Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy

Microscopic structure and dynamics of air/water interface by computer simulations—comparison with sum-frequency generation experiments

“…It is thereby free, and protrudes out of the water phase, confirming earlier predictions from computations. 11 Roughly the same picture of the existence of the free OH bonds at the interface has since been consistently observed in SFG experiments by Richmond, 5 Eisenthal, 12 Allen, 13 and Gan et al, 14 and also in computer simulations. 3,[15][16][17][18][19] The structure of water at the hydrophobic organic/water interfaces was shown to be similar to that at the air/water interface.…”

Absolute Orientation of Molecules at Interfaces

“…Initially, we set the molecular plane in the ZY plane of the laboratory coordinates, the symmetry axis of the -CN is along both the z molecular axis and the Z laboratory axis (negative), and, finally, the X and x axes are perpendicular to the molecular plane, as shown in Figure 8A. Thus, in the laboratory coordinates, the initial coordinates of the methyl group C 3 axis expressed in terms of the direction cosines with the laboratory axes (l CH 3 …”

“…As the first step, the molecular coordinates are rotated by an angle of θ 1 ) 53°around the X axis such that the orientational angle of the nitrile group with respect to the surface normal is 53°as obtained from null angle measurements ( Figure 8B). This rotation operation R X (θ 1 ) changes the spatial coordinates of the methyl group C 3 axis from its initial orientation (0, cos 30°, cos 60°) to the orientation (l CH 3 The final molecular configuration after the second rotation around the molecular z axis, bringing the C3 axis of the CH3 group to its measured orientation of 23°with respect to the interface normal Z. The normal to the molecular plane, given by the molecular coordinate x, is found to be 70°with respect to the surface normal.…”

“…[1][2][3] Because the molecular interface is only a few molecules thick, interfacial populations are very small. As a consequence, a useful optical technique must be interface specific and sensitive to composition, orientation, and structure at the interface.…”

“…2,3,[5][6][7][8] Orientation measurements of chemical groups at interfaces and surfaces were realized soon after the SFG technique was applied to the study of interfaces and surfaces. 9 Recently, the orientational angle with respect to the surface normal and the twist angle relative to the plane comprising the surface normal and the symmetry axis of the isopropyl headgroup of a leucine molecule at the air/water interface was measured.…”