Quantum Beats and Coherence Decay in Degenerate States Split by Solvation.

Coherent dynamics of degenerate quantum states symmetry-broken on the femtosecond timescale is found to exhibit the phenomenon of quantum beats. Frequency-resolved and polarization-selective heterodyned transient grating spectroscopy enabled us to retrieve the oscillation pattern characteristic of the beating in systems undergoing ultrafast dynamical processes. This methodology applies to the general phenomena of coherence dynamics which is important in any ultrafast multidimensional spectroscopy. A particular application to the vibrational spectroscopy of coherence in the degenerate normal modes of the tricyanomethanide anion solvated in water is explored in this study. The relaxation of the cross-polarization transient grating anisotropy is shown to reflect the loss of the vibrational coherence, which is caused by ultrafast dynamics of the water solvation shell.