THE CLASSICAL LANGEVIN RATE-CONSTANT FOR ION-MOLECULE CAPTURE - WHEN,IF AT ALL, IS IT CONSTANT

Using the statistical adiabatic channel model (SACM) to estimate the r ate of capture of an ion by a spherically symmetrical molecule, and in cluding quantization of the orbital momentum, leads to the reduced cap ture rate constant that depends on two dimensionless parameters charac terizing the system. The form of such a dependence indicates, however, that for all of the typical ions as well as for all of the typical sp herical molecules the capture rate constant approximates to the classi cal Langevin rate constant. The rate can only be appreciably faster if the system features a very small reduced mass, for instance, a therma l electron plus a molecule. Also the importance of two quantum effects , the overbarrier reflection from the potential barrier and the tunnel ling through this barrier, was examined with the use of three differen t barriers to approximate the potential barrier for the polarization c omplex. The functions that describe this barrier realistically, the sy mmetrical Eckart function and the Dirac comb, indicate that any contri bution to the capture rate constant for all the ion-molecule systems c onsidered from the former effect can be ignored over the whole range o f temperatures. The latter effect, tunnelling through the barrier, is important but only from the systems that feature reduced masses so sma ll as to become physically unrealistic. The physically realistic syste ms do not undergo any tunnelling unless the temperature is extremely l ow, decreasing below the characteristic rotational temperature of the molecule.