The fifth-order contribution to the oscillations in photon echoes of anharmonic vibrators

Vibrational photon echo experiments on the CO stretching mode of carbon monoxide bound to hemoglobin revealed pronounced oscillations with a beat frequency which corresponds to the anharmonicity of the vibrator. Intensity-dependent measurements show that the oscillatory signal is due to interference between the third- and fifth-order polarization. Such oscillations cannot occur in the third-order signal unless the inhomogeneous width exceeds the anharmonicity. An expression is derived which simultaneously describes the intensity dependence, the phase, and the damping of such oscillatory signals.