https://doi.org/10.1351/goldbook.08588
Combination transition that does not start at the ground state and in which at least one vibration decreases and at least one vibration increases its degree of excitation; i.e., a transition in which more than one vibrational quantum number changes and at least one becomes smaller.
Note:
A difference band or difference tone results from a difference transition.
A difference band or difference tone results from a difference transition.