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Spectroscopy is the study of spectra.

Spectroscopy is often used in physical and analytical chemistry for the identification of substances, through the spectrum emitted or absorbed. A device for recording a spectrum is a spectrometer. Spectroscopy can be classified according to the physical quantity which is measured or calculated or the measurement process.

Spectroscopy is also heavily used in astronomy. See astronomical spectroscopy.

Table of contents
1 Physical quantity measured
2 Measurement process

Physical quantity measured

The type of spectroscopy depends on the physical quantity measured. Normally, the quantity that is measured is an amount or intensity of something.

Measurement process

Different types of spectroscopy use different measurement processes:

Two main types of spectroscopy

Absorption Spectroscopy uses the range of electromagnetic spectra in which a substance absorbs. It is more commonly used. The sample is vaporised and then light of a particular frequency is passed through the vapour. After calibration, the amount of absorption can be related to the concentrations of various metal ions. The method can be automated and is widely used to measure concentrations of ions such as sodium and calcium in blood. ( Atomic Absorption Spectroscopy )

Emission Spectroscopy uses the range of electromagnetic spectra in which a substance radiates. It requires the substance to be vaporised at high temperatures by placing it in a spark gap.

Common types of spectroscopy

X-ray spectroscopy and X-ray crystallography When X-rays of sufficient frequency (energy) interact with a substance, inner shell electrons in the atom are excited to outer empty orbitals, or they may be removed completely, ionizing the atom. The inner shell "hole" will then be filled by electrons from outer orbitals. The energy available in this de-excitation process is emitted as radiation (fluorescence) or will remove other less-bound electrons from the atom (Auger effect). The absorption or emission frequencies (energies) are characteristic of the specific atom. In addition, for a specific atom small frequency (energy) variations occur which are characteristic of the chemical bonding. With a suitable apparatus, these characteristic X-ray frequencies or Auger electron energies can be measured. X-ray absorption and emission spectroscopy is e.g. used in chemistry and material sciences to determine elemental composition and chemical bonding.

X-ray crystallography is a process in which X-rays are shone onto crystals at a certain angle. The wavelength of the X-rays is known and so the distance apart of the crystal planes can be calculated. Combining all information enables crystal structure to be detected.

Visible spectroscopy

Many atoms emit or absorb visible light. In order to obtain a fine line spectrum, the atoms must be in a gas phase. This means that the substance has to be vaporised. Spectrum is studied in absorption or emission.

UV spectroscopy

All atoms absorb in the UV region because photons are energetic enough to excite outer electrons. If the frequency is high enough, Photoionisation takes place.

Photoemission spectroscopy

Less frequently used / combined spectroscopy