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Electromagnetic spectroscopy

Electromagnetic spectroscopy is the spectroscopy of electromagnetic spectra which arise out of atoms absorbing and emitting quanta of electromagnetic radiation. It is done with the aid of a spectrometer (spectroscope).

Types of electromagnetic spectroscopy

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.

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, phosphorescence. Generally, emission spectroscopy deals with visible light and shorter wavelengths, since fluorescence is less likely to happen with long wavelengths. See also: spontaneous emission.

Examples:

Absorption spectroscopy

Absorption spectroscopy is the study of electromagnetic radiation spectra absorbed by atoms or molecules that change energy levels; the atoms usually positioned between a radiation source and the observer. 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:

Vibrational spectroscopy - absorption of infrared radiation, see infrared spectroscopy; often used as an analytical tool
Atomic absorption - often used as an analytical tool
UV/visible spectroscopy - absorption of ultraviolet and visible light; often used as an analytical tool
Mossbauer spectroscopy - Measures the absorption of gamma rays by atoms bound in a solid as a function of gamma-ray energy. This is not an analytical technique; it is a means to understand certain microscopic processes in matter.

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.

Examples

The spectrum of sunlight

Matter reflects, absorbs or scatters regions of the electromagnetic radiation shown upon it. Depending on the Correlated Color Temperature of the light source, you will perceive the object to be of a differing color. Man has attempted to utilize Plank's Law to assign a specific Correlated Color temperature to each light source sold in your store. Each bulb measured, was assigned a Correlated Color Temperature CCT in kelvins; 2800 K is a living room light, 6000 K is a bright sunny day. "Correlated" is used because all is compared back to a perfect black body radiator.

The higher the temperature, the shorter (and bluer) the average visible 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 chemical 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.

Absorption in the atmosphere

The material in Earth's atmosphere absorbs some of the sunlight passing through it. This has been measured at sea level and various altitudes. Estimates were made of the likely spectrum of sunlight above the atmosphere and the absorption within the atmosphere. Actual measurements above the atmosphere required spacecraft which were able to take such readings. These efforts are illustrated in the following images.