Warm water vibrates for longer

Dutch researcher Arjan Lock has investigated the behaviour of vibrating water molecules. Using ultra-short laser pulses, he found that hydrogen atoms in water molecules vibrate for longer at higher temperatures. This is abnormal because in the majority of substances a vibration lives shorter at higher temperatures.

Lock studied the OH-stretch vibration in water. He found that the lifetime of the OH-stretch vibration, a vibration of a hydrogen atom with respect to the oxygen atom, is extremely short in water, just 0.26 picoseconds (0.26 millionth, millionth of a second). The energy is then transferred from the OH-stretch vibration to a bend vibration in water.

At a higher temperature the lifetime of the vibration increases. This is completely contrary to the expected behaviour because in the majority of substances, the duration of the vibration is shorter at higher temperatures. In water however, higher temperatures weaken the hydrogen bonds and as a result of this the lifetime of the vibration increases.

After a certain period of time, the hydrogen atom will stop vibrating with respect to the oxygen atom and the vibrational energy will then be transferred to other movements. The time span in which that occurs is termed the lifetime of the vibration. If the molecule has a hydrogen bond, the frequency of the OH-stretch vibration decreases and the lifetime of the vibration changes.

The lifetime of the vibration is a measure of the strength of the hydrogen bonds. Hydrogen bonds are weak bonds between the hydrogen atom in one molecule and the oxygen atom in another molecule. These bonds bind the individual water molecules together.

Lock used a special ultrafast infrared laser for his experiments. This laser provides extremely short light pulses: 0.2 picoseconds. As these are slightly shorter than the duration of the vibrations, they can be used to carefully follow the behaviour of the vibrations.

In the experiments the researcher used two light pulses. The first energy-rich pulse causes the molecules to vibrate. With the second pulse Lock could examine how many molecules were still vibrating at a certain point in time after the first pulse. A water molecule that is still vibrating will not absorb the energy from the second light pulse. By measuring how much light passes through the water, the number of water molecules still vibrating can be determined.

The research was funded by the Netherlands Organisation for Scientific Research.