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内容記述 |
In the doctoral thesis of Mr. Kafle, the results of the study on the absolute total photoionization cross section σ<small>abs,I</small> of gaseous C<sub><small>60</sub></small> in the photon energy <i>hv</i> range from 25 to 120 eV are presented. The measurements were carried out using photoionization mass spectrometry in combination with synchrotron radiation. Absolute partial cross sections σ<small>abs</small>(z+) (z = 1-3) for the formation of the ions in a charge state z from C<sub><small>60</sub></small> were evaluated by considering the absolute detection efficiencies of photoions in different charge states. Then the absolute total photoionization cross section (σ<small>abs,I</small>) was obtained by summing up all of the σ<small>abs</small>(z+) and compared with available experimental data in the literature. For instance, the, σ<small>abs,I</small> obtained from his study takes the value of 401 Mb at <i>hν</i> = 25.5 eV. This value is a little smaller than σ<small>abs,A</small> determined by Jaensch and Kamke using the gas chamber technique [R. Jaensch and W. Kamke, Mol. Mater. 13 (2000) 143] (～450 ± 213 Mb at 25.5 eV). Moreover, the σ<small>abs,I</small> curve obtained from his study was combined with the photoabsorption cross section curves of C<sub><small>60</sub></small> at <i>hv </i>= 3.5 - 26 eV in the literature (specifically, the data of Jaensch and Kamke at <i>hv</i> = 11.4 - 25 eV, and of Yasumatsu <i>et al. </i> at <i>hv</i> = 3.5 - 11.4 eV [J. Chem. Phys. 104 (1996) 899] were used), after appropriate alterations of the vapor pressure are taken into account. The oscillator strengths are computed from the composite curve to be 178.5 and 230.5 for the <i>hv</i> ranges from 3.5 to 40.8 eV and from 3.5 to 119 eV, respectively. These oscillator strengths agree well with those expected from the Thomas-Kuhn- Reiche sum rule and 60 times the photoabsorption cross section of a carbon atom. Moreover, the σ<small>abs,I</small> curve obtained from his study behaves similarly to the relative photoionization cross section curve reported by Reinköster <i>et al.</i> [J. Phys. B, 37(2004) 2135].<br /> When a fullerene, in particular C<sub><small>60</sub></small> molecule, absorbs a photon of energy ～ 41 eV or above, a highly excited ion of C<sub><small>60</sub></small> is produced and then dissociates into smaller ionic and neutral fragments. In his doctoral thesis, description is also made on a design of a new version of photofragment imaging spectrometer, which will be applied to observe the momentum distributions of ionic fragments from large molecules, clusters, and fullerenes. The apparatus consists of several components: circular electrodes, a time-of-flight drift tube, a potential-switcheable <i>mass gate, ion reflector,</i> and a position sensitive detector (PSD). The velocity focusing lens system of Eppink- Parker type [Eppink and Parker, Rev. Sci. Instrum. 68, 3477 (1997)] realizes high resolution of the photofragment images. Moreover, the mass <i>gate</i> is incorporated inside the tube in order to separate fragment ions with a particular cluster size (e.g.C<sub><small>58</sub></small><sup>+</sup>) from those with other sizes (e.g. C<small><sub>60</small></sub><sup>+</sup>, and C<small><sub>56</small></sub><sup>+</sup>).<br /> The optimum arrangement and dimensions of the components are determined from the results of ion trajectories of C<sub><small>56</sub></small><sup>+</sup>,C<sub><small>58</sub></small><sup>+</sup> and C<sub><small>60</sub></small><sup>+</sup> simulated by using the SIMION software. The calculated images of C<sub><small>58</sub></small><sup>+</sup> ions show that kinetic-energy resolution of 10 meV is achievable. Moreover, a linear dependence between (quasi-linear relation) the y-component of the momentum (Py) and that of the displacement on PSD (Δy) is obtained. This observation allows one to transform the displacement of ionic fragment on PSD to obtain velocity and spatial distribution of a desired fragment.<br /> The present momentum imaging spectrometer has been constructed and installed in the end station of beam line 2B in the UVSOR facility. The preliminary experimental results on Kr sample at room temperature (Kr<sup>+</sup>and Kr<sup><small>2</sup></small><sup>+</sup>were focused on to the PSD) obtained using this spectrometer show that ions produced in the ionization region can be focused at the center of the PSD (within 2mm), if image defocusing due to thermal energy of the molecule is omitted. These experimentally observed results guides us that one of the objectives of the present design is fulfilled. Thus, present momentum imaging spectrometer can be utilized to obtain reliable velocity distributions of the fullerene fragments. From the close analysis of photofragment images, one will be able to decide on which mechanism dominates the fragmentation of fullerene ions between sequential loss of C<sub><small>2</sub></small> unit (C<sub><small>60-2n+2</sub><sup>z+</sup></small> → C<sub><small>60-2n</sub></small><sup>+</sup> + C<sub><small>2</sub></small>) and single-step two-fragment fission (C<sub><small>60</sub></small><sup>+</sup> → C<sub><small>60-2n</sub></small><sup>+</sup>+C<sub><small>2n</sub></small>)of the parent C<sub><small>60</sub></small><sup>+</sup>ions. Because three- dimensional velocity distributions are expected to considerably differ for different mechanisms. |
要旨・審査要旨 (299.9 kB)
