Surface structure of water from soft X-ray second harmonic generation

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Studying the hydrogen bond (H-bond) network at the water surface is vital for natural and industrial processes.
• Existing experimental techniques struggle with the interfacial sensitivity required for direct H-bond network analysis.

Purpose of the Study:

• To investigate the electronic structure of the liquid water/vapor interface using a novel spectroscopic method.
• To understand the role of hydrogen bonding in the unique properties of water's surface.

Main Methods:

• Employed soft X-ray second harmonic generation (SXSHG) spectroscopy.
• Utilized flat liquid sheet microjets and attosecond soft X-ray pulses from the LCLS X-ray free electron laser.
• Performed first-principles electronic structure calculations.

Main Results:

• The SXSHG spectrum of the water surface differs significantly from bulk water's X-ray absorption spectrum.
• A distinct electronic environment at the interface was identified, with the SXSHG peak shifted by several eV compared to the bulk.
• Calculations demonstrated the sensitivity of SXSHG to H-bond configurations, particularly single H-bond acceptors prevalent at the surface.

Conclusions:

• SXSHG spectroscopy provides elemental and surface selectivity, accessing the electronic structure of interfacial water.
• The water-vapor interface possesses a unique electronic structure governed by specific hydrogen bond arrangements.
• This technique offers new insights into the microscopic structure and dynamics of liquid interfaces.