Electromagnetic spectroscopy
Electromagnetic spectroscopy a.k.a. spectrophotometry is the spectroscopy of electromagnetic spectra which arise out of atoms absorbing and emitting quanta of electromagnetic radiation.
Electromagnetic spectroscopy involves the use of a spectrophotometer.
Types of electromagnetic radiation measured
This can be in any range of wavelengths:
Electromagnetic spectroscopy can be classified into narrower fields as discussed below, though in some spectroscopic techniques, several processes may be happening at the same time.
Types of electromagnetic spectroscopy
Emission spectroscopy
Emission spectroscopy is the study of electromagnetic radiation spectra given off by atoms or molecules that undergo a transition to a lower energy level. Such a process is called fluorescence or, under certain conditions, spontaneous emission.
Examples:
Absorption spectroscopy
Absorption spectroscopy is the study of electromagnetic radiation spectra absorbed by atoms or molecules that change energy levels. Often, it is used as an analytical technique; specific chemical compounds have a specific absorption spectrum that acts as a fingerprint. Moreover, the amount of absorption is related to the amount of absorbing compound. Absorption spectroscopy can be used to determine the concentration of chemical compounds in samples (see molar absorptivity).
Examples of absorption spectroscopy:
Other techniques
Electromagnetic radiation can interact with matter in ways other than simple absorption and emission, such as in the following techniques:
- Circular dichroism spectroscopy - measures effects of a sample on the polarization of light.
- Magnetic circular dichroism
- Nuclear magnetic resonance (NMR) - measures the resonant absorption of radiofrequency radiation by nuclei in a strong magnetic field. Absorption peaks correspond to transitions in the nuclear spin states of the sample molecule(s).
- Electron spin resonance - similar to NMR, but looking at electrons.
- Raman spectroscopy - A molecule can absorb a part of the energy of a photon, which results in a change in frequency (or wavelength) of the photon. The amount of absorbed energy corresponds to an infrared transition in the molecule, even though the photon might have a visible-light wavelength.
- Stark spectroscopy - measures effects of electrical fields on the spectra.
Application example: the spectrum of sunlight
Matter emits electromagnetic radiation with a spectrum that depends on temperature (see: black body, Wien's law). The higher the temperature, the shorter the average wavelength. The sun, which has a temperature around 6000 K, emits most strongly in the visible light. However, certain wavelengths are missing from the solar spectrum, which is the result of elements in the chromosphere of the sun that have resonant transitions at those wavelengths. From the exact wavelengths of these missing parts of the spectrum, or absorption lines, we can deduce which elements are present in the sun. The fact that these elements have absorbed the radiation indicates that the chromosphere is cooler than the photosphere.
However absorption spectra can not give us information about the
abundance of the various elements. This is because Hydrogen
and Helium (the main constituents of the sun) need much more
energy to excite them enough to absorb radiation than other
elements (such as Calcium) present. So even though H and He
are more abundant, a much smaller percentage of them get excited
enough to produce a high intensity. To get a better understanding of abundance of these elements it is necessary to study the emission spectrum of elements in the chromosphere. It is only possible to assess this when the photosphoric radiation is totally obscured during an eclipse. At this time the emission spectrum of the chromosphere is highly dominated by hydrogen, which is the main constituent of the sun.
