Echo-peak shift fails to resolve the liquid–glass phase transition

The echo-peak shift (EPS) technique, frequently used to study ultrafast solvation dynamics, can be remarkably insensitive to changes in systems’ dynamical properties. This is illustrated by comparing the results of EPS and time-resolved photon echo experiments on temperature-dependent optical dephasing in a glass-forming liquid. The observed failure of the EPS technique appears to be a fundamental problem for systems with strong inhomogeneous contribution to the line broadening.