Performance improvement of mobility management in IP-based wireless networks

博士論文

In the doctoral thesis of Mr. Kafle, the results of the study on the absolute total photoionization cross section σabs,I of gaseous C₆₀ in the photon energy hv 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 σabs(z+) (z = 1-3) for the formation of the ions in a charge state z from C₆₀ were evaluated by considering the absolute detection efficiencies of photoions in different charge states. Then the absolute total photoionization cross section (σabs,I) was obtained by summing up all of the σabs(z+) and compared with available experimental data in the literature. For instance, the, σabs,I obtained from his study takes the value of 401 Mb at hν = 25.5 eV. This value is a little smaller than σabs,A 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 σabs,I curve obtained from his study was combined with the photoabsorption cross section curves of C₆₀ at hv = 3.5 - 26 eV in the literature (specifically, the data of Jaensch and Kamke at hv = 11.4 - 25 eV, and of Yasumatsu et al. at hv = 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 hv 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 σabs,I curve obtained from his study behaves similarly to the relative photoionization cross section curve reported by Reinköster et al. [J. Phys. B, 37(2004) 2135]. When a fullerene, in particular C₆₀ molecule, absorbs a photon of energy ～ 41 eV or above, a highly excited ion of C₆₀ 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 mass gate, ion reflector, 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 gate is incorporated inside the tube in order to separate fragment ions with a particular cluster size (e.g.C₅₈⁺) from those with other sizes (e.g. C₆₀⁺,and C₅₆⁺).The optimum arrangement and dimensions of the components are determined from the results of ion trajectories of C₅₆⁺,C₅₈⁺ and C₆₀⁺ simulated by using the SIMION software. The calculated images of C₅₈⁺ 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. 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⁺and Kr²⁺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<br />dominates the fragmentation of fullerene ions between sequential loss of C₂ unit (C_60-2n+2^z+ → C_60-2n⁺ + C₂) and single-step two-fragment fission (C₆₀⁺ → C_60-2n⁺+C_2n)of the parent C₆₀⁺ions. Because three- dimensional velocity distributions are expected to considerably differ for different mechanisms.<br />

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