ACTIVATION-ENERGIES FOR THE FRAGMENTATION OF THIOPHENE IONS BY SURFACE-INDUCED DISSOCIATION

We have improved our previously described method for extracting activa tion energies of fragmentation for polyatomic ions from surface-induce d dissociation (SID) data [Wainhaus, S. B.; et al. J. Am. Chem. Sec. 1 997, 119, 4001]. Our method analyzes the energy-resolved mass spectra and the kinetic energy distribution spectra of the parent and fragment ions that scatter off the surface. It extracts the activation energie s by integrating over the distribution of the initial ion energy and t he energy transferred to the surface, taking into account both the ave rage value and the width of these distributions. The new method gave i mproved activation energies for SiMe3+ --> SiMex+ (x = 0-2) fragmentat ion at a hexanethiolate-covered gold surface. We then used our data an alysis method to analyze the activation energies for the fragmentation of thiophene ions at the hexanethiolate-covered gold surface. The act ivation energies for the formation of C2H2S+, CHS+, and C3H3+ from C4H 4S+ were found to be 4.6 +/- 0.7, 6.9 +/- 0.7, and 6.5 +/- 0.7 eV, res pectively. Our activation energy results followed the trend in the val ues from threshold photoelectron photoion coincidence data. However, t he SLD values were similar to 50% higher than the threshold photoelect ron photoion coincidence values; this discrepancy mostly resulted from delayed dissociation. This model may be used to extract quantitative activation energies from SID data once certain ongoing issues are reso lved in future papers. Molecular dynamics simulations were also perfor med to assist in the data analysis and to test the assumptions of ener gy transfer in this system. Qualitative agreement in energy transfer w as found between the experiments and simulations.