Collisions of low-energy molecules
Dutch scientists in Berlin and Nijmegen are collaborating in developing a new method in order to investigate collisions between neutral molecules and atoms at low velocities. The unusual aspect of this experiment is chiefly because, for the first time, it is possible to change the velocity between unloaded molecules and atoms from high to very low, as at a chosen velocity the spreading of the velocity will be extremely small. Comparison to the most accurate theory in this field that describes exactly what happens at a collision, provided a suitable similarity. The researchers will publish their results in Science on 15 September 2006.
A collision experiment at which particles are being shot under the best possible conditions, is one of the more fundamental ways to study the collision partners. Measuring the condition of the particles after the collision may teach us something about these particles and their interactions. One of the most significant experimental variables to this is the velocity of the collision partners compared to one another.
The collision velocity determines the amount of energy that is available for any change in the particle, caused by the collision. The limit of extremely high energies is being investigated by experiments in particle accelerators, for example, at CERN in Geneva. That makes it possible to scatter charged particles into the smallest particles possible by shooting at them. In the experiment with low-energy molecules at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin, exactly the limit of extremely low energy is being investigated. Consequently, the molecules do not scatter, but remain intact and can be studied as a whole.
To change collision energy
Gerard Meijer and fellow scientists, originally at Radboud University Nijmegen, later on at the FOM-Institute for Plasma Physics Rijnhuizen, Nieuwegein and nowadays at the Max-Planck-Gesellschaft in Berlin, have developed a method that makes it possible to change the collision energy, a very important parameter. In the past few years it became possible to change the velocity of neutral molecules between 600 and 30 metres per second that usually was about 500 metres per second, by switching on and off high-electrostatic fields at the right moment. This procedure will change the kinetic energy of the molecules very precisely and consequently, the energy that is needed for the collision.
Researchers may use this in order to expose details of the collisions that in the past were hidden because of the high velocity and the large velocity distribution.
In order to demonstrate the new method the researchers have been examining the collisions between OH-molecules (a so-called radical that is very important in atmospheric chemistry) and xenon atoms. There is a chance that the OH-molecule will be rotating after the collision. However high the chance and however fast the rotation, depends on the available collision energy. The interesting thing is that the quantum structure of the colliding molecules is clearly visible when measured at very large collision energies. The result is that not just every rotation velocity may occur. Now, it has been experimentally measured with a very high accuracy at what threshold values to the collision energy and to what extent rotation will be taking place. A comparison to the most accurate theory in this field by the theoretical quantum chemist Gerrit Groenenboom in Nijmegen provided a suitable similarity. This indicates that the theoretical method used, is describing the physics of the collision process very well.
New experiments are possible
This new method makes a large range of experiments possible. Think, for example, of developing the collision experiment in Berlin, at which the velocity of not only one scattering partner will be changed, but of both. In order to do so, it is necessary to set up two inhibitors for neutral particles crossed to one another. This could improve the accuracy at which the collision energy can be changed to another dimension, so that even more details of the quantum structure in molecules will come to light. A different significant application for the molecular inhibitor can be found in chemistry. Also to chemical reactions apply that, if only little energy is available, the reactions do not expire at all or that quantum mechanical consequencies will dominate the behaviour of the reactions. The inhibitor may be an ideal means for investigating reactions in similar circumstances.
The article is entitled Near-threshold Inelastic Collisions with the Use of Molecular Beams with a Tunable Velocity. The authors are Joop Gilijamse, Steven Hoekstra, Bas van de Meerakker, Gerrit Groenenboom and Gerard Meijer.
For more inormation, please contact Professor Dr. Gerard Meijer, Frits-Haber-Institut der Max-Planck-Gesellschaft, Berlin, phone: ++49 30 8413 5600 or Dr. Bas van de Meeerakker, Frits-Haber-Institut der Max-Planck-Gesellschaft, Berlin, phone: ++49 30 841 357 38.