|
内容記述 |
Two major topics of his thesis are (i) development of the velocity map<br /> imaging (VMI) spectrometer for investigation of the photoion images of<br />fullerenes, and (ii) simulation studies of possible processes for C<small>60</small>fragmentation. <br />There are two plausible photofragmentation pathways of excited C<small>60</small> cations, <br />namely the stepwise C<small>2</small>-loss and direct fission processes. Several groups have<br />tried to distinguish these two processes by measuring the total average kinetic<br />energy release (KER) in the decompositkxr of C<small>60</small><sup>+</sup> into C<small>56</small><sup>+</sup> and smaller<br />fragments. Such efforts, however, have met with failure. The portion of the KER<br />partitioned to the ionic fragments was found to be a few tens of <br />millielectronvolts and is comparable to or smaller than the average thermal<br />energies of neurtal C<small>60</small> molecules in an effusive beam. The above two processes<br />therefore give C<small>56</small><sup>+</sup> and smaller fragments with similar average kinetic energies<br />in the laboratory system. Obviously, we must measure a precise 3D velocity<br />distribution, speed and angular distributions, to gain helpful clue to decide on<br />which process is more dominant. For this purpose we have developed a VMI<br />spectrometer that is very sensitive to thermal ions having small translational<br /> energies.<br /> Our VMI spectrometer is based on a time-of-flight (TOF) technique for<br />the fragment ions produced by irradiation of synchrotron radiation. Its basic<br />performance has been experimentally tested by using five rare gases at photon<br />energy <i>hv</i>=35 eV. The 3D velocity distributions were reconstructed by using<br />the inverse Abel transformation (IAT) from measured 2D images projected<br />on a position-sensitive detector(PSD) to the cross- sectional images in the<br />perpendicular plane of the spectrometer. Using the speed distributions extracted<br />perpendicular plane of the spectrometer. Using the speed distributions extracted<br />from these cross-sectional images, we have evaluated the temperatures by the<br />least-squares fit of the data points to the Maxwell-Boltzmann distribution. The<br />best fitted curves of the lighter three rare gases are in reasonable agreement<br />with the Maxwell-Boltzmann distributions at the temperature <i>T</i>= 300K. The<br />temperatures obtained by the fittings are 282,272 and 295 K for He, Ne ard Ar, <br />respectively. Small deviations from the expected value of 300 K can be<br />accounted for by systematic errors peculiar to the numerical image processing in<br />the IAT. For Kr and Xe the agreement is much worse mainly due to smaller<br />signal-to-background ratios. Furthermore, the best fitted curve of Xe appears to<br />shift by 60 K in the direction of lower speed as compared to the expected<br />distribution at 300 K. This shift can be explained as that the raw images of Xe<br />includes not only Xe<sup>+</sup> but also Xe<sup>2+</sup> signal counts.<br /> We have simulated the images of five rare gases at 300 K to compare with<br />the experimentally obtained images. From the simulated projections on the PSD<br />we have obtained the cross-sectional images and speed distributions. The<br />temperature of He is evaluated to be 287 K from the last-squares fit of the data<br />points of the simulated speed distribution to the Maxwell-Boltzmann<br />distribution. Similar simulations were executed for Ne and Ar. All the<br />temperatures were found to be in good agreement with those from the<br />experimental images. Moreover, a close inspection of the simulated images<br />revealed that the defocusing effect due to a definite ionization volume can be<br /> well reproduced by introducing two Gaussian functions as (a) 2σ<small>x</small>, = 0.2 and <br />2σ<small>y</small>= 2.8 mm when oven with thickness monitor was not installed inside the<br />experimental vacuum chamber, <small>(b)</small>2σ, <small>x</small>=1.7 and 2 σ <small>y</small>=3.2mm when oven<br />with thickness monitor was installed. Here, σ <small>x</small> and σ <small>y</small> are corresponding to the <br />standard deviations of two Gaussian functions which are called defocusing<br /> parameters.<br /> A novel simulation method has been established for the image of the 3D<br />velocity distributions of C<small>56</small><sup>+</sup> produced by dissociative photoionization of C<small>60-</small>.<br />We calculated the arrival positions of C<small>56</small><sup>+</sup> ions, the spatial density functions, <br />and the projections on the PSD. The 2D cross-sectional images were derived<br /> from the projected images of C<small>56</small><sup>+</sup> ions produced through the stepwise C<small>2-</small> and<br />C<small>4-</small>-loss processes. At <i>T</i>= 0 K a marked difference in the image pattern could be<br />seen between the two processes but it is almost smeared out under bulk<br />conditions of C<small>60</small> at <i>T</i>= <i>273</i>K owing to the convolution of the thermal velocity<br />of nascent parent C<small>60</small><sup>+</sup> ions. In contrast, a remarkable difference at <i>T</i>=0 K were<br />found to remain event at <i> T</i>=785K for the C<small>56</small><sup>+</sup>formation in the C<small>60</small> beams, <br /> because the transverse velocity of the beam is extremely low. The difference in<br />the image pattern between the two processes permits conclusive<br />evidence on which mechanism dominates photofragmentation of C<small>60</small> in the<br />extreme UV region. We therefore consider that the present VMI spectrometer<br />will be available for future studies of the excited-state dynamics of fullerene<br />ions. Experimentally the image of C<small>56</small><sup>+</sup> might be contaminated by the<br />background dark counts due to impurities such as water, air, and organic<br />compounds. We have tried to remove the background counts from the measured<br />2D image by means of deconvolution using the low-pass and Wiener filters. |
要旨・審査要旨 (294.7 kB)
