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Physical Chemistry

Quantum Description Of Two-Molecule Collisions

New experiments probe dynamics with unprecedented detail

by Elizabeth K. Wilson
November 27, 2012 | APPEARED IN VOLUME 90, ISSUE 48

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Credit: Ludwig Scharfenberg
A Stark decelerator controls a beam of HO∙ radicals, which collide with a beam of NO∙ radicals.
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Credit: Ludwig Scharfenberg
A Stark decelerator controls a beam of HO∙ radicals, which collide with a beam of NO∙ radicals.

To understand phenomena such as combustion and atmospheric chemistry, scientists need to know quantum state dynamics of atomic and molecular scattering. But such descriptions tax even modern computers and experimental methods. Until now, the most complex type of collision scientists have been able to characterize is that of a homonuclear diatomic molecule such as H2. But in a new experiment, theorist Gerrit C. Groenenboom and experimentalist Sebastiaan Y. T. van de Meerakker of Radboud University Nijmegen and colleagues have probed the dynamics of collisions between two diatomic radicals—HO∙ and NO∙—something that computers still can’t do in detail (Science, DOI: 10.1126/science.1229549). The group collided beams of the radicals and observed their rotational and spin-orbit inelastic scattering cross sections. To the researchers’ surprise, they found that the experimental results agreed well with predictions of a relatively simple theoretical model that only treats long-range interactions and avoids the extreme complexity of close-range effects. Until computational power catches up, this simple theoretical treatment could be used to explore other complicated collisions involving radicals that play a vital role in areas such as astrochemistry.

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