Thermoionization and dissociation of fullerenes and endohedral fullerenes.

摘要

Electron emission and unimolecular dissociation of energetically excited fullerenes and endohedral fullerenes are studied with mass spectrometry. Three experimental approaches have been developed for these studies. These are UV laser excitation time-of-flight mass spectrometry (TOF-MS), laser desorption and ionization mass spectrometry (LDI-MS), and ion-molecule collision mass spectrometry (IMC-MS).; Our first experimental effort is to investigate delayed electron emission from multiphoton excited C₆₀ molecules. A lower limit of the probability of electron emission from multiphoton excited C₆₀ molecules is determined to be about 2.6%. This result indicates that electron emission is not merely parasitic to dissociation.; The second experimental effort is the study of metallofullerene formation. Threshold energies were found in the formation of potassium metallofullerenes and sodium metallofullerenes. The large thresholds indicate that the alkali metallofullerene products have endohedral structure. Sodium ion implantation into C₆₀ films could be a potential method of producing metallofullerenes in macroscopic quantity.; The ion-molecule collision experiments characterize the dynamical and physical processes in energetically excited fullerenes. Modeling the breakdown curve of Na@C₆₀+ in the collision of C₆₀ with sodium ions enables us to extract the activation energy for loss of C ₂ from Na@C₆₀+, which gives the value of 10.17 ± 0.02 eV. The result indicates that the encaged sodium ion does not significantly change the C-C binding in the fullerene shell.; Changing the internal energy of the target C₆₀ molecules causes a shift in the breakdown curve of Na@C₆₀+. Based on this phenomenon, a method of measuring the heat capacity of C₆₀ is introduced. The heat capacity is determined to be 0.0126 ± 0.0014 eV/K for temperatures of 500°C ≤ T ≤ 570°C.; The C₆₀+ ion product in the collision of C ₆₀ with potassium ions is modeled as thermionic emission. The C ₆₀+ intensity reaches its maximum at 57 eV. At this collision energy the efficiency of energy transfer reaches about 78%. This high efficiency can be explained using the two-stage collision model.