Ultrafast IR spectroscopy of ice-water phase transition

Superheating and bulk melting of neat H2O ice are studied using 2-color IR spectroscopy with sub-picosecond pulses. The data are compared with similar experiments in 15 M HDO in D2O. After calibration of our spectroscopic thermometer a maximum superheating of the solid phase up to 330 ± 10 K is observed after ultrafast temperature jumps. For energy depositions beyond the limit of superheating a strong evidence for partial melting in two steps is observed. The ultrafast thermal melting leads to formation of water clusters in the excited ice lattice and is followed by secondary melting at the generated phase boundaries. The latter process is found to consume energy amounts in agreement with the latent heat of melting and is accompanied by an accelerated temperature and pressure decrease of the residual ice component. The bottleneck of the initial melting process seems to be the thermalization time of the hydrogen-bonded network of 7.0 ± 1 ps.