Spectroscopy is the study and measurement of spectra produced by matter that interacts with or emits electromagnetic radiation. Initially, spectroscopy was defined as the study of the interaction between radiation and matter as a function of wavelength. Now, spectroscopy is defined as any measurement of a quantity as a function of wavelength or frequency. During a spectroscopy experiment, electromagnetic radiation in a certain wavelength range passes from a source through a sample containing compounds of interest, resulting in absorption or emission. During absorption, the sample absorbs energy from the light source. During emission, the sample emits light of a different wavelength than that of the source.
In absorption spectroscopy, the compounds of the sample are excited by electromagnetic radiation supplied by a light source. Its molecules absorb energy from electromagnetic radiation, are excited, and jump from a low-energy ground state to a higher energy excitation state. A detector on the opposite side of the sample, usually a photodiode, records the absorption of wavelengths of the sample and determines the extent of their absorption. The spectrum of wavelengths absorbed by a sample is known as the absorption spectrum, and the amount of light absorbed by a sample is its absorbance.
Each molecule in a sample will only absorb wavelengths with energies corresponding to the energy difference of the current transition. In simpler terms, this means that a molecule bouncing from ground state 1 to excited state 2 with an energy difference ΔE will allow other wavelengths to pass until it absorbs radiation from a wavelength corresponding to ΔE.
The light that passes through the photodiode without any absorption is called Stray Radiant Energy or stray light. The absorption resulting from the energy difference between the two states is called the absorption line, and a collection of absorption lines forms an absorption spectrum.
The frequency of each absorption line in an absorption spectrum tells us the molecular structure of the sample and can be affected by factors such as stray light, ambient temperature and electromagnetic fields.