Cross-revival of molecular wave packets

We predict that vibration-rotation coupling in diatomic molecules will lead to cross-revivals of vibrational wave packets---provided lifetime effects and collisions can be eliminated. Analytical conditions for full cross-revivals of systems governed by the minimal Hamiltonian exhibiting rotation-vibration coupling are found and the influence of higher-order cross terms is studied. Finally, generality of the phenomenon of molecular cross-revival is investigated by doing numerical calculations for several molecular potentials and temperatures. For expansion-cooled molecules strong cross-revivals are obtained in all cases. In the particular example of cold Li₂ in the A¹Σᵤ⁺ state nearly complete cross-revivals are observed. These cross-revivals survive thermal averaging and reach at an initial temperature of 900 K an amplitude which is close to half the beat amplitude at zero time delay. The ``rotational cooling'' mechanism facilitating this, by reducing the effective rotational temperature of the wave packet, is explained in terms of the molecular rotational constants and laser wavelength bandwidth.