The dominant fragmentation pathways in multiphoton ionization of alkyl iodides

The dominant sequence of reactions in the dissociation of vis/UV multiphoton pumped alkyl iodides is determined by comparison with the observed final fragmentation pattern. In the computational procedure, the parent ion and each fragment ion are allowed a wide range of possible dissociation processes. The maximum entropy formalism is then used to determine the relative importance of the different pathways. It is found that under realistic conditions each ion has but one or two major dissociation processes, with the others being significantly less important. It is therefore possible to draw a "tree" of reactions connecting the parent ion (the root) to all final, observed, ions. To obtain a match to the observed fragmentation pattern it was found necessary to assume that intermediate ions also absorb the laser radiation to an extent depending on intensity and wavelengths. Since the mean number of photons absorbed by each ion is used as a parameter, the agreement with any particular fragmentation pattern is, in itself, only supporting evidence. The primary reasons for confidence in the proposed mechanism are based on the following observations: (1) very nearly the same sequence of reactions emerges as the dominant "tree" irrespective of wavelength or intensity, (2) the higher branches of the tree for the higher homologues are found to be the same as the tree of the lower ones, and (3) that ions absent in the observed mass spectra can usually not be made to be present in the computed spectra even by a wide variation in the absorption pattern suggests that the proposed mechanism is indeed robust.