Band Systems, Bands, and Branches

A rotation-vibration-electronic spectrum is a collection of transitions between an upper state, always denoted by a single prime, with energy 

An electronic band system will consist of (v, v ) vibrational bands, and each vibrational band will contain many rotational lines, which may be organized into rotational branches, one branch for each value of A J — J — J . Whenever a spectroscopist wants to indicate a change in anything (energy, a molecular constant, or a quantum number) using the symbol A, it is always implied that the lower state quantity ( ) is subtracted from the upper state quantity ( ), for example, 

It is conventional, in both spoken and written spectroscopy-ese , always to say or write the quantum numbers of the upper state first (Mulliken, 1930 Jenkins, 1953 Mulliken, 1955 Sheppard, et al, 1987 Schutte, et al, 1997). 

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