Specialized bacteria trigger ice formation by controlling the molecular structure and energy transfer in interfacial water.

Time-resolved photoelectron imaging was used to investigate the dynamical evolution of the initially prepared S(1) (ππ*) excited state of phenol (hydroxybenzene), catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene), and hydroquinone (1,4-dihydroxybenzene) following excitation at 267 nm. Our analysis was supported by ab initio calculations at the coupled-cluster and CASSCF levels of theory. In all cases, we observe rapid (<1 ps) intramolecular vibrational redistribution on the S(1) potential surface. In catechol, the overall S(1) state lifetime was observed to be 12.1 ps, which is 1-2 orders of magnitude shorter than in the other three molecules studied. This may be attributed to differences in the H atom tunnelling rate under the barrier formed by a conical intersection between the S(1) state and the close lying S(2) (πσ*) state, which is dissociative along the O-H stretching coordinate. Further evidence of this S(1)/S(2) interaction is also seen in the time-dependent anisotropy of the photoelectron angular distributions we have observed. Our data analysis was assisted by a matrix inversion method for processing photoelectron images that is significantly faster than most other previously reported approaches and is extremely quick and easy to implement.

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright 1 L a and 1 L b electronic states of 1 ππ* character and spectroscopically dark and dissociative 1 πσ* states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited 1 L a state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived 1 L b state or the 1 πσ* state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the 1 πσ* state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.

The interfacial structure of water in contact with TiO2 is the key to understand the mechanism of photocatalytic water dissociation as well as photoinduced superhydrophilicity. We investigate the interfacial molecular structure of water at the surface of anatase TiO2, using phase-sensitive sum frequency generation spectroscopy together with spectra simulation using ab initio molecular dynamic trajectories. We identify two oppositely oriented, weakly and strongly hydrogen-bonded subensembles of O–H groups at the superhydrophilic UV irradiated TiO2 surface. The water molecules with weakly hydrogen-bonded O–H groups are chemisorbed, i.e. form hydroxyl groups, at the TiO2 surface with their hydrogen atoms pointing toward bulk water. The strongly hydrogen-bonded O–H groups interact with the oxygen atom of the chemisorbed water. Their hydrogen atoms point toward the TiO2. This strong interaction between physisorbed and chemisorbed water molecules causes superhydrophilicity.

The surfactant sodium dodecyl sulfate (SDS) is widely used as a detergent for both domestic and industrial applications. It forms a self-assembled monolayer on the surface of water. We report a microscopic model for the interaction between the surfactant and water and between water molecules at the interface, revealed using static and time-resolved two-dimensional sum frequency generation spectroscopy. Two distinct subensembles of water in the presence of this negatively charged SDS surfactant have been identified: those close to the SDS headgroup having fairly isolated O-H groups, i.e., localized O-H stretch vibrations, and those whose O-H stretch vibrations are delocalized, i.e., shared between multiple O-H bonds. The two subensembles are coupled, with subpicosecond energy transfer occurring between them. This is markedly different from O-H bonds at the air-water interface, which are less heterogeneous, and indicates that the water molecules that interact with the surfactant headgroups have hydrogen-bonding properties different from those of water molecules interacting with the other water molecules.