Where does overtone intensity come from?

The factors that influence overtone intensity are examined using realistic potential and dipole moment functions that model the acetylenic stretching vibration. It is found that mechanical anharmonicity is more important than electrical anharmonicity, especially at higher overtone levels but a quantitative treatment requires both contributions which are not simply additive. It is found numerically that the inner wall of the potential dominates in determining overtone intensity. This can be justified by heuristic arguments and by the properties of the sensitivity functional of the transition dipole matrix elements to changes in V(r). It is suggested that absolute overtone intensities could be useful in determining the inner walls of hydrogen stretching vibrations.