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Details of Grant 

EPSRC Reference: EP/C015975/1
Title: Polarisation spectroscopy: a highly promising new tool for the study of collisional energy transfer
Principal Investigator: McKendrick, Professor KG
Other Investigators:
Costen, Professor ML
Researcher Co-Investigators:
Project Partners:
Department: Sch of Engineering and Physical Science
Organisation: Heriot-Watt University
Scheme: Standard Research (Pre-FEC)
Starts: 01 November 2005 Ends: 31 July 2009 Value (£): 334,999
EPSRC Research Topic Classifications:
Gas & Solution Phase Reactions Scattering & Spectroscopy
EPSRC Industrial Sector Classifications:
No relevance to Underpinning Sectors
Related Grants:
Panel History:  
Summary on Grant Application Form
Molecules in gases are in continual rapid motion and inevitably collide with one another. When they do, energy is transferred between them. Establishing the factors that control the flow of energy is crucial to understanding many important, real-world processes. Some examples include combustion (e.g. the burning of natural gas on a domestic cooker or of petrol in a car's engine); the maintenance of the ozone layer in the upper atmosphere; and the technical plasmas used to etch silicon wafers that are the basis of the vast semiconductor industry.The transfer of energy is a complex but fascinating problem. If the molecules were all spherical, it would be rather like 3-dimensional snooker. Only the velocities of the molecules (speed and direction) would continually change. However, real molecules have more complicated shapes, so as well as the velocities changing, the rotational (tumbling in space) and vibrational (oscillation of the atoms connected by chemical bonds) energies also change as a result of collisions. It is also possible for the motions of the electrons within the molecules to be affected, leading to a change of electronic state with a different energy.This work is concerned with ways in which these exchanges of energy can be detected. We are developing a new method to do this known as polarisation spectroscopy . It makes use of some of the special properties of laser light. A pulse of light from one laser is carefully chosen to have the right wavelength (i.e. colour) to interact with the chosen type of molecule under study. The light is also polarised . This is the same property that's exploited in polarising sunglasses. It means that the light waves are oscillating in a particular plane, or other more complicated but carefully controlled arrangements in space. The molecules that interact with the light are left no longer rotating randomly, but in specific planes determined by the polarisation of the light. This can be measured by a pulse of light from a second laser, which is also polarised. As it passes through the sample of molecules prepared by the first pulse, its own polarisation state is affected in a way that can be detected.The crux of our experiments is to delay the arrival of the second pulse of light, so that the molecules suffer collisions in the time between it and the first pulse. These collisions may change the amount of energy the molecules have, either in rotation, vibration or electronic motion. This means that they will no longer interact in the same way with the second pulse because they will have changed the wavelength at which they would naturally absorb light. In addition, even if they have only been tipped over in the collision but otherwise keep the same amount of energy, this will still affect how they interact with the second pulse because of its polarisation. We therefore have a method for measuring very carefully what happens to the molecules during a collision. We have chosen to study the small, highly reactive free radical molecules NO, OH and CH that control the chain reactions in combustion. To understand precisely what the results of the experiments are telling us about their collisions with other molecules, we compare what we measure with theoretical predictions. The most elementary theories are models , treating the collisions as the interaction of simple ball and stick-like objects. These are good for developing a physical intuition because the results are relatively easy to grasp. For the most accurate comparisons, full quantum mechanical calculations are carried out. We are then testing the ability of the most rigorous theories currently available to predict the behaviour of real molecules.
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Organisation Website: http://www.hw.ac.uk