Spectral lines
A spectral line is a dark or bright line present in a uniform and continuous electromagnetic spectrum. The spectral lines result from the interaction between a quantum system (generally atoms, but sometimes also molecules or atomic nuclei) and electromagnetic radiation.

In a quantum system, energy cannot take arbitrary values: only precise energy levels are possible. It is said that the energy of the system is quantified. The changes of state will thus correspond to precise energy values corresponding to the difference of energy between the final and the original level.

If the energy of the system decreases by a quantity DeltaE, an electromagnetic quantum of radiation, called photon, will be emitted at the frequency u given by the Planck-Einstein relation:

  • DeltaE = hu where H is the Planck's constant.

    Similarly, if the system absorbs a photon of frequency ?, its energy will increase by a hu quantity. As the energy of the system is quantified, it will be the same for the frequency of the photons emitted or absorbed by the system. This explains why the quantum system spectrum consists of number of discrete lines rather than of a continuous spectrum where all the frequencies are present in variable quantity.

    A hot gas will cool by emitting photons. The spectrum observed will then consist of luminous lines on a dark background. These are emission lines. Alternatively, if the gas is cold and lit by a continuous source, the gas will absorb photons and the spectrum will consist of dark lines on a luminous background: these are absorption lines.

    The emission and absorption lines are very specific to each chemical, and can be used to easily identify the chemical composition of any medium able to transmit light (generally, it will be a gas). They also depend on the physical conditions of the gas and are then useful to determine the chemical composition and physical state of stars and other astronomical bodies which couldn't be analyzed by any other means.

    In the real world, the lines do not have a perfectly determined frequency but are spread out over a frequency band. The reasons of this widening are multiple:

  • Doppler widening: the Doppler effect causes a shift towards the red or the blue according to whether the source moves away or get close to the observer. In a gas, all the particles are moving in all directions, which causes a widening of the spectral lines. As the speed of the particles depends on their temperature: the higher the temperature of gas is, the larger the differences in speed are, and the broader the lines are. This effect is typically 1000 times more intense than natural widening.
  • Natural widening: the principle of uncertainty connects the lifespan ?T of an excited state and the precision of its energy level ?E, thus the same excited level will have slightly different energies in various atoms. This effect is rather weak.
  • Collisional widening: the collision between particles (atoms or molecules) modifies slighty their energy levels, from where the widening of the lines. The amount of this effect depends on gas density.

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