A vibrating oxygen-hydrogen (O-H) bond on a water molecule can transfer energy to another molecule one hundred times more rapidly than physicists expected. Sander Woutersen and Huib Bakker of the FOM-Institute for Atomic and Molecular Physics in Amsterdam, the Netherlands, have used ultrafast laser pulses to excite the vibrational modes of water molecules and then probe how the vibrational energy gets redistributed throughout the liquid. Their results could explain why water is used to transfer energy between proteins and other biomolecules (Nature 402 507).
As a water molecule is excited by the laser pulse, it can only drop back to a less excited state by releasing the excess energy. This energy causes the O-H bonds to stretch and vibrate. As the bonds vibrate they knock other water molecules, transferring energy to a different O-H bond. This energy transfer can take place within one-tenth of an attosecond (10-19 seconds). Woutersen and Bakker used two 200 femtosecond infrared pulses – one that was strong enough to excite the molecules and another, lower intensity pulse to probe them. The pulses were fired at ultra thin layers of pure water (H2O), or a mixture of ‘heavy water’ (D2O) and H2O water. The deuterium mixture enabled the rate of transfer of energy between the O-H bonds to be calculated by comparing the results with the pure water sample.