When the spectrum of an unknown substance is excited by flame, as in the flame test, or by an arc, spark, or other suitable method, a quick analysis with a spectrograph is usually sufficient to determine the presence or absence of any particular element. Absorption spectra are frequently useful in identifying chemical compounds.
Suitable ionisation detectors detect spectra beyond the ultraviolet region, of X rays and gamma rays. Gamma-ray spectra are useful in neutron-activation analysis. In this technique, a specimen is irradiated with neutrons in a nuclear reactor and becomes radioactive, emitting gamma rays. The spectra of these gamma rays serve to identify minute quantities of certain chemical elements in the specimen. Along with more conventional types of spectroscopy, this technique is valuable in crime detection.
Raman spectroscopy, discovered in 1928 by the Indian physicist Sir Chandrasekhara Venkata Raman, has had widespread recent application in theoretical chemistry. Raman spectra are formed when, under certain conditions, light in the visible or ultraviolet region is first absorbed, then is reemitted at a lower frequency after causing molecules to rotate or vibrate. Two magnetic methods of spectroscopy at the radio-frequency region of the spectrum, longer than the infrared band, are valuable in providing chemical information on molecules and showing their detailed structure. These methods are nuclear-magnetic resonance (nmr) and electron-paramagnetic resonance (epr), the latter also being called electron-spin resonance (esr). These methods depend on the fact that electrons and protons spin like little tops. To align the spins, the specimen is placed in a magnetic field. Electrons or protons in the specimen “flip” over, reversing their spin axes, when the proper amount of radio-frequency power is supplied.

Astrophysical Applications
The distance at which a spectroscope may be placed from the source of light is unlimited. Thus, spectroscopic analysis of the light of the sun permits an accurate chemical analysis of the constituents of the sun. The Fraunhofer lines were discovered and named early in the 19th century after their discovery as absorption lines in the spectrum of the sun; a secondary discovery was that these same lines could be produced on the earth. The element helium was discovered on the sun and named many years before its presence on the earth was detected. More recently, spectroscopic study of the sun has given strong indirect evidence for the existence of a negative hydrogen ion.