Spectroscopic notation provides a way to specify atomicionization states,atomic orbitals, andmolecular orbitals.

Spectroscopists customarily refer to the spectrum arising from a given ionization state of a givenelement by the element's symbol followed by aRoman numeral. The numeral I is used for spectral lines associated with the neutral element, II for those from the first ionization state, III for those from the second ionization state, and so on.^[1] For example, "He I" denotes lines of neutralhelium, and "C IV" denotes lines arising from the third ionization state, C³⁺, ofcarbon. This notation is used for example to retrieve data from theNIST Atomic Spectrum Database.

Beforeatomic orbitals were understood, spectroscopists discovered various distinctive series of spectral lines in atomic spectra, which they identified by letters. These letters were later associated with theazimuthal quantum number,ℓ. The letters, "s", "p", "d", and "f", for the first four values ofℓ were chosen to be the first letters of properties of the spectral series observed inalkali metals. Other letters for subsequent values ofℓ were assigned in alphabetical order, omitting the letter "j"^[2][3][4] because some languages do not distinguish between the letters "i" and "j":^[5][6]

letter	name	ℓ
s	sharp	0
p	principal	1
d	diffuse	2
f	fundamental	3
g		4
h		5
i		6
k		7
l		8
m		9
n		10
o		11
q		12
r		13
t		14
u		15
v		16
...		...

This notation is used to specifyelectron configurations and to create theterm symbol for theelectron states in a multi-electron atom. When writing a term symbol, the above scheme for a single electron'sorbital quantum number is applied to the total orbital angular momentum associated to an electron state.^[4]

The spectroscopic notation of molecules uses Greek letters to represent the modulus of the orbital angular momentum along the internuclear axis.The quantum number that represents this angular momentum is Λ.

Λ = 0, 1, 2, 3, ...

Symbols: Σ, Π, Δ, Φ

For Σ states, one denotes if there is a reflection in a plane containing the nuclei (symmetric), using the + above. The − is used to indicate that there is not.

For homonuclear diatomic molecules, the index g or u denotes the existence of acenter of symmetry (or inversion center) and indicates the symmetry of thevibronic wave function with respect to the point-group inversion operationi. Vibronic states that are symmetric with respect toi are denoted g forgerade (German for "even"), and unsymmetric states are denoted u forungerade (German for "odd").

Formesons whose constituents are a heavyquark and its own antiquark (quarkonium) the same notation applies as for atomic states. However, uppercase letters are used.

Furthermore, the first number is (as in nuclear physics)${\displaystyle n=N+1}$ where${\displaystyle N}$ is the number of nodes in the radial wave function, while in atomic physics${\displaystyle n=N+\ell +1}$ is used. Hence, a 1P state in quarkonium corresponds to a 2p state in an atom orpositronium.

• Atomic physics
• Spectroscopy

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