Excess Hydrogen Bond at the Ice-Vapor Interface around 200 K

Phase-resolved sum-frequency generation measurements combined with molecular dynamics simulations are employed to study the effect of temperature on the molecular arrangement of water on the basal face of ice. The topmost monolayer, interrogated through its nonhydrogen-bonded, free O-H stretch peak, exhibits a maximum in surface H-bond density around 200 K. This maximum results from two competing effects: above 200 K, thermal fluctuations cause the breaking of H bonds; below 200 K, the formation of bulklike crystalline interfacial structures leads to H-bond breaking. Knowledge of the surface structure of ice is critical for understanding reactions occurring on ice surfaces and ice nucleation.

Figure 1

(a) Schematic of the $DAA$, $DDA$, and $DDAA$ water molecules at the basal ice-air interface with its perfect form. (b) Simulated SFG spectra in the free O-H region at 200 K with various cross-correlation cutoffs $r_t$ along with the experimental result.

Figure 2

Temperature dependence of (a) the experimentally measured free O-H stretch $\mathrm{Im}{\chi }^{(2)}$ features; (b) the simulated $\mathrm{Im}{\chi }^{(2)}$ features with $r_t=0\AA$. The amplitudes of the peaks in the simulated spectra (b) are set to equal those in the experimental data (a). All spectra are offset by increments of 0.1 for clarity. (c) The free O-H stretch peak frequency vs temperature. The lines are to guide the eye. Error bars represent the 95% confidence intervals.

Figure 3

(a) Simulated fraction of different interfacial water species ($DAA$, $DA$, $AA$, $DDA$, and $DDAA$ water molecules) and total number of water molecules with free O-H groups ($DAA+DA+AA$) at the topmost monolayer of the ice surface. Error bars are smaller than the size of the symbols. (b) Variation of the intramolecular H-O-H angle of the DAA water molecules. The inset exhibits the H-O-H angle vs. temperature. Error bars represent 95% confidence intervals. (c) Temperature variation of the peak area of the experimentally measured free O-H SFG responses and the simulated fractions of water molecules with free O-H groups at the surface. (d) Schematics of the freezing process of ice for the basal surface of ice. This side view of the ice surface reveals that, with increasing temperature, the crystalline hexagonal structure present at 150 K starts to melt around 200 K; around this temperature the number of H-bonds is maximal, and the number of free O-H groups minimal.