Absorption Spectrum vs Emission Spectrum
Absorption and emission spectra of a species help to identify those species and provide a lot of information about them. When absorption and emission spectra of a species are put together, they form the continuous spectrum.
What is Absorption Spectrum?
An absorption spectrum is a plot drawn between absorbance and wavelength. Sometimes instead of wavelength, frequency or wave number can also be used in the x axis. Log absorption value or the transmission value is also used for the y axis in some occasions. Absorption spectrum is characteristic for a given molecule or an atom. Therefore, it can be used in identifying or confirming the identity of a particular species. A colored compound is visible to our eyes in that particular color because it absorbs light from the visible range. Actually, it absorbs the complementary color of the color we see. For example, we see an object as green because it absorbs purple light from the visible range. Thus, purple is the complementary color of green. Likewise, atoms or molecules also absorb certain wavelengths from the electromagnetic radiation (these wavelengths are not necessarily to be in the visible range). When a beam of electromagnetic radiation passes through a sample containing gaseous atoms, only some wavelengths are absorbed by the atoms. So when the spectrum is recorded, it consists of a number of very narrow absorption lines. This is known as an atomic spectrum, and it is characteristic to a type of atom. The absorbed energy is used to excite ground electrons to upper levels in the atom. This is known as electronic transition. The energy difference between the two levels is supplied by the photons in the electromagnetic radiation. Since the energy difference is discreet and constant, the same kind of atoms will always absorb the same wavelengths from the given radiation. When molecules are excited with UV, visible and IR radiation, they undergo three different types of transitions as electronic, vibrational and rotational. Because of this, in molecular absorption spectra, absorption bands appear instead of narrow lines.
What is Emission Spectrum?
Atoms, ions, and molecules can be excited to higher energy levels by giving energy. The lifetime of an excited state is generally short. Therefore, these excited species have to release the absorbed energy and come back to the ground state. This is known as relaxation. The release of energy may take place as electromagnetic radiation, heat or as both types. The plot of released energy versus wavelength is known as the emission spectrum. Each element has a unique emission spectrum, as like it has a unique absorption spectrum. So radiation from a source can be characterized by emission spectra. Line spectra occur when the radiating species are individual atomic particles that are well separated in a gas. Band spectra occur due to the radiation of molecules.
What is the difference between Absorption and Emission Spectrums?
• Absorption spectrum gives the wavelengths, which a species would absorb in order to excite to upper states. Emission spectrum gives the wavelengths a species would release when coming back to the ground state from the excited state.
• Absorption spectrum can be recorded upon supplying radiation to the sample whereas emission spectrum can be recorded in the absence of a radiation source.