Identification of a wagging vibrational mode of water molecules at the water/vapor interface

Abstract

An improved time correlation function description of sum frequency generation (SFG) spectroscopy was applied to theoretically describe the water/vapor interface. The resulting spectra compare favorably in shape and relative magnitude to extant experimental results in the O-H stretching region of water. Further, the SFG spectra show a well-defined intermolecular mode at 875 cm(-1) that has significant intensity. The resonance is due to a wagging mode localized on a single water molecule. It represents a well-defined population of water molecules at the interface that, along with the free O-H modes, represent the dominant interfacial species.

Vibrational Line Shapes, Spectral Diffusion, and Hydrogen Bonding in Liquid Water

“…Terrific and informative vibrational sum-frequency generation experiments [177][178][179][180][181][182][183][184][185][186] on this problem have been performed over the last 10 or 15 years, leading to a good understanding of how dynamics and hydrogen bonding differ at this interface from in the bulk [187][188][189][190][191][192]. A significant amount of theoretical work has been completed on this system as well [71,110,140,[193][194][195][196][197][198][199][200][201]. Very recently, ultrafast surface-selective vibrational spectroscopy experiments have also appeared [202][203][204], and are in need of a good theoretical description.…”

Theoretical Modeling of Interface Specific Vibrational Spectroscopy: Methods and Applications to Aqueous Interfaces

“…Intermediate polarization conditions refer to light with a polarization vector rotated somewhere between the S and the P polarization. This method is effective eff,SSP (2) ) sin( IR ) L yy (ω SFG ) L yy (ω vis ) L zz (ω IR ) yyz (16) eff,SPS (2) ) sin( vis ) L yy (ω SFG ) L zz (ω vis ) L yy (ω IR ) yzy (17) eff,PSS (2) ) sin( SFG ) L zz (ω SFG ) L yy (ω vis ) L yy (ω IR ) zyy (18) eff,PPP (2) ) -cos( SFG ) cos( vis ) sin…”

“…As pointed out by Morita and Hynes, 5 orientational information can also be deduced from the relative signs of the imaginary mode line shapes given knowledge of the signs of the prefactors in eqs 47 and 48 (the dipole and polarizability derivatives). Figure 15 presents the real and imaginary parts of the susceptibility in the O-H stretching region, from approximately 3000 to 3600 cm -1 calculated by Perry et al 3 Careful examination of the spectrum reveals three separate modes in this region centered at 3195, 3325, and 3400 cm -1 . Remarkably, this agrees very well with previous experimental work that deconvoluted the spectrum in this region.…”

“…To calculate or, equivalently, the system response function, R (2) , it is necessary to develop a microscopic description of it. Furthermore, it is desirable to represent the response function in a form amenable to calculation and one that can exploit the power of MD interfacial simulations.…”

“…We use eff (2) to denote the effective susceptibilitysunlike ijk (2) , eff (2) explicitly accounts for the Fresnel factors. The S and P indices on eff,R γ (2) denote how the fields ω SFG , ω vis , and ω IR , respectively, are polarized, i.e., S or P. In the above expressions, because of the chosen experimental geometry, three of the polarization conditions (SSP, SPS, and PSS) directly probe single susceptibility tensor components while the PPP condition has components of all unique allowed Cartesian tensor elements.…”