TY - JOUR A1 - Melani, Giacomo A1 - Nagata, Yuki A1 - Wirth, Jonas A1 - Saalfrank, Peter T1 - Vibrational spectroscopy of hydroxylated alpha-Al2O3(0001) surfaces with and without water BT - an ab initio molecular dynamics study JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Using gradient- and dispersion-corrected density functional theory in connection with ab initio molecular dynamics and efficient, parametrized Velocity-Velocity Autocorrelation Function (VVAF) methodology, we study the vibrational spectra (Vibrational Sum Frequency, VSF, and infrared, IR) of hydroxylated alpha-Al2O3(0001) surfaces with and without additional water. Specifically, by considering a naked hydroxylated surface and the same surface with a particularly stable, "ice-like" hexagonal water later allows us to identify and disentangle main spectroscopic bands of OH bonds, their orientation and dynamics, and the role of water adsorption. In particular, we assign spectroscopic signals around 3700 cm(-1) as being dominated by perpendicularly oriented non-hydrogen bonded aluminol groups, with and without additional water. Furthermore, the thin water layer gives spectroscopic signals which are already comparable to previous theoretical and experimental findings for the solid/(bulk) liquid interface, showing that water molecules closest to the surface play a decisive role in the vibrational response of these systems. From a methodological point of view, the effects of temperature, anharmonicity, hydrogen-bonding, and structural dynamics are taken into account and analyzed, allowing us to compare the calculated IR and VSF spectra with the ones based on normal mode analysis and vibrational density of states. The VVAF approach employed in this work appears to be a computationally accurate yet feasible method to address the vibrational fingerprints and dynamical properties of water/metal oxide interfaces. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5023347 SN - 0021-9606 SN - 1089-7690 VL - 149 IS - 1 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Melani, Giacomo A1 - Nagata, Yuki A1 - Campen, Richard Kramer A1 - Saalfrank, Peter T1 - Vibrational spectra of dissociatively adsorbed D2O on Al-terminated alpha-Al2O3(0001) surfaces from ab initio molecular dynamics JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Water can adsorb molecularly or dissociatively onto different sites of metal oxide surfaces. These adsorption sites can be disentangled using surface-sensitive vibrational spectroscopy. Here, we model Vibrational Sum Frequency (VSF) spectra for various forms of dissociated, deuterated water on a reconstructed, Al-terminated α-Al2O3(0001) surface at submonolayer coverages (the so-called 1-2, 1-4, and 1-4′ modes). Using an efficient scheme based on velocity-velocity autocorrelation functions, we go beyond previous normal mode analyses by including anharmonicity, mode coupling, and thermal surface motion in the framework of ab initio molecular dynamics. In this way, we calculate vibrational density of states curves, infrared, and VSF spectra. Comparing computed VSF spectra with measured ones, we find that relative frequencies of resonances are in quite good agreement and linewidths are reasonably well represented, while VSF intensities coincide not well. We argue that intensities are sensitively affected by local interactions and thermal fluctuations, even at such low coverage, while absolute peak positions strongly depend on the choice of the electronic structure method and on the appropriate inclusion of anharmonicity. Y1 - 2019 U6 - https://doi.org/10.1063/1.5099895 SN - 0021-9606 SN - 1089-7690 VL - 150 IS - 24 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Melani, Giacomo A1 - Nagata, Yuki A1 - Saalfrank, Peter T1 - Vibrational energy relaxation of interfacial OH on a water-covered alpha-Al2O3(0001) surface BT - a non-equilibrium ab initio molecular dynamics study JF - Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies N2 - Vibrational relaxation of adsorbates is a sensitive tool to probe energy transfer at gas/solid and liquid/solid interfaces. The most direct way to study relaxation dynamics uses time-resolved spectroscopy. Here we report on a non-equilibrium ab initio molecular dynamics (NE-AIMD) methodology to model vibrational relaxation of OH vibrations on a hydroxylated, water-covered alpha-Al2O3(0001) surface. In our NE-AIMD approach, after exciting selected O-H bonds their coupling to surface phonons and to the water adlayer is analyzed in detail, by following both the energy flow in time, as well as the time-evolution of Vibrational Density of States (VDOS) curves. The latter are obtained from Time-dependent Correlation Functions (TCFs) and serve as prototypical, generic representatives of time-resolved vibrational spectra. As most important results, (i) we find a few-picosecond lifetime of the excited modes and (ii) identify both hydrogen-bonded aluminols and water molecules in the adsorbed water layer as main dissipative channels, while the direct coupling to Al2O3 surface phonons is of minor importance on the timescales of interest. Our NE-AIMD/TCF methodology is powerful for complex adsorbate systems, in principle even reacting ones, and opens a way towards time-resolved vibrational spectroscopy. Y1 - 2021 U6 - https://doi.org/10.1039/d0cp03777j SN - 1463-9076 SN - 1463-9084 VL - 23 IS - 13 SP - 7714 EP - 7723 PB - Royal Society of Chemistry CY - Cambridge ER -