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.
|Symmetric stretching mode, v1
|Bending mode, v2
|Asymmetric stretching mode, v3|
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.
|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
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.
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.
|Sun with sunspots
Click for larger image (6K)
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.
Monitor and optimize the performance of internal combustion engines.
Hot water emissons have been detected from forest fires for which they give a clear signature. This can be used for early warning systems using satellites.
Keeping track of ships, aircrafts, helicopters and tanks using their hot water emission from their exhaust gases.
In astronomy water vapour is the most important absorber of infrared star light in the atmosphere of oxygen rich cool stars.
Atmospheric anormalities and modification of atmospheric models.