G. Dujardin

Double photoionization of SO2 and fragmentation spectroscopy of SO2++ studied by a photoion-photoion coincidence method

Abstract Doubly charged sulphur dioxide cations (SO 2 ++ ) are produced by photoionization with synchrotron radiation from ACO in the excitation-energy range 34–54 eV. A new photoion-photoion coincidence (PIPICO) experiment is described in which coincidences between photoion fragments originating from the dissociation of the doubly charged parent cation are counted. This PIPICO method enables us to study the fragmentation of individual electronically excited states of SO 2 ++ and to determine the corresponding absolute double-photoionization partial cross sections as a function of the excitation energy. A tentative assignment of the three observed α, β and γ SO 2 ++ states is given. The dissociation processes of the α and β states into the products SO + + O + are found to be non-statistical in nature; the γ state dissociates completely into three atomic fragments S + + O + + O. Three main observed features of the double-photoionization cross-section curves are discussed in the text: appearance potentials, linear threshold laws, and constant double-photoionization cross sections relative to the total ionization cross section at high energies.

Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2 × 1)

We report results on the self-assembly of silicon nanoribbons (NRs) on the (2 × 1) reconstructed Au(110) surface under ultra-high vacuum conditions. Upon adsorption of 0.2 monolayer (ML) of silicon, the (2 × 1) reconstruction of Au(110) is replaced by an ordered surface alloy. Above this coverage, a new superstructure is revealed by low energy electron diffraction (LEED), which becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons all oriented along the [1¯10] direction as revealed by LEED and scanning tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In addition, the silicon atoms show signatures of two chemical environments corresponding to the edge and center of the ribbons.

Atomic-scale imaging of insulating diamond through resonant electron injection.

The electronic properties of insulators such as diamond are of interest not only for their passive dielectric capabilities for use in electronic devices, but also for their strong electron confinement on atomic scales. However, the inherent lack of electrical conductivity in insulators usually prevents the investigation of their surfaces by atomic-scale characterization techniques such as scanning tunnelling microscopy (STM). And although atomic force microscopy could in principle be used, imaging diamond surfaces has not yet been possible. Here, we demonstrate that STM can be used in an unconventional resonant electron injection mode to image insulating diamond surfaces and to probe their electronic properties at the atomic scale. Our results reveal striking electronic features in high-purity diamond single crystals, such as the existence of one-dimensional fully delocalized electronic states and a very long diffusion length for conduction-band electrons. We expect that our method can be applied to investigate the electronic properties of other insulating materials and so help in the design of atomic-scale electronic devices.

Photoion–fluorescence photon coincidence study of radiative and dissociative relaxation processes in VUV photoexcited SO2. Fluorescence of SO+2, SO+, and SO

Line photoexcitation sources in the range 10.2–21.2 eV are used to produce excited electronic states of SO2 and/or SO+2. Photoion–fluorescence photon coincidence measurements show that SO+2 states in the 16 eV region fluoresce at λ<370 nm, with a quantum yield φF(? 2B2+? 2A1)∼6×10−5, in competition with predissociation. The products of predissociation of excited states of SO+2 are identified by their TOF mass spectra and/or dispersed optical emission. SO+(A 2Π–X 2Π), SO(B 3Σ−–X 3Σ−), and SO(A 3Π–X 3Σ−) emissions are observed when the excited products are formed with ionic or (super) excited states of SO2. The measured lifetime of the SO+A 2Π state is 2.4±0.4 μs. The results are discussed in relation to photoion–photoelectron measurements and in the context of unimolecular dissociation models.

Discovery of an emission in NO

Doubly charged NO++ ions were produced by double photoionization of neutral nitric oxide molecules with the synchrotron radiation from ACO as a photon source of variable energy in the 35–68 eV range. Two stable states of NO++ were observed in the double photoionization spectrum with energies of 38.6 and 40.0 eV. An emission from NO++ was discovered by using a new photoion–photon of fluorescence coincidence (PIFCO) experiment. The fluorescence efficiency was measured as a function of the excitation energy, indicating that the emitting state, whose onset energy was observed at 42.5 eV, is a stable or slowly predissociating state of NO++. On the basis of the comparison with calculated energies, the three stable states of NO++ with energies at 38.6, 40.0, and 42.5 eV were assigned to, respectively, the X 2Σ+, A 2Π, and B 2Σ+ electronic states. The detected emission was ascribed to a B 2Σ+→X 2Σ+ emission around 3170 A.

Imaging Molecular Orbitals by Scanning Tunneling Microscopy on a Passivated Semiconductor

Decoupling the electronic properties of a molecule from a substrate is of crucial importance for the development of single-molecule electronics. This is achieved here by adsorbing pentacene molecules at low temperature on a hydrogenated Si(100) surface (12 K). The low temperature (5 K) scanning tunneling microscope (STM) topography of the single pentacene molecule at the energy of the highest occupied molecular orbital (HOMO) tunnel resonance clearly resembles the native HOMO of the free molecule. The negligible electronic coupling between the molecule and the substrate is confirmed by theoretical STM topography and diffusion barrier energy calculations.

Fluorescence quantum yields and lifetimes of nine fluorobenzene cations. Electronic state relaxation processes

Abstract Recent improvements are described concerning the photoion—fluorescence photon coincidence technique developed for measuring quantum yields and lifetimes of optical emission of excited states of molecular ions. Measurements were made an C6F+6, C6F5H+, the three C6F4H+2 ions, 1,2,4-C6F3H+3, 1,3,5-C6F3H+3, 1,3,5-C6F3D+3 and 1,3-C6F2H+4 using 21.22, 16.85 and 15.1 eV photoionization. Time of flight mass spectra indicate that some fragment ions may be formed via a predissociation process. Parent ion B state emission quantum yields range from 0.003 to 0.5 except for the 1,3,5-trifluorobenzene ions. For the latter, apparent quantum yields greater than unity appear to be due to partial population of B by cascade from a higher C state. After correcting for this process the quantum yields of the 1,3,5-trifluorobenzene cations are found to be 0.87 ± 0.13. The results are interpreted in terms of the large molecule, statistical limit case of radiationless transition models.

Evolution and properties of nanodiamond films deposited by direct current glow discharge

Nanocrystalline carbon films possessing a prevailing diamond character are deposited by a direct current glow discharge chemical vapor deposition method using a 9:91 vol % methane to hydrogen gas mixture. In the present work the evolution and properties of nanodiamond films deposited by this method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nanodiamond character, a graphitic phase is formed. After the nanodiamond phase is stabilized, near edge x-ray adsorption fine structure measurements proved the predominant diamond character of the film to be about 80%. By electron energy loss spectroscopy analysis the sp2-like character of the nanodiamond grain boundaries has been determined. The nanodiamond films were found to be thermally stable up to temperatures of ∼950 °C as established by vacuum heating. By scanning electron microscopy and atomic force microscopy the ...

Sensitivity of near-edge x-ray absorption fine structure spectroscopy to ion beam damage in diamond films

In the present work, we study the sensitivity of the near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to ion induced defects in polycrystalline diamond films. The ion bombardment of hydrogenated films is performed using 30 keV Xe+ ions at room temperature for doses ranging from 2×1013 ions/cm2, producing local point defects, to 2×1015 ions/cm2, which results in almost complete amorphization of the diamond surface. Auger electron spectroscopy measurements are not sensitive to the modifications induced by the lowest implantation dose. Whereas partial electron yield (PEY) NEXAFS measurements, applied in surface and bulk-sensitive modes, using 35, 15, and 8 eV secondary electrons, respectively, reveals the formation of a defective structure and gradual deterioration of diamond in the near-surface region. From PEY NEXAFS spectra measured using 15 eV secondary electrons, the position of C(1s) binding energy is measured. The x-ray photoelectron spectra of the samples were measured using an inciden...

Valence and inner shell electronic processes in dissociative double photoionization of CH3I

Abstract The dissociative double photoionization of CH 3 I has been studied in the 28–62 eV photon energy range by using the photoion-photoion coincidence (PIPICO) method. At lower energies ( 3 I 2+ cations were produced by direct double photoionization involving the ejection of two valence electrons. Six channels for the dissociation of CH 3 I 2+ into two or three fragments were detected and analysed in detail. Their appearance potentials were measured, giving evidence that, at least, four electronic states of CH 3 I 2+ are involved in these fragmentation processes. At higher energies ( > 50eV), we observed resonant double photoionization associated with the electronic transitions from the I(4d) core levels to the lowest unoccupied a * 1 level and to the molecular Rydberg orbitals 6p and 7p of CH 3 I. The shape resonance of the I(4d → ef) transition was also observed in the double photoionization spectra. Doubly charged CH 3 I 2+ ions produced via the I(4d → ef) shape resonance were found to specifically dissociate as compared to those produced via the I(4d → a * 1 ) resonance or by direct double photoionization.