Spectroscopic notation

Spectroscopic notation provides various ways to specify atomic ionization states, as well as atomic and molecular orbitals.

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Ionization states

Spectroscopists customarily refer to a given ionization state of a given element by giving the element's symbol followed by a Roman numeral specifying the ionization state. The numeral I is used for the neutral element, II for the first ionization state, III for the second ionization state, and so on.^[1] For example, H II denotes singly ionized hydrogen, H⁺; He I denotes neutral helium; and C IV denotes the third ionization state, C⁺⁺⁺, of carbon.

Atomic and molecular orbitals

Before atomic 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 the azimuthal quantum number, l. The letters, "s", "p", "d", and "f", for the first four values of l were chosen to be the first letters of properties of the spectral series observed in alkali metals. Letters for subsequent values of l were assigned in alphabetical order, omitting the letter "j":^[2][3][4]

letter	name	l
s	sharp	0
p	principal	1
d	diffuse	2
f	fundamental	3
g		4
h		5
...		...

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

Mnemonic

One mnemonic phrase for remembering the sequence S. P. D. F. G. H. ... is "Sober Physicists Don't Find Giraffes Hiding In Kitchens Like Mine". One is expected to remember that after the M, the letters in the sequence are alphabetical.

There are a few other mnemonic phrases for remembering the sequence, for example:

• Some Poor Damn Fool
• Smart People Don't Fail
• Silly People Drive Fast

Another mnemonic is: if you don't memorize the proper order, you will get a SPeeDy F!

Molecular Spectroscopic Notation

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 momentum is the 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 Diatomics, the index g or u denotes the existence of an inversion center.

See also

• principal quantum number
• azimuthal quantum number
• electronic configuration
• term symbol

References