Atomic Absorption and
Emission Spectra

As we have noted in the section on the Bohr atom, isolated atoms can absorb and emit packets of electromagnetic radiation having discrete energies dictated by the detailed atomic structure of the atoms. When the corresponding light is passed through a prism or spectrograph it is separated spatially according to wavelength, as illustrated in the following image.

Separation of light by a prism according to wavelength

Continuum, Emission, and Absorption Spectra

The corresponding spectrum may exhibit a continuum, or may have superposed on the continuum bright lines (an emission spectrum) or dark lines (an absorption spectrum), as illustrated in the following figure.

Continuous, emission, and absorption spectra

Origin of Continuum, Emission, and Absorption Spectra

The origins of these three types of spectra are illustrated in the following figure.

Sources of continuous, emission, and absorption spectra

Kirchhoff's Laws

Thus, emission spectra are produced by thin gases in which the atoms do not experience many collisions (because of the low density). The emission lines correspond to photons of discrete energies that are emitted when excited atomic states in the gas make transitions back to lower-lying levels.

A continuum spectrum results when the gas pressures are higher, so that lines are broadened by collisions between the atoms until they are smeared into a continuum. We may view a continuum spectrum as an emission spectrum in which the lines overlap with each other and can no longer be distinguished as individual emission lines. BLACKBODY IS AN EXAMPLE OF CONTINUUM EMISSION (energy at all frequencies).

An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum.

Hydrogen Emission and Absorption Series

Hydrogen emission series
The spectrum of hydrogen is particularly important in astronomy because most of the Universe is made of hydrogen. Emission or absorption processes in hydrogen give rise to series, which are sequences of lines corresponding to atomic transitions, each ending or beginning with the same atomic state in hydrogen. Thus, for example, the Balmer Series involves transitions starting (for absorption) or ending (for emission) with the first excited state of hydrogen, while the Lyman Series involves transitions that start or end with the ground state of hydrogen; the adjacent image illustrates the atomic transitions that produce these two series in emission.

Because of the details of hydrogen's atomic structure, the Balmer Series is in the visible spectrum and the Lyman Series is in the the UV. The following image illustrates some of the transitions in the Balmer series.

The Balmer spectrum of hydrogen

The Balmer lines are designated by H with a greek subscript in order of decreasing wavelength. Thus the longest wavelength Balmer transition is designated H with a subscript alpha, the second longest H with a subscript beta, and so on.

Molecular Spectra

In addition to spectra associated with atoms and ions, molecules can interact with electromagnetic radiation and give rise to characteristic spectra. Because of basic atomic and molecular structure, the spectra associated with molecules typically involve infrared wavelengths. In addition, because molecules are usually fragile, molecular spectra are important mostly in objects that are relatively cool such as planetary atmospheres, the surfaces of very cool stars, and various interstellar regions.