1. Introduction
This article outlines the main concepts of atomic structure, with
some emphasis on terminology and notation. Atomic radiation is
discussed, in particular the wavelengths, intensities, and shapes of
spectral lines, and a few remarks are made regarding continuous
spectra. We include updated tabulations of ionization energies for the neutral
atoms and transition probabilities for persistent lines of selected neutral
atoms. Some sources of additional atomic spectroscopic data are
mentioned.
Experimental techniques and the details of atomic theoretical methods are not
covered in this article; these and a number of other subjects pertinent to
atomic spectroscopy are treated in one or more of at least fifteen chapters in
the volume Atomic, Molecular, and Optical Physics Handbook,
G.W.F. Drake, Ed. (AIP Press, Woodbury, NY, 1996) and in references below.
2. Frequency, Wavenumber, Wavelength
The photon energy due to an electron transition between an upper atomic level
k (of energy E_{k}) and a lower level i
is
ΔE = E_{k}  E_{i} = hν
= hcσ = hc/λ_{vac} ,

(1)

where ν is the frequency, σ the wavenumber in vacuum, and
λ_{vac} the wavelength in
vacuum. The most accurate spectroscopic measurements are determinations of
transition frequencies, the unit being the Hertz
(1 Hz = cycle/sec) or one of its multiples. A measurement of any
one of the entities frequency, wavenumber, or wavelength (in vacuum) is an
equally accurate determination of the others since the speed of light is
exactly defined. The most common wavelength units are the nanometer (nm), the
Ångström (1 = 10^{1} nm) and the
micrometer (µm). The SI wavenumber unit is the inverse meter, but in
practice wavenumbers are usually expressed in inverse centimeters:
1 cm^{1} = 10^{2} m^{1},
equivalent to 2.997 924 58 × 10^{4} MHz.
In addition to frequency and wavenumber units, atomic energies are often
expressed in electron volts (eV). One eV is the energy associated with each of
the following quantities:
2.417 989 40(21) × 10^{14} Hz
8 065.544 45(69) cm^{1}
1 239.841 91(11) nm^{ }
11 604.505(20) K (kelvin)^{ }
1.602 176 53(14) × 10^{19} J (joule)

We note that the basic unit of temperature, the kelvin, is equivalent to about
0.7 cm^{1}, i.e., the value of the Boltzmann constant k
expressed in wavenumber units per kelvin is
0.695 035 6(12) cm^{1}/K. One reason for citing this
particular equivalency involving the internationally accepted symbol for the
kelvin (K) [1] is to suggest
that use of the letter K as a symbol for 1 cm^{1}
(1 kayser) should be discontinued.
The unit of energy in the system of atomic units (a.u.) often used for
theoretical calculations is the hartree, which is equal to 2 rydbergs. The
rydberg for an atom having nuclear mass M is
1 Ry = R_{M} = M(M + m_{e})^{1}
R_{∞} ,

(2)

with
R_{∞} =
m_{e}cα^{2}/(2h) =
10 973 731.568 525(73) m^{1} ,

(3)

which is equivalent to 13.605 692 3(12) eV. The Rydberg constant
R_{∞} is thus the limit value for infinite nuclear mass.