The dynamics of melting and crystallization of ices by pump-probe experiments based on ultrafast T-jump

The melting mechanisms on a sub-microscopic scale have been addressed by many experimental and theoretical works covering a sub-nanosecond time window. On the other hand, macroscopic observations of phase transitions, such as the freezing process, take place in a millisecond time window, limited in time by thermal exchange with the environment. The access to the intermediate time interval is difficult to realize both through experimental and theoretical techniques, and it is still completely unexplored. The coverage of this gap is the purpose of this study. In this thesis the characterization of the melting and crystallization dynamics of water ices (ice Ih and VI) and water-based crystalline structures, such as clathrate hydrates of methane and argon, is reported. The melting dynamics of pure water ices Ih and VI have been experimentally investigated within the entire mesoscopic regime from the first nanoseconds after the melting onset to the complete recrystallization of the sample, by combining the Thermal Jump (T-Jump) technique and the time-resolved Mie scattering spectroscopy. The experiment exploits a pump-probe technique realized by combining the T-Jump source (pump), responsible for the increase of the sample temperature and thus the melting, and a continuous-wave laser used as probe. The crystal sample, kept at static temperature and pressure conditions, is homogeneously and instantaneously heated by an energetic picosecond infrared pulse resonant with a vibrational combination band of water (T-Jump). By monitoring the scattering at the melt-ice interfaces inside the ice sample, the dimensional evolution of the molten domains created by the IR pulse in the crystal lattice has been followed. The experiment gives an unexpected result, revealing a bulk melting of the sample which is not instantaneous (up to the microseconds time scale), characterized by a regime of molten domains dimensions ranging from tens of nanometers up to around 1.5 microns. In order to proceed in the dynamics investigation of water-based crystalline structures, such as clathrate hydrates which have more complex structures than pure water, an homogeneous initial sample is a fundamental requirement to investigate their melting dynamics. The development of a synthesis technique for hydrates and a loading procedure in the diamond or sapphire anvil cell has been thus necessary. A working procedure to synthesize clathrate hydrates in a high-pressure apparatus, especially designed for this purpose, has been developed. The characterization by FTIR and Raman spectroscopy of melting and recrystallization of these clathrate hydrates has also been performed in order to investigate the possibility of studying the dynamics of these phase transitions with pump and probe techniques. In order to access the first nanosecond after the pump pulse, the T-Jump technique has been coupled to the transient infrared (TRIR) absorption spectroscopy to monitor the ice (and potentially clathrate) destruction and formation in time with the necessary time resolution by probing the combination band of water related to bending and libration modes. This experiment allows to access the investigation of melting dynamics from 100 ps after the pump pulse, thus filling the gap with the experiment based on the Mie scattering technique. The melted sample portion as a function of time has been directly quantified, revealing the maximum fraction of the probed ice volume that can be melted: it is about 33%, and is reached in around 3 ns. The TRIR experiment, together with the previous method based on Mie scattering, allows to follow the melting dynamics from 100 ps after the pump excitation (T-Jump) to tens of milliseconds, allowing to monitor the entire dynamics from the beginning of melting to recrystallization in bulk water samples, and allowing the experimental characterization of the growth process of the nucleated liquid up to the recrystallization at the thermodynamic conditions of melting.