Water

One of our research interests is the water molecule, H2O , in different environments and its interaction with radiation. The water molecule is build from one oxygen atom and two hydrogen atoms. The molecule has three degrees of vibrational and rotational freedom. The three figures beneath show the 3 possible vibrational motions. They are labelled with v1, v2 and v3 and are often called the normal modes of vibration of the molecule. The figures also show the energy of one quantum of vibration.

Figure: H2O molecule showing symmetric 
stretching mode. Figure: H2O molecule showing bending mode. Figure: H2O molecule showing asymmetric 
stretching mode.
Symmetric stretching mode, v1
3657 cm-1
Bending mode, v2
1595 cm-1
Asymmetric stretching mode, v3
3756 cm-1

Normal modes involve the participation of all parts of the molecule. A vibration localized in a single OH bond is called a local mode. This gives a better model for high levels of excitation than the normal mode model. The transition from normal mode to local mode is a manifestation of the mixing between vibrational states which also leads to the concept of polyads.

When two modes lie close in energy they can mix. The energy for the v3 mode is about half the energy of the v1 and v2 modes. So 2n quanta of bend can mix with n quanta of stretch. The number n is called the polyad number. A polyad is a group of states that are close in energy.

But the molecule cannot only vibrate. It can also rotate. To describe all possible rotations of the water molecule we need to define 3 rotational axes. By convention the axes are labelled so that the rotational constant A is biggest and C is smallest.

Figure: H2O molecule showing rotation axis A. Figure: H2O molecule showing rotation axis B. Figure: H2O molecule showing rotation axis C.
Rotational axis A Rotational axis B Rotational axis C

The water molecule is an asymmetric top molecule. Hence the degeneracy in the vibrational and rotational states is lifted resulting in many allowed transitions. This gives rise to spectra with little or no readily discernable structure.

The group have used Variational methods to analyse spectra of sunspots and laboratory emission spectra. This work has significantly increased our knowledge about the water molecule. The next image shows one of these spectra and with some identified transitions.

Click for larger image of
 spectrum. (15K)

Sunspots are centres of intense solar magnetic activity. They are significantly cooler than the surrounding areas of the Sun's atmosphere an so appear as dark patches on the Sun's surface. They also have a more complicated spectra than the main body of the sun. The two images beneath show the sun with sunspots and a sunspot close-up.

Figure: The sun with sunspots. Click image for
larger image (6K). Use the browser's [Back] to get back here again. Figure: Sunspot closeup. Click image for larger image
(77K). Use the browser's [Back] to get back here again.
Sun with sunspots
Click for larger image (6K)
Sunspot closeup.
Click for larger image (77K)

The temperature of the sun's atmosphere is around 5500°C. In contrast, sunspots have a temperature of about 3000°C. Hence for different regions of the sun's atmosphere there is a change in chemistry. At 5500°C only simple two atom molecules can form. The rest are blasted apart the second they are formed. One of the stable molecules in this environment is the OH radical, which is quite common in the sun's atmosphere. In cooler regions H2O molecules can form from OH radicals. At ~3700°C half of the OH radicals have become water molecules.

Spectra of hot water are very different from the spectrum of cold water vapour found in the Earth's atmosphere. This allows astronomers to observe hot water spectra through windows in the Earth's atmosphere. Astronomers label these windows with different letters. Especially the N-window (10-20µm) is of interest for us. Here pure rotational transitions of hot water occur and can be investigated. You can find some of our data on water in our ftp-archive ftp.tampa.phys.ucl.ac.uk/pub/water

The research on hot water is for use in many other research fields. The following list gives a short overview over some applications.


For further reading please refer to