one would have the difference in the energy gap of these two processes comes from the isotope effect on the difference in the v 3 to v 2 energy spacing for both isotopic species the 3 u2192 2 de excitation process will be a few hundred cm u2212 1 out of resonance with the 0 u2192 1 excitation process because of the large anharmonicity of the c h stretch the small fractional difference in the energy deficit of the process between the 13 c and 12 c species is significantly less than the simple case of co 2 shown above it is difficult to imagine that such a small fractional difference in energy gap could lead to a substantial difference in vibrational energy transfer rates in cf 3 h the key to understanding the phenomenon of collisional enhanced isotope selectivity exploited in this invention is to realize that the initially prepared state of cf 3 h is not a pure v ch 3 stretch state vibrational state mixing in these molecules has been extensively studied and is well understood j segall et al j chem phys 86 634 1987 each overtone level with n ch stretch quanta is characterized by a polyad of n 1 strongly coupled states consisting of combinations of stretch and bend excitations thus for n 3 one has the group of coupled states designated v s v b gt for the number of stretch and bend quanta respectively 3 0 gt 2 2 gt 1 4 gt and 0 6.