Spectroscopy

Spectroscopy refers to a group of physical methods that decompose a radiation according to a certain property such as wavelength, energy, mass, etc.. The resulting intensity distribution is called spectrum.

Spectrometry is the quantitative measurement of spectra using a spectrometer. The recording method is called spectrography and the recording (graphical representation) itself is called a spectrogram, but is often simply referred to as "the spectrum" in technical jargon. Spectroscopes are used for visual observation of optical spectra, as was first done by Isaac Newton when he discovered the composition of white light from spectral colors in the 17th century.

The radiation studied covers the entire range of electromagnetic waves and mechanical waves such as sound and water waves, as well as particle beams, e.g. from electrons, ions, atoms or molecules. Spectroscopy is used to study the properties of the radiation itself, to find out the properties of the radiation source (emission spectroscopy) or to study the properties of a transport medium located between the source and the spectrometer (absorption spectroscopy). In particular, spectrometry can be used to determine the nature and concentration of emitting or absorbing substances.

If a spectrum shows sharp and separated intensity maxima, it is generally called a line spectrum, otherwise it is called a continuous spectrum. Spectra of these two basic types are often mixed. The name line spectrum is explained by the fact that the first optical spectral apparatuses received light from an illuminated narrow slit that was mapped to a specific location on the screen depending on the wavelength, so that a bright line was formed for each intensity maximum (see figure).

For example, the energy or wavelength spectrum of thermal radiation is of the continuous type with a broad maximum, from the position of which one can also read the temperature of the radiating body. On the other hand, the light emitted or absorbed by atoms shows a line spectrum by which one can clearly identify the chemical elements to which the atoms belong (spectral analysis according to Kirchhoff and Bunsen, 1859). The mass spectrum of a substance when examined with a mass spectrometer is also a line spectrum, indicating the masses of the molecules present in the substance or, where appropriate, their fragments. Both types of line spectra show high sensitivity and are therefore routinely used in chemical analyses to detect admixtures of foreign substances at the lowest concentration.

Spectrography is used in a wide variety of forms, for example in medicine, forensic chemistry, forensic toxicology and forensic biology. Spectroscopic observations of the line spectra of atoms and molecules provided decisive impetus for the development of atomic physics and quantum mechanics. The high precision with which many of their spectral lines can be measured allows, among other things, the precise verification of laws of nature, the determination of natural constants and the definition of the base units meter and second.

List of spectroscopy types and methods in analytics

1. Atomic spectroscopy - measurements of the properties of individual atoms, especially their electron energy levels
• Atomic Absorption Spectroscopy (AAS/OAS)
• Flame Technology
• Graphite Tube Technology
• Hydride Technology
• Atomic emission spectrometry (AES/OES)
• Inductively coupled plasma (ICP-OES)
• Microwave Plasma Torch AES (MPT-AES)
• Atomic fluorescence spectroscopy (AFS)
• Gamma spectroscopy
• Disturbed gamma-gamma angle correlation (PAC spectroscopy)
• Mößbauer spectroscopy (based on the Mößbauer effect)
• Electron Spectroscopy
• Photoelectron spectroscopy with X-rays (XPS)
• Photoelectron spectroscopy with UV light (UPS)
• Angle-resolved photoelectron spectroscopy (ARPES)
• Auger electron spectroscopy (AES)
• Electron Energy Loss Spectroscopy (EELS)
• X-ray spectroscopy (XRS)
• X-ray fluorescence analysis (XRF)
• X-ray diffraction (XRD)
• X-ray absorption spectroscopy (XAS)
• Glow discharge spectroscopy (GDOES)
2. Molecular spectroscopy - measurements of the properties of single molecules, especially valence electron energy levels and molecular vibrations and rotations
• Frequency modulation spectroscopy
• Fluorescence spectroscopy
• Single molecule fluorescence spectroscopy
• Fluorescence correlation spectroscopy
• Vibrational spectroscopy
• Infrared spectroscopy (IR)
• Ultraviolet spectroscopy (UV)
• Raman spectroscopy
• Terahertz spectroscopy
• Nuclear magnetic resonance spectroscopy (NMR, also high frequency spectroscopy)
• CIDNP spectroscopy (also NMR-CIDNP spectroscopy)
• Electron spin resonance (ESR/EPR)
• Electron Nuclear Double Resonance (ENDOR)
• Microwave spectroscopy
• UV/VIS spectroscopy (UV/Vis)
3. Solid state spectroscopy - measurements of the properties of whole solids (like crystals), especially their band structure details
• Absorption or transmission spectroscopy
• Reflectance spectroscopy
• photoconductive spectroscopy, see photoelectric effect#internal photoelectric effect
4. Impedance spectroscopy (dielectric spectroscopy)
5. Laser spectroscopy
• Cavity-ring-down spectroscopy (CRDS, also CRLAS)
• Laser Induced Fluorescence (LIF)
• Ultrafast spectroscopy - measurements of the details of fast processes, especially chemical reactions

Spectroscopy in astronomy

The absorption lines in the solar spectrum were named after Josef Fraunhofer, who discovered them in 1813. But it was not until 1859 that Gustav Kirchhoff and Robert Bunsen were able to explain the nature of these lines as fingerprints of elements in the solar atmosphere. In the following further development of spectral analysis, William Huggins (USA) and Angelo Secchi (Vatican Observatory), among others, succeeded in the systematic investigation of stellar spectra and the temperature-dependent classification of stars.

Spectral analysis of the light from the Sun and other stars showed that the celestial bodies are composed of the same elements as the Earth. However, helium was first identified by spectroscopy of the sun's light. One of the solar spectral lines could not be assigned to any known substance for decades, so that until the earthly occurrence was proven, it was assumed that an unknown element existed on the sun (Greek: Helios).

Other classical successes of astronomical spectral analysis are

• the detection of the Doppler effect on stars (see also radial velocity)
• and (around 1920) on galaxies (see redshift),
• the exact photographic analysis of the star Tau Scorpii by Albrecht Unsöld in 1939
• of magnetic fields on sun and bright stars (Zeeman effect)
• and especially the determination of stellar temperatures and spectral classes (see also Hertzsprung-Russell diagram and stellar evolution).

The associated measuring instruments ("spectral apparatus") of astrospectroscopy are:

• the spectroscope and the spectrometer (both visual)
• the spectrograph (photographic or with sensors)
• the monochromator and the interference spectrometer
• the frequency comb

Memorial plaque for Kirchhoff in Heidelberg