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.
The mass-to-charge ratio of molecules and atoms are studied in mass spectrometry. Note that a mass spectrometer does not measure the kinetic energy of particles: all particles have the same known kinetic energy (or an integer multiple thereof, depending on the charge). It is disputable whether this field strictly is a type of spectroscopy.
The number of molecules or atoms or quantum-mechanical states to which the frequency or energy parameter applies.
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.
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.
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.
Fourier transform is an efficient method for collecting various spectra. The use of Fourier transform in spectroscopy is called Fourier transform spectroscopy. It is frequently applied to infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy.