Month: April 2013

Tweet One of the useful things the Rabett used to do was to explain what happens to the energy when a molecule, say CO 2 (carbon dioxide) although you could also say H 2 O (water vapor) or CH 4 (methane) absorbs light. For the purpose of this post, the photon would be in the infrared region of the spectrum.  This is an evergreen for two classes of bunnies Bunnies who don’t realize that the molecule can also emit light.  This is a popular one amongst organikers and analytical chemists whose experience with IR spectroscopy is in an absorption spectrum for analysis of samples Bunnies who think that the only way that an excited molecule can get rid of the energy is to emit a photon.    For every CO 2 molecule there are roughly 3000 2500 other molecules in the same volume of air.  When a CO 2 molecule collides with one of the other molecules, almost certainly an oxygen or nitrogen molecule, energy transfer occurs.  Each CO 2 molecule can be described as having translational, vibrational and rotational energy and the same is true of the collision partner.  Any collision can in principle change the amount of any of these forms of energy by any amount subject to conservation of energy and momentum.  The probability of this happening depends on the relative translational energy of the collision, the relative orientation of the molecules, their distance of closest approach and the distribution of energy in each of the collision partners prior to the collision.  The detailed study of such effects is called collision dynamics or molecular dynamics.  Fortunately, we can take thermal averages over many of these variables, either theoretically or experimentally which makes life, theory and experiments much simpler and a hell of a lot less expensive and time consuming.  That sort of thing usually goes under the rubric of reaction (when there is one) kinetics or energy transfer studies when there isn’t.