{"id":78,"date":"2023-02-23T01:21:38","date_gmt":"2023-02-23T06:21:38","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/?post_type=chapter&p=78"},"modified":"2025-01-30T15:07:51","modified_gmt":"2025-01-30T20:07:51","slug":"atomic-absorption-and-emission","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/chapter\/atomic-absorption-and-emission\/","title":{"raw":"Atomic Absorption and Emission","rendered":"Atomic Absorption and Emission"},"content":{"raw":" \r\n

Recall that molecules absorb UV-visible light to undergo a HOMO-to-LUMO electronic transition, and may then emit light to relax back to their ground electronic state (e.g.<\/em> fluorescence). Similar phenomena occur with individual atoms for transitions between atomic orbitals. Such absorption and emission are useful for elemental analysis.<\/p>\r\n\r\n

Spectroscopy<\/h2>\r\n

The electronic transitions in atoms correspond to transitions of valence electrons between s<\/em>, p<\/em>, d<\/em>, and f<\/em> orbitals (as applicable for the element). For example, sodium absorbs and emits yellow light that corresponds to 3s\u21923p and 3p\u21923s transitions, respectively. Lithium absorbs and emits red light that corresponds to 2s\u21922p and 2p\u21922s transitions. Most atoms, including sodium and lithium, have multiple absorption and emission lines that correspond to various transitions between orbitals.<\/p>\r\n \r\n\r\n\"Atomic\r\n

Unlike molecules, atoms do not have vibrational and rotational states. The transitions are thus purely electronic. Atoms are also found only in the gas phase, undergoing far fewer collisions than molecules in solution. Consequently, atomic spectra are line spectra (cf.<\/em> band spectra for molecules) with widths less than 0.01 nm (cf. <\/em>more than 10 nm for molecular bands), and the absorption and emission lines are at the same wavelength. Atomic spectra have multiple lines across the UV-visible-NIR region. Multiple sources of broadening contribute to the finite width of the spectral lines (e.g.\u00a0<\/em>Heisenberg uncertainty principle, Doppler broadening, pressure broadening, effects of electric and magnetic fields).<\/p>\r\n

In theory, the Beer-Lambert Law is valid for atomic absorption and calibration plots are ideally linear; however, absorption coefficients for elements are not used in practice because the atomization process (vide infra<\/em>) is variable in its efficiency between samples.\u00a0Like molecules, atoms also have selection rules for allowed transitions.<\/p>\r\n\r\n

Atomization<\/h2>\r\n

A significant practical difference between molecular spectroscopy and atomic spectroscopy is that molecular samples are easy to find, whereas special effort is usually required to produce an atomic sample. Energy must be applied to convert a molecular or other condensed-phase sample into an atomic population. This energy is applied in the form of heat.<\/p>\r\n

For atomic absorption spectroscopy (AAS)<\/strong>, a ground-state atomic population is desired. The amount of heat (i.e.\u00a0<\/em>energy) applied must be sufficient for atomization, but not so much as to produce a significant population of excited-state atoms. For atomic emission spectroscopy (AES)<\/strong>, the amount of heat applied must be sufficient for atomization and electronic excitation, but not for ionization. The ratio of ground-state and excited-state atoms is determined by the temperature, according to the Boltzmann distribution<\/em>. The Saha equation<\/em> is an analog of the Boltzmann distribution that describes the ionization as a function of temperature.<\/p>\r\n

As an analytical method, atomic absorption and emission provide information about the elemental composition of a sample. There is no bonding in the measured atomic population, so the methods do not provide molecular information.<\/p>\r\n \r\n\r\n

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Connections<\/h3>\r\n