8. Effect of the surface on energy transfer in collision
Both the average energy transfer and width of the internal energy distribution deposited in ion–surface collision are affected by the surface properties. For example, the percentage of translational-to-vibrational (T → V) energy transfer is in the range of 18–28% for FSAMs and 8–12% for HSAMs [13], [89], [92], [103] and [112]. Similar internal energy distributions were reported for collisions of small ions with HSAM, carboxylic acid–terminated SAM (COOH–SAM), HOPG, and hydrocarbon-covered metal surfaces [92]. The average T → V energy transfer is affected mainly by the surface’s “effective mass” of the surface given by the mass of the chemical moiety representing an immediate collision partner in ion–surface collisions [89] and [92]. Comparison of the energy transfer in collisions of peptides with relatively soft SAMs and relatively stiff LiF and diamond surfaces demonstrated that surface stiffness has a major effect on the width of the internal energy distribution [89]. Specifically, substantially broader internal energy distributions are deposited by collisions with stiff surfaces. These experimental observations are in excellent agreement with the results of classical trajectory simulations by Hase and co-workers [20] and [103]. The simulations indicate that differences in the T → V transfer efficiency originate from different scattering dynamics of ions colliding with stiff and soft surfaces. For example, less than 10% of the precursor ion’s initial kinetic energy is transferred to the surface, and more than 70% is converted into the translational energy of scattered ions in collisions with stiff surfaces compared to ∼60% and ∼30%, respectively, for collisions with soft HSAM surfaces [20] and [103]. Although, as discussed earlier, the average T → V transfer efficiency in collisions with SAM surfaces is independent of the incidence angle, the average energy deposition into the precursor ion’s internal modes in collisions with a crystalline diamond (1 1 1) surface decreases as the incidence angle increases from 0° to 45° [106]. These studies indicate that the internal energy distribution deposited into the projectile ion is controlled predominately by the surface properties.