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 SpectraThe 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 SpectraThe origins of these three types of spectra are illustrated in the following figure.
|Sources of continuous, emission, and absorption spectra|
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.
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|
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.