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# Atomic spectral line

In physics, atomic spectral lines are of two types:

• An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength. A spectrum of many such photons will show an emission spike at the wavelength associated with these photons.
• An absorption line is formed when an electron makes a transition from a lower to a higher discrete energy state, with a photon being absorbed in the process. These absorbed photons generally come from background continuum radiation and a spectrum will show a drop in the continuum radiation at the wavelength associated with the absorbed photons.

The two states must be bound states in which the electron is bound to the atom, so the transition is sometimes referred to as a "bound–bound" transition, as opposed to a transition in which the electron is ejected out of the atom completely ("bound–free" transition) into a continuum state, leaving an ionized atom, and generating continuum radiation.

A photon with an energy equal to the energy difference between the levels is released or absorbed in the process. The frequency ν at which the spectral line occurs is related to the photon energy E by Planck's law E = hν where h is Planck's constant.

## Emission and absorption coefficients

The emission of atomic line radiation may be described by an emission coefficient ε with units of energy/time/volume/solid angle. ε dt dV dΩ is then the energy emitted by a volume element dV in time dt into solid angle dΩ. For atomic line radiation:

$\epsilon = \frac{h\nu}{4\pi}n_2 A_{21}\,$

where n2 is the density of emitting atoms, A21 is the Einstein coefficient for spontaneous emission, which is fixed for any two energy levels. By Kirchhoff's law, the absorption characteristics in a region of space are closely tied to its emission characteristics, so we must mention the absorption coefficient as well. The absorption coefficient κ will have units of 1/length and κ' dx gives the fraction of intensity absorbed for a light beam at frequency ν while traveling distance dx. The absorption coefficient is given by:

$\kappa' = \frac{h\nu}{4\pi}~(n_1 B_{12}-n_2 B_{21}) \,$

where Iν is the spectral intensity of radiation at (and near) frequency ν, n1 is the density of absorbing atoms, and B12 and B21 are the Einstein coefficients for absorption and induced emission respectively. Like the coefficient A21, these are also constant for any two energy levels.

In the case of local thermodynamic equilibrium, the densities of the atoms, both excited and unexcited, may be calculated from the Maxwell-Boltzmann distribution, but for other cases, (e.g. lasers) the calculation is more complicated.

The above equations have ignored the influence of the spectral line shape. To be accurate, the above equations need to be multiplied by the (normalized) spectral line shape, in which case the units will change to include a 1/Hz term.

## The Einstein coefficients

In 1916, Albert Einstein proposed that there are essentially three processes occurring in the formation of an atomic spectral line. The three processes are referred to as spontaneous emission, induced emission and absorption and with each is associated an Einstein coefficient which is a measure of the probability of that particular process occurring.

### Spontaneous emission

Main article: Spontaneous emission

Spontaneous emission is the process by which an electron "spontaneously" (i.e. without any outside influence) decays from a higher energy level to a lower one. The process is described by the Einstein coefficient A21 which gives the probability per unit time that an electron in state 2 with energy E2 will decay spontaneously to state 1 with energy E1, emitting a photon with an energy E2E1 = hν. If ni is the number density of atoms in state i  then the change in the number density of atoms in state 1 per unit time due to spontaneous emission will be:

$\left(\frac{dn_1}{dt}\right)_{A_{21}}=A_{21}n_2$

### Stimulated emission

Main article: Stimulated emission

Stimulated emission (also known as induced emission) is the process by which an electron is induced to jump from a higher energy level to a lower one by the presence of electromagnetic radiation at (or near) the frequency of the transition. The process is described by the Einstein coefficient B21 which gives the probability per unit time per unit energy density of the radiation field, that an electron in state 2 with energy E2 will decay to state 1 with energy E1, emitting a photon with an energy E2E1 = hν. The change in the number density of atoms in state 1 per unit time due to induced emission will be:

$\left(\frac{dn_1}{dt}\right)_{B_{21}}=B_{21}n_2 I(\nu)$

where I(ν) is the spectral intensity of the radiation field at the frequency of the transition.

Stimulated emission is one of the fundamental processes that led to the development of the laser.

### Photoabsorption

Main article: Absorption (optics)

Absorption is the process by which a photon is absorbed by the atom, causing an electron to jump from a lower energy level to a higher one. The process is described by the Einstein coefficient B12 which gives the probability per unit time per unit energy density of the radiation field, that an electron in state 1 with energy E1 will absorb a photon with an energy E2E1 = hν and jump to state 2 with energy E2. The change in the number density of atoms in state 1 per unit time due to absorption will be:

$\left(\frac{dn_1}{dt}\right)_{B_{12}}=-B_{12}n_1 I(\nu)$

## Detailed balancing

The Einstein coefficients are fixed probabilities associated with each atom, and do not depend on the state of the gas of which the atoms are a part. Therefore, any relationship that we can derive between the coefficients at, say, thermal equilibrium will be valid universally. At equilibrium, we will have a simple balancing, in which the net change in the number of any excited atoms is zero, being balanced by loss and gain due to all processes. With respect to bound-bound transitions, we will have detailed balancing as well, which states that the net exchange between any two levels will be balanced. This is because the probabilities of transition cannot be affected by the presence or absence of other excited atoms. Detailed balance requires that the change in time of the number of atoms in level 1 due to the above three processes be zero:

$0=A_{21}n_2+B_{21}n_2I(\nu)-B_{12}n_1 I(\nu)\,$

Along with detailed balancing, we may use our knowledge of the equilibrium energy distribution of the atoms, as stated in the Maxwell-Boltzmann distribution , and the equilibrium distribution of the photons, as stated in Planck's law of black body radiation to derive universal relationships between the Einstein coefficients.

From the Maxwell-Boltzmann distribution we have for the number of excited atomic specie i:

$\frac{n_i}{n}= \frac{g_i e^{-E_i/kT}}{Z}$

where n is the total density of the atomic specie, excited and unexcited, k is Boltzmann's constant, T is the temperature, gi is the degeneracy of state i, and Z is the partition function. From Planck's law of black body radiation we have for the spectral intensity at frequency ν

$I(\nu)=\frac{F(\nu)}{e^{h\nu/kT}-1}$

where:

$F(\nu)=\frac{2h\nu^3}{c^2}$

where c is the speed of light and h is Planck's constant. Note that in some treatments, the blackbody energy density is used rather than the intensity, in which case:

$F(\nu)=\frac{8\pi h\nu^3 }{c^3}$

Substituting these expressions into the equation of detailed balancing and remembering that E2E1 = hν yields:

$A_{21}g_2e^{-h\nu/kT}+B_{21}g_2e^{-h\nu/kT}\frac{F(\nu)}{e^{h\nu/kT}-1}= B_{12}g_1\frac{F(\nu)}{e^{h\nu/kT}-1}$

The above equation must hold at any temperature, so that the three Einstein coefficients are interrelated by:

$\frac{A_{21}}{B_{12}}=\frac{g_1}{g_2}~F(\nu)$

and

$\frac{B_{21}}{B_{12}}=\frac{g_1}{g_2}$

When this relation is inserted into the original equation, one can also find a relation between A12 and B12, involving Planck's law.

## Oscillator strengths

Oscillator strength f12 is defined by:

$a_{12}=\frac{\pi e^2}{m_e c}\,f_{12}$

where e is the electron charge and me is the electron mass. This allows all three Einstein coefficients to be expressed in terms of the single oscillator strength associated with the particular atomic spectral line:

$A_{21}=\frac{8\nu^2 \pi^2 e^2}{m_e c^3}~\frac{g_1}{g_2}~f_{12}$
$B_{12}=\frac{4\pi^2 e^2}{m_e h\nu c}\,f_{12}$
$B_{21}=\frac{4\pi^2 e^2}{m_e h\nu c}~\frac{g_1}{g_2}~f_{12}$