The Patterns and Routes of Fragmentation of the Irradiated

Abstract

This thesis describes detailed experimental studies of patterns and routes of fragmentation of C60 in the condensed form and as powder. The fragmentation has been investigated in collisional and ablative environments using continuous and pulsed, ions and electron beams. Two different types of experimental setups are used for the study of fragmentation in the two modes. Continuous mode fragmentation by Cs+ is studied using mass spectrometry. The effects of incident particle's energy as well as dose variation on fragmentation patterns are investigated. Threshold energies for the emission of only C2 as a fragment in mass spectra are found to be different for C60 fullerite and powder samples. The pulsed and continuous mode fragmentations lead to ablation and collisional sputtering studies. Pulsed electrons and Ne+ induced fragmentation of C60 cages are studied by in situ emission spectroscopy. Source of Negative Ions by Cesium Sputtering, SNICS is employed to investigate collisions of Cs+ with C60 that led to its fragmentation. The Cs+ sputtered clusters, from the C60 acting as cathode surface are mass analyzed to identify the patterns and routes of the cage fragmentation. Two regimes of fragmentation of C60 cluster are observed by variation of E(Cs+) and showed significant difference among the emitted fragments. In the case of thermal regime, C60 undergoes fragmentation without destruction of the cage structure (Cs+-C60). The C60 cage however, shrinks by emitting one or more C2s. In the collisional regime the interaction of the incident particle with the cage constituents are more pronounced (Cs+-Cx). Effect of Cs+ dose variation on fragmentation patterns of C60 is also investigated using SNICS. Continuous irradiation by Cs+ is monitored by C2's emitted intensity. C2 is found to be the major fragment even from the destroyed C60 cages after heavy dose of Cs+ ions. The ablative experiments are performed in pulsed discharge ion source. Fragmentation of C60 is compared with that of graphite. Two set of experiments are performed in this mode; one using electrons and the other using Ne+ ions. Due to the polarity and mass difference, the two incident particles produce different fragmentation patterns. In-situ emission spectroscopy observed the emitted fragments', excited states. The vibrational temperature of C2 molecule emitted as fragment in result of electron ablation is found to xvii be ~ 12000 K. For the case of Ne+ induced fragmentation, the emitted C2 is found to be in non-LTE state. For both collisional and ablative experiments, C2 is found to be the dominant fragment at all bombarding energies and in all target conditions. Mass spectra observed C2 as the major fragment and emission spectroscopy provides clues to the details of the fragmentation route via C2 ejection. Structural changes in the irradiated C60 after bombardment of these particles are investigated by XRD and Raman spectroscopy. Cs+ induced damage in the C60 clusters resulted in amorphization of the structure. The cage structure is destroyed as a result of heavy Cs+ dose. Pulsed Ne+ and electron ablation of C60 powder samples also resulted in breakage of the cages. Mass spectra of the heavily damaged and reformed fullerite continue to show C2 as the main fragment with C1, C3 and C4 as minor fragments. Diagnostics include emission, FTIR and Raman Spectroscopy, XRD, AFM, mass spectrometry.