Abstract
Vibrational sum-frequency generation (vSFG) spectroscopy allows the study of the structure and dynamics of interfacial systems. In the present work, we provide a simple recipe, based on a narrowband IR pump and broadband vSFG probe technique, to computationally obtain the two-dimensional vSFG spectrum of water molecules at the air–water interface. Using this technique, to study the time-dependent spectral evolution of hydrogen-bonded and free water molecules, we demonstrate that at the interface, the vibrational spectral dynamics of the free OH bond is faster than that of the bonded OH mode.
Highlights
MethodsWe have conducted a PIMD simulation in the canonical (NVT) ensemble consisting of 125 water molecules using the flexible q-TIP4P/F water model developed by Habershon and c oworkers[37]
We have demonstrated a computationally efficient and inexpensive approach for obtaining the 2D-vSFG spectrum of interfacial water molecules and applied it to study the hydrogen-bond dynamics of free and hydrogen-bonded water molecules
Summary
We have conducted a PIMD simulation in the canonical (NVT) ensemble consisting of 125 water molecules using the flexible q-TIP4P/F water model developed by Habershon and c oworkers[37]. Earlier studies have shown that the flexible q-TIP4P/F model is well suited to include nuclear quantum effects (NQE) within structural, dielectric and dynamical, as well as spectroscopic properties of liquid w ater[37,38,39]. The ring-polymer contraction scheme with a cutoff value of σ = 5 Å was used to reduce the computationally expensive part of electrostatic forces calculation to a single Ewald s um[37,40]. While a p = 32 ring polymer bead was employed, the computationally expensive electrostatic calculations were contracted to the centroid. The effective mass of the ring-polymer beads is adjusted by modifying the elements of the Parrinello–Rahman mass matrix so as to recover the correct dynamics of the centroids and have the integration time-step close to the ionic resonance limit. Interfacial water molecules were obtained using the identification of truly interfacial molecules (ITIM) a lgorithm[44]
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