Hot band

In molecular vibrational spectroscopy, a hot band is a band centred on a hot transition, which is a transition between two excited vibrational states, i.e. neither is the overall ground state.^[1] In infrared or Raman spectroscopy, hot bands refer to those transitions for a particular vibrational mode which arise from a state containing thermal population of another vibrational mode.^[2] For example, for a molecule with 3 normal modes, ${\displaystyle \nu _{1}}$, ${\displaystyle \nu _{2}}$ and ${\displaystyle \nu _{3}}$, the transition ${\displaystyle 101}$ ← ${\displaystyle 001}$, would be a hot band, since the initial state has one quantum of excitation in the ${\displaystyle \nu _{3}}$ mode. Hot bands are distinct from combination bands, which involve simultaneous excitation of multiple normal modes with a single photon, and overtones, which are transitions that involve changing the vibrational quantum number for a normal mode by more than 1.