Metastable decay of molecular fragment ions sputtered from hydrocarbon polymers under keV ion bombardment

To investigate the metastable decay processes for molecular ions sputtered from hydrocarbon polymers with different degrees of unsaturation, polyisobutylene, polybutadiene, and polystyrene thin films were bombarded by 15 keV, Ga+ ions and the secondary ions were mass and energy analyzed by means of a time-of-flight spectrometer. In general, the kinetic energy distributions show that an important fraction of the secondary ions is detected with an energy deficit, due to the dissociation of their metastable parents in the linear part of the spectrometer. The analysis of the energy spectra leads to propose two types of metastable decay reactions: fast, unindentified dissociation in the acceleration section of the spectrometer and delayed H and H-2 losses in the drift section of the spectrometer. The interpretation of the results in the frame of the unimolecular reactions theory indicates that the decay rates of these reactions are in the range 10(7)-10(8) s(-1) (fast decay) and 10(4)-10(5) s(-1) (H and H, loss), which corresponds to half lives of 10 ns to 0.1 mu s and 10 to 100 mu s, respectively. On average, the fraction of ions produced in the vacuum increases with the mass of the daughter ions. When comparing the three polymers, the metastable decay for the lowest mass range (0 < m/z < 100) is increasing when decreasing unsaturation (polyisobutylene > polybutadiene > polystyrene). The important analytical issue of these unimolecular decomposition reactions is addressed, too. (C) 1999 Elsevier Science B.V.