Communication: Ultrafast vibrational dynamics of hydrogen bond network terminated at the air/water interface: A two-dimensional heterodyne-detected vibrational sum frequency generation study

Abstract

Ultrafast vibrational dynamics of hydrogen bond network at the air/water interface is revealed by two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy. Three diagonal peaks are clearly observed in the 2D HD-VSFG spectrum, which correspond to the negative and positive hydrogen-bonded OH stretch bands and the positive “free” OH stretch band in the steady-state HD-VSFG spectrum. A diagonally elongated bleaching lobe of the hydrogen-bonded OH at 0 ps indicates that it is partly inhomogeneously broadened. This diagonal elongation vanishes in a few hundred femtoseconds, implying the ultrafast spectral diffusion in the hydrogen-bonded OH band. Off-diagonal cross peaks between each OH oscillator are clearly observed instantaneously within the time resolution of 0.2 ps, suggesting that they are vibrationally coupled through anharmonicity and/or energy transfer.

Extreme surface propensity of halide ions in water

“…Given that no ion pairing occurs for the studied salts 35,36 , that the volume taken by a bare Na þ ion is B6/B11 times smaller than that of a bare Cl À /I À ion (ionic radius of sodium: 1.02 Å), and that the sodium ions are hardly present in the probed surface layer 11,15,16,24 , we neglect the effect of the cation in calculating interfacial concentrations. We derive an effective interfacial concentration of chloride of (2.0/1.81) 3 Our findings are of fundamental importance for processes in atmospheric chemistry. Specifically, it has recently been shown that significant levels of atmospheric iodine originate from the interfacial chemistry at the oceans interfaces, and is associated with ozone reduction 5 .…”

“…We measure the vibrational energy relaxation dynamics of the OD groups at the surface of salt solutions using time-resolved IR-pump/SFG-probe spectroscopy 3,[21][22][23] . In this technique, an IR excitation (pump) pulse excites OD groups at a specific vibrational frequency, and the effect of that excitation is followed in time with the SFG probe pair (for details see Methods).…”

“…I n the past years, much progress has been made in the elucidation of the structure and dynamics of water molecules at the pure water-air interface [1][2][3] . However, the vast majority of water on earth is not pure water, but rather contains salts.…”

Solving the “Magic Angle” Challenge in Determining Molecular Orientation Heterogeneity at Interfaces

“…[40][41][42][43][58][59][60][61] Therefore, the orientation analysis presented here can be directly applied to these spectral regimes. In addition, since only the diagonal cuts of HD 2D VSFG are analyzed to obtain the orientation heterogeneity, in principle, the same method can be applied to IR pump-SFG probe experiments 38 when there is no coupling between vibrational modes.…”

“…This new advancement solves the long-standing "magic angle" challenge in 1D VSFG spectroscopy. With the growing popularity of HD 2D VSFG spectroscopy in the surface science community, [39][40][41][42][43][44]47,49,[58][59][60][61][62][63] this new method will contribute significantly in determining the molecular conformations at interfaces.…”