Kinetic energy distributions of secondary molecular ions from thin organic films under ion bombardment

Langmuir-Blodgett films of tricosenoic acid deposited on gold have been bombarded by Ga+ and Cs+ ions and secondary ion mass spectra were measured by a Time-of-Flight spectrometer. The energy distributions of atomic ions are found to follow the Sigmund-Thompson law whereas, for the molecular CxHy positive and negative ions, the energy distributions have a much more intriguing structure. Within a given CiHy cluster, the energy distributions become broader for the ions that are more unsaturated. This effect is more important for the smaller clusters. The more saturated ions which have a structure close to that of the original molecule have the thinner energy distributions and these are only slightly dependent of the fragment mass. The results are tentatively interpreted on the basis of the chemical and molecular structure of the fragments as compared to that of the original hydrocarbon chains.