10 Atomic Absorption and Emission

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. fluorescence). Similar phenomena occur with individual atoms for transitions between atomic orbitals. Such absorption and emission are useful for elemental analysis.

Spectroscopy

The electronic transitions in atoms correspond to transitions of valence electrons between s, p, d, and f orbitals (as applicable for the element). For example, sodium absorbs and emits yellow light that corresponds to 3s→3p and 3p→3s transitions, respectively. Lithium absorbs and emits red light that corresponds to 2s→2p and 2p→2s transitions. Most atoms, including sodium and lithium, have multiple absorption and emission lines that correspond to various transitions between orbitals.

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. band spectra for molecules) with widths less than 0.01 nm (cf. 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. Heisenberg uncertainty principle, Doppler broadening, pressure broadening, effects of electric and magnetic fields).

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) is variable in its efficiency between samples. Like molecules, atoms also have selection rules for allowed transitions.

Atomization

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.

For atomic absorption spectroscopy (AAS), a ground-state atomic population is desired. The amount of heat (i.e. 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), 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. The Saha equation is an analog of the Boltzmann distribution that describes the ionization as a function of temperature.

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.

Connections

• AAS and AES are atomic spectroscopies that mainly utilize UV-visible light, similar to the molecular spectroscopies discussed in Ch. 6 (absorption) and Ch. 8 (fluorescence).
• As with the molecular spectroscopies (Ch. 6, Ch. 8), atomic absorption and emission are associated with electronic transitions.

Post-Reading Questions

1. Is atomic spectroscopy useful for identifying and quantifying molecules or elements?
2. Why do atomic spectra feature lines instead of bands?
3. Sodium has absorption lines at approximately 589.0 nm and 589.6 nm. At what wavelengths are the corresponding emission lines?
4. What happens during atomization and why is atomization necessary?
5. What distribution determines the ratio of ground-state and excited-state atoms at a given temperature?

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Be able to draw diagrams illustrating atomic absorption and emission.
• Sketch atomic spectra and explain the origin of the line widths.
• Explain and quantify the role of heat in determining ground-state and excited-state atomic populations.
• Explain why AAS and AES are more easily able to measure multiple analytes in one sample than molecular absorption or fluorescence spectroscopy.