Spectral line

As spectral lines or resonance lines are called sharply separated from each other lines of the spectrum emitted (emission lines) or absorbed ( absorption lines ) of electromagnetic waves, in the strict sense within the wavelength range of visible light ( light spectrum). Spectral lines are characterized by wavelength, line intensity and line width. The cause of the spectral lines is excited by light electronic transitions in atoms or molecules.

Spectral lines occur in the instrumental Atomic Spectroscopy ( such as nuclear magnetic resonance spectroscopy ) or the flame color.

Basics

A spectral line is the light of a well-defined frequency which is emitted or absorbed by an atom or a molecule due to a quantum mechanical transition. It is used to distinguish different types of atoms. The frequency of a spectral line is determined by the energy of the emitted or absorbed photon, which is determining the difference between the energies of the quantum -mechanical states.

An emission line is obtained by changing from a higher to a lower energy level, such as when an electron from the excited state to the ground state. In this case, a photon is emitted. This can happen either spontaneously (spontaneous emission), or such as the laser, are excited by light of suitable frequency (stimulated emission). It shows itself in the spectrum as a bright line.

An absorption curve obtained by resonance absorption of a suitable photon of the incident light, whereby a transition is induced from a lower to a higher energy level, such as when an electron is excited by the photons from the valence band into the conduction band (cf. the photoelectric effect). When relapse into the lower energy level, photons are emitted isotropically in any direction, so that (if a sufficient number of absorbing atoms are present)

• To observe a dark line in the continuous spectrum of the translucent light ( Fraunhofer lines).
• A bright emission line is seen against a dark background when analyzing the exiting laterally from the gas lamp.

The name " spectral " goes back historically to the fact that in conventional spectrometers input a gap exists that determines the form on the screen or in the eye of the beholder. The name was transferred later to the shape of a recorded spectrum with the characteristic peaks in the curve.

Emission profiles

The light of a spectral line does not contain a frequency, but a ( narrow ) frequency range. The width of this area is called the line width. The line width of an emission line is made up of several contributions:

History

Absorption lines were first discovered in 1802 by William Hyde Wollaston in 1813 and, independently of him, by Joseph von Fraunhofer in the solar spectrum. These dark lines in the solar spectrum are also called Fraunhofer lines.

The spectral lines were one of the effects that contributed to the development of quantum mechanics. A bound electron in an atom could emit electromagnetic waves of arbitrary frequencies according to classical electrodynamics; the existence of discrete lines was classically unexplainable. The discovery that the frequencies of the spectral lines of the hydrogen atom proportional to an expression of the form ( 1/n2 - 1/m2 ) with integers m and n are led to the concept of the quantum number and brought Niels Bohr finally on his Bohr model of the atom, the first - now obsolete - quantum mechanical model of the atom. Modern quantum mechanics can predict the spectral lines of atoms with very high accuracy.