Search

We present experimental results that show the secondary-ion yield depends on the structure and orientation of incident cluster ions. A beam of carbon-cluster ions ${{\mathrm{C}}_{n}}^{+}$ ($n\ensuremath{\le}4$) with an energy of 0.9 MeV per atom, which were obtained from a tandem electrostatic accelerator, was incident on a glycine target deposited on a carbon foil. The structure and orientation of the cluster ions after passing through the target were determined by the Coulomb explosion imaging method, and the positive secondary ions were simultaneously measured by time-of-flight mass spectrometry. When looking at the orientation dependence of linear structured ${{\mathrm{C}}_{2}}^{+}, {{\mathrm{C}}_{3}}^{+}$, and ${{\mathrm{C}}_{4}}^{+}$ projectiles, parallel orientations with respect to the beam direction were found to enhance the secondary-ion yield compared with perpendicular orientations. Additionally, comparison of the structure dependence of ${{\mathrm{C}}_{3}}^{+}$ and ${{\mathrm{C}}_{4}}^{+}$ projectiles with linear and ring structures showed that the secondary-ion yield was larger for linear structures. Interestingly, there was a clear correlation between the secondary-ion yield and the deposited energy density in the cluster-ion track. The energy density was calculated with electronic stopping power for incident cluster ions using linear-response dielectric theory and the cross section of the track of the incident cluster considering its geometrical structure and orientation. These results suggest that the deposited energy density in the cluster-ion track is a key parameter in the secondary-ion emission process induced by swift cluster-ion impacts.

In this study, the recurrent fluorescence of naphthalene ions was investigated using an electrostatic ion beam trap. The number of photons from the recurrent fluorescence on the ${D}_{2}\ensuremath{\rightarrow}{D}_{0}$ transition was measured as a function of time, after the production of naphthalene ions by electron impact. The emission of photons was observed over a time range of approximately 10 ms. The variation in the number of photons for such a long time range could be explained by conducting simulations using previously reported theoretical rate constants of both the fragmentation and the recurrent fluorescence of vibrationally hot naphthalene ions.

The photodissociation of the monoanions of chlorophylls $a$ and $b$ in the gas phase were studied using an electrostatic storage ring. The number of neutral particles produced by the photoabsorption of the ions was measured as a function of time after irradiating the ions with a laser pulse with wavelengths in the region 410\ensuremath{-}660 nm. Fast and slow decay components were observed in the photodissociation spectra of the chlorophylls, and the lifetimes of these components increased with an increase in the wavelength of the irradiating laser pulse. It is shown that the slow decay component was due to one-photon excitation whereas the fast decay component was due to two-photon excitation. A statistical model was used to simulate the decay processes, and the simulated decay spectra were in good accordance with the experimentally determined ones. The activation energies of the decay processes of the two chlorophylls were derived using the model, and were found to be approximately 0.6 eV for both. The measured wavelength spectra were markedly different from the photoabsorption spectra of the chlorophylls in the solution phase for the $B$-band-to-$Q$-band ratio.

Electron-impact detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ was studied with the TARN II storage ring (at the Institute for Nuclear Study, University of Tokyo) and the associated high-resolution electron beam. The relative cross sections were measured for the relative energies between electron and ion from 0 to 60 eV in search of the ${\mathrm{H}}^{2\mathrm{\ensuremath{-}}}$ resonances that were reported earlier. No evidence was found for the existence of these resonance states. \textcopyright{} 1996 The American Physical Society.

The vibrational cooling of ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{D}}_{2}^{+},$ having no dipole moment, has been studied at the storage ring TARN II. A two-dimensional imaging technique was used to measure the kinetic-energy release (KER) in the dissociative recombination of ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{D}}_{2}^{+}$ as a function of the storage time at a relative energy of 0 eV. The vibrational-level populations of the ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{D}}_{2}^{+}$ beams were estimated from the KER data. The estimation shows that the relative population of ${\mathrm{H}}_{2}^{+}$ ions in their vibrational ground states reaches more than 90% at a storage time of 26 s. This result is in agreement with the recent result in TARN II [T. Tanabe et al., Phys. Rev. Lett. 83, 2163 (1999)]. The relative population of ${\mathrm{D}}_{2}^{+}$ ions in their vibrational ground states is estimated to be more than 90% for a storage time of 10 s.