The Persistence of Molecular Vibration in Collisions

Abstract

The dispersion of sound is theoretically interpreted by the idea that the exchange of energy in collisions between molecular translation and rotation on one side and vibration on the other side is much slower than, for example, between the various translational degrees of freedom. It seemed desirable to find a direct optical method of observing this hypothetical effect. Apparatus has been developed in this laboratory for the purpose of investigating molecules present in very small concentration, like radicals, by their absorption spectra; with this method, relative concentrations of molecules are determined by comparing intensities of certain rotational lines. This test was applied to the higher vibrational levels of the I2 molecule. Their concentration was artificially increased beyond the small amount present in thermal equilibrium by an electric discharge of brief duration. By varying the phase between an electric switch and optical shutter, snapshots of the absorption spectrum could be taken at different intervals after the discharge was interrupted. The result was that iodine molecules raised from the ground level to the first excited vibrational level are able to persist in this level through several thousand collisions with other iodine molecules before losing their vibration.