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Angle-dispersive X-ray diffraction and Fourier transform infrared spectroscopy have been employed to study the phase diagram of urea crystal beyond 15 GPa and at temperatures in excess of 400 K. Previously reported Bridgman phase II was structurally characterized for the first time, and it is discovered that it coincides with room-temperature phase IV. Large metastability P-T regions were identified for all phases in the sequence I-III-IV-V, ascribed to the difficulty to disrupt the H- bonding network, a prerequisite to accomplish the molecular rearrangement necessary for the structural transformation. High-temperature studies and use of a hydrostatic compression medium allows the thermodynamic boundaries of phase III, and partly of phase IV, to be identified therefore making a considerable step forward in the knowledge of the phase diagram of urea. (Graph Presented).

The room-temperature reactivity of liquid methanol induced by two- photon absorption of near UV photons (350 nm) was studied as a function of pressure. Different chemical reactions were triggered by the radical species produced through the population of the lowest electronic excited singlet state because of its dissociative character. Experiments were performed at room temperature between 0.1 and 1.8 GPa on CH3OH and between 0.2 and 1.5 GPa on CD3OH. Different irradiation cycles were performed at constant pressure conditions, and FTIR and Raman spectra were measured to monitor the reaction evolution. Methoxymethanol and methylformate were the main products and the only ones detected in all the experiments. Ethylene glycol formed only at low pressure (0.2-0.3 GPa), whereas small amounts of methane, water, and unsaturated (C=C) species were also detected independently of the reaction pressure. Only dissociation along the O-H and C-O coordinates was relevant in the investigated pressure range. Ethylene glycol, methoxymethanol, and methylformate derive from the dissociation channel involving the O-H bond cleavage, whereas methane and unsaturated species come from the dissociation along the C-O bond. The comparison of the results obtained for the two isotopomers at the different investigated pressures allowed the identification of three different reactive paths that, starting from the methoxy radical, lead to the formation of the main products. The important effect of pressure on the reaction evolution could suggest a modification of the potential energy surface of the lowest electronic excited state along the O-H coordinate on increasing pressure.