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.