Water molecules as efficient infrared antennae
Water molecules on the surface of a water mass exchange energy with each other extremely rapidly. Researchers from the FOM Institute AMOLF observed this with a new spectroscopic technique they have developed. Using this technique they could follow the transfer of vibrational energy between water molecules on the surface in real-time at the very short timescale of femtoseconds (10-15 s). This result is important for a better understanding of the physical and chemical processes on water surfaces, such as cell membranes, atmospheric water droplets and surfaces in electrochemical cells. The researchers published their results online Nature Chemistry on 2 October 2011.
Water surfaces
Water is a unique liquid that is characterised by the strong interaction between the molecules – the so-called hydrogen bridges. One unusual characteristic of water is the ability of its molecules to rapidly exchange energy in the form of vibrations. This energy exchange is particularly relevant for chemical processes in water. However, nothing is known about the energy exchange between water molecules on surfaces, where processes vital for membrane biology, electrochemistry and atmospheric chemistry occur.
New technique
Using a new two-dimensional spectroscopic technique, the researchers managed to measure the energy exchange on the water surface. They transferred energy to water molecules by means of an ultrashort infrared light pulse of 100 femtoseconds. A water molecule consists of an oxygen atom (O), bonded to 2 hydrogen atoms (H). Such a bond can vibrate due to the energy from the infrared pulse. The researchers can excite vibrations of specific water molecules, for example only those with strong hydrogen bridges, by very precisely changing the frequency of the infrared pulse.
This vibration of a water molecule contains lots of energy and can 'hop' from one water molecule to another. Consequently water molecules on the surface with weak hydrogen bonds also becoming excited over the course of time. The researchers measured how quickly this occurred on the surface by varying the delay between the first excitation pulse and the two detecting pulses.
Antennae
The energy transfer in the upper layer was found to take place over a timescale of several hundred femtoseconds (fs, 1 fs=10-15 s). This was slower than in the rest of the water because water molecules on the surface can only transfer energy downwards into the water body. The researchers also observed that the outermost water molecules with a single OH group pointing freely into the air were highly efficient at capturing energy and conducting this into the water (see Figure 1). If a free OH group is caused to vibrate by the correct infrared wavelength it quickly converts the energy into a vibration of the other OH group in the same molecule, which can then rapidly transfer the energy to other water molecules. Such free OH groups therefore serve as highly efficient antennae for infrared light.
The water surface can therefore act as an extremely efficient absorber and distributor of energy. This property is important for processes that occur at water surfaces and release large quantities of energy locally. Such processes include the functioning of active membrane proteins and chemical reactions catalysed by water surfaces, for example on water drops and ice particles in the atmosphere.
Contact
Mischa Bonn (workgroup leader AMOLF)
Reference
Ultrafast vibrational energy transfer at the water-air interface revealed by two-dimensional surface vibrational spectroscopy
Zhen Zhang 1, Lukasz Piatkowski1, Huib J. Bakker1 and Mischa Bonn1,2, Nature Chemistry, 2011.
1 FOM Institute AMOLF.
2 Max Planck Institute for Polymer Research, Mainz, Germany.
DOI: 10.1038/NCHEM.115