.
fluorescence
The absorption of light at one wavelength followed by emission at the same or (usually) longer wavelength(s). Emission line. Bowen fluorescent mechanism.
flux
The rate at which energy crosses a unit area, watt per square metre.
forbidden line
Atomic (bound-bound) transition that violates the La Porte parity rule. Transition with low probability and unlikely to be seen in the laboratory.
Fraunhofer line
A solar (or often stellar) absorption line.
free-free
An atomic transition between the continuum states of an atom*. Classically, an electron in the field of an ion will be (de)accelerated and hence radiate, bremsstrahlung radiation.
gamma rays
Electromagnetic radiation of highest energy, above that of X-rays. The boundary is fuzzy, I say above 500keV.
interstellar dust
Micron sized grains (carbon/silicon/ices) in interstellar space. Not gas! Dark nebulae, interstellar reddening.
interstellar gas
Gas in interstellar space. Mostly hydrogen and helium. Not dust! Typical density is an atom per cc but can be hundreds of times more or less. H I, H II, GMC, gas cloud.
interstellar line
Sharp often multiple lines superimposed on stellar spectra seen through interstellar gas (clouds).
ion
An atom or molecule that has lost (or sometimes gained) an electron. Atoms may lose more than one electron and become multiply ionized. Fe+,Fe II, Ni++,Ni III, Co++++,Co V...
ionization
Collisional or radiative process wherein an atom* loses an electron. Bound-free. Continuum.
ionization limit
Ionization energy. The energy required to remove an electron from and atom*.
ionization potential
The energy required to ionize an atom in its normal (ground) state.
J
Angular momentum quantum number. Seen as subscript in term for example the 7/2 in ⁵D_7/2. Permitted lines obey "J changes by zero or one, but not zero to zero".
level
Atomic* energy level. One of the bound energies associated with a particular atom*. A level often consists of several states having nearly the same energy, degenerate states. You can find tables of energy levels and levels are often shown in Grotrian or energy level diagrams.
light
Strictly electromagnetic radiation visible to the human eye but often used to describe the rest of or other parts of the electromagnetic spectrum.
line
An emission or absorption feature arising from a bound-bound atomic* transition. Fraunhofer line, nebular line, forbidden line,... No continuum line!
line broadening
A mechanism that gives a finite width to spectral lines. Damping, Doppler, microturbulence, macroturbulence, rotational, instrumental.
line profile
A graph of intensity vs. wavelength or frequency.
micron
Unit of measurement, a millionth of a metre, m = 1.E-6 metre, one micrometre.
opacity
Absorbing power of a medium. Gives rise to extinction.
parity
A property of any quantum mechanical state (or mathematical function) that describes the function after (mirror) reflection. Even states (functions) are symmetric - identical after reflection (like a cosine function) while odd states (functions) are antisymmetric (like a sine function). Quantum mechanical states have definite parity and the parity attribute is sometimes called a "good quantum number".
parity rule
La Porte's rule states that parity must change for a permitted atomic transition. Violation of this rule gives rise to "forbidden lines".
phosphorescence
Fluorescence that occurs between atomic levels near the ground level up to the first excited level having parity opposite to that of the ground level. Forbidden line. Delayed fluorescece.
photoionization
Ionization by radiation. Photodissociation (molecules).
photon
(Exchange) particle responsible for Coulomb force. No charge, no mass, energy equals Planck's constant (h) times frequency, spin one (boson), propagates at the speed of light in vacuum. "Light". "Electromagnetic radiation". "Particle nature of light". Photon energy is related to frequency by E=hn.
proton
Basic constituent of nucleus. Nucleus of hydrogen atom. Ionized hydrogen. Charge +e, mass 1.6726485E-27[kg], spin one-half (fermion), one down plus two up quarks (UUD).
redshift
Doppler shift, a frequency or wavelength shift caused by motion. v=cDl/l
recombination
Joining of electron and ion. Opposite of ionization.
recombination spectrum
Emission spectrum observed in ionized gas. Not exactly fluorescence, excited levels are populated when ion recombines to some excited level.
spectrogram
A photograph of a spectrum taken through a (usually slit) spectrograph.
spectrograph
An instrument for recording the spectrum of an object.
spectroscope
An instrument for viewing the spectrum of an object. (Can also be used as spectrograph).
spectroscopy
A scientific field that employs the study of the spectrum emitted by objects.
spectrum
A graph or plot of intensity vs. wavelength or frequency. Rainbow.
spectral analysis
in astronomy, the study of the composition and structure of bodies via spectroscopy.
spin
An intrinsic property of subatomic particles. Angular momentum, usually measured in units of "h-bar", plancks constant divided by two pi. Electrons, protons, neutrons and quarks all have "spin one-half" and are called fermions (obey the laws of Fermi statistics). Photons have "spin-one" and are called bosons (obey Bose-Einstein statistics).
state
Information about a system at some instant. Usually given in some state representation. (Not necessarily all information or the best representation!) Time averaged dynamial variables are often called stationary states and might include energy, parity, and information regarding angular momentum. Often the terms state and level are loosely used but levels are made of states and not vice-versa.
Stefan's (Stefan-Boltzmann) law
The power radiated per square metre of surface at temperature T is proportional to the fourth power of T.
synchrtron radiation
Radiation emitted by relativistic charged partiles in a magnetic field. Nonthermal radiation.
transition
Refers to a change in the energy state of an atom*. May be due to collisional excitation/deexcitation, radiative absorption/emission, or spontaneous decay (the usual emission or fluorescence we "see").
Wien's law
Formula that relates the temperature of a black body to the wavelength of maximum emission (wavelength spectrum).
X-ray
Region of the electromagnetic spectrum above extreme UV (about one nm, "soft" X-rays) and below "soft" gamma rays (about 500keV, "hard" X-rays).