Speaker
Description
Understanding water interactions with metal oxide surfaces [1] (such as aluminum oxide, Al$_\mathrm{2}$O$_\mathrm{3}$) is of importance both for fundamental reasons and for technological applications.
To unravel the details of such interactions, spectroscopic methods in the form of vibrational (IR) spectroscopy or, as surface-sensitive tool, Vibrational Sum Frequency (VSF) generation spectroscopy [2,3] are powerful experimental techniques which have been applied with great success.
However, the interpretation of vibrational spectra is typically hard and requires theoretical support. On the simplest level of theory, simple Normal Mode Analysis (NMA) is performed in order to do so.
This approach lacks inclusion of anharmonicities, thermal motion, and spectroscopic selection rules, however, which can be decisive. These features are accessible, in principle, by (classical) correlation function approaches [4] which can be evaluated by Ab Initio Molecular Dynamics (AIMD).
Here we apply recently proposed, efficient methods based on velocity-velocity autocorrelation functions (VVACFs) [5] to compute vibrational spectra (IR and VSF) of hydroxylated α-Al$_\mathrm{2}$O$_\mathrm{3}$(0001) surfaces with and without additional water [6]. We compare the validity of NMA and of Vibrational Density of States (VDOS) curves to predict / interprete IR and VSF spectra at finite temperature which we determine by VVACF-based AIMD. Further, a detailed assignment of vibrational signals is given, with special emphasis on the key role played by surface OH bonds, their dynamical behaviour and the effects brought in by H$_\mathrm{2}$O adsorption. When possible, a connection to recent experiments [7,8] is made.
Further theoretical efforts are spent to address the vibrational relaxation and lifetimes of surface OH-species, whose dynamics is followed by pump-probe VSF-measurements. We again employ AIMD simulations to investigate the structural properties of hydroxylated α-alumina surface which can influence the energy pathways of excited OH-bonds. We then propose lifetimes in qualitative agreement with time-resolved experiments [9].
References
[1] Olle Bjørneholm et al, Chem. Rev., 116(13):7698, 2016.
[2] Fivos Perakis et al, Chem. Rev., 116(13):7590, 2016.
[3] Paul A. Covert et al, Ann. Rev. Phys. Chem., 67(1):233, 2016.
[4] B. J. Berne et al, Adv. Chem. Phys., 27:64, 1970.
[5] Tatsuhiko Ohto et al, J. Chem. Phys., 143(12):124702, 2015.
[6] Giacomo Melani et al, submitted, 2018.
[7] Luning Zhang et al, J. Am. Chem. Soc., 130(24):7686, 2008.
[8] Yujin Tong et al, J. Chem. Phys., 114(12):054704, 2015.
[9] Aashis Tuladhar et al, J. Phys. Chem. C, 121:5168, 2017.
