Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 1 / A3 / a / iii / Solution

The second vibrational overtone is $v=0\to 3$, with $\Delta v=3$, near three times the fundamental wavenumber. It requires higher-order departures from the linear-dipole harmonic model and is ordinarily weaker again. These are qualitative intensity trends; resonances can redistribute intensity between nearby vibrational states.