In liquids, the molecules are constantly moving and changing their orientations. These motions are very rapid, one typically measures the time of these processes in picoseconds (10^-12  s) or femtoseconds (10^-15 s).
The most important liquid is water. Although the simple molecular formula (H O) suggests that its properties can be understood easily, its molecular dynamics are very complicated. The main reason for this is the fact that every water molecule can be linked to four other neighboring water molecules by means of hydrogen bonds. These hydrogen bonds are not permanent, but are constantly formed and broken. Thus, in liquid water, a complicated and rapidly changing network of water molecules exists.
We can follow the rapid dynamics of the molecules in a liquid in time with a laser system that delivers very short and intense pulses. The wavelength of these pulses is chosen such that it excites vibrations in the molecules under study (for example, water). From the behaviour of these molecular vibrations, we learn more about the changing of the hydrogen-bond network, how energy is transfered from one molecule to another, and how fast the molecules move.

From pump-probe experiments, we found that the energy of the vibration is transfered in 740 femtoseconds to one of the hydrogen bonds that connect the water molecule to neighboring water molecules. In addition, we found that the constantly changing hydrogen bond network causes the frequency of the OH stretch vibration to change on a timescale of about 600 femtoseconds.
It is also possible to look at the direction in which the water molecules vibrate, by varying the polarization of the laser light. In this type of experiment, the so-called anisotropy is measured, as depicted below:

At time zero, directly after excitation by the pump pulse, the anisotropy has its maximum value (left picture). After some time, however, the anistropy becomes smaller and smaller, which means that the molecules vibrate in random directions (right picture). This is due to two processes:

• Reorientation
  The water molecules move and tumble at random. These random changes in their orientation make the anisotropy disappear.
• Förster energy transfer
  An excited vibrating water molecule can transfer its energy to a neighboring water molecule that was initially not vibrating. In this process, the total number of vibrating molecules does not change. This should not be confused with the separate process where vibrational energy `disappears' to the hydrogen bond network.

  
  Förster energy transfer

To distinguish these two processes, one can note that the Förster transfer depends on the average distance between the water molecules. Therefore, we dissolved the water (H O) in heavy water (D O). Förster energy transfer is only possible from one OH group to another OH group, so we see that the measurements depend on the H O concentration:

Anisotropy decay in H O/D O mixtures

At the lowest H O concentrations (0.2%), the distance between the OH groups is so large that only the random changes in orientation of water molecules are visible in the measurements, which is a relatively slow process. We found that a water molecule typically needs 4 picoseconds to change its orientation.
In pure normal water, the OH groups are very close to each other. The initially high anisotropy disappears extremely rapidly due to the efficient Förster transfer in this case.
It is interesting to know at which distance between two OH groups the Förster process becomes important. From the measurements at intermediate concentrations, we found that this distance, the Förster radius, is 22 X 10^-10  meters. This is comparable to the size of a water molecule.

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