Nils Mårtensson

Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution and intramolecular thermalization of excited states occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution, to show that electron transfer from an aromatic adsorbate to a TiO2 semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes.

Physisorbed, chemisorbed and dissociated O2 on Pt(111) studied by different core level spectroscopy methods

Abstract For O 2 Pt (111) we have found four different adsorption phases which are formed at different substrate temperatures. At about 25 K the oxygen molecules physisorb on the surface. Two chemisorbed phases are observed at 90 and 135 K, respectively. An atomic phase, characterized by a sharp (2 × 2) LEED pattern, exists at a temperature above 150 K. Different spectroscopic techniques have been used to characterize the four different adsorption states: XPS studies of adsorbate and surface core level shifts, UPS, NEXAFS, autoionization and Auger spectroscopy. We conclude that oxygen adsorbs in two different molecular chemisorbed states which can be considered to be precursors for the thermally activated atomization process. The first of these molecular states is weakly chemisorbed at 90 K. It is adsorbed in a hollow site with a saturation coverage of 0.23 (molecules per Pt surface atom). We have identified this phase as a superoxo-like configuration. The second phase is more strongly bonded to the Pt substrate. It is characterized by a longer and weaker molecular σ bonding due to more charge transfer from the metallic substrate to the antibonding molecular 1 π g orbitals than for the first chemisorbed phase. With a coverage of 0.15 the oxygen molecules seem to be adsorbed in hollow or hollow-bridge sites. We have characterized this phase as a peroxo-like configuration of the oxygen molecule. For atomic oxygen on platinum we have found a coverage of 0.25 (oxygen atoms per Pt surface atom) and a threefold adsorption site, in agreement with previous studies. We discuss the XAS results according to a model for the density of states induced by the hybridization of the 2p atomic orbitals with the 6sp states and 5d band of the metal.

A very high resolution electron spectrometer

Abstract The construction of a new electron energy analyzer for photoelectron spectroscopy is described. The analyzer is a full hemisphere with a mean radius of 200mm. The spectrometer incorporates highly stable voltage supplies and is equipped with a multidetection system. The electron lens can be operated in different modes, optimizing transmission, spatial resolution or angular resolution. An angular resolution of better than 0.2° can be obtained. UV excited Xe5p spectra recorded in the gas phase show that the energy resolution is better than 2.7 meV at 2eV analyzer pass energy.

Adsorption of Bi-Isonicotinic Acid on Rutile TiO2 (110)

Bi-isonicotinic acid ~2,28-bipyridine–4,48-dicarboxylic acid! is the ligand of several organometallic dyes, used in photoelectrochemical applications. Therefore the atomic scale understanding of the bonding of this molecule to rutile TiO2(110) should give insight into the crucial dye–surface interaction. High resolution x-ray photoelectron spectroscopy ~XPS!, near edge x-ray absorption fine structure ~NEXAFS!, and periodic intermediate neglect of differential overlap ~INDO! calculations were carried out on submonolayer bi-isonicotinic acid rutile TiO2(110). Data from multilayers is also presented to support the submonolayer results. For a multilayer, XPS shows that the carboxyl groups remain in the ~pristine! protonated form, and NEXAFS show that the molecular plane is tilted by 57° with respect to the surface normal. For the submonolayer, the molecule bonds to the rutile TiO2(110) surface via both deprotonated carboxyl groups, with a tilt angle of 25°, and additionally an azimuthal orientation of 44° with respect to the @001# crystallographic direction. The adsorbant system was also investigated by quantum mechanical calculations using a periodic INDO model. The most stable theoretical adsorption geometry involves a twist around the molecular axis, such that the pyridine rings are tilted in opposite directions. Both oxygen atoms of each carboxyl group are bonded to five-fold coordinated Ti atoms ~2M-bidentate!, in excellent agreement with the experimental results.

Overlayer structure from adsorbate and substrate core level binding energy shifts: CO, CCH3 and O on Pt(111)

Abstract By combining high resolution photoemission measurements of adsorption induced binding energy shifts of both adsorbate and substrate core levels the nature and distribution of adsorption sites in the CO/Pt(111) system can be determined. The existence of different surface shifted components demonstrates the local character of the surface core level shift. This is used to study the O/Pt(111) (2 × 2) and CCH3/Pt(111) “(2 × 2)” phases. The intensity relations of the different surface peaks suggest O/Pt(111) to be a true (2 × 2) phase, while the CCH3/Pt(111) “(2 × 2)” phase is proposed to consist of three equivalent (2 × 1) domains.

Experimental L and M Core Level Binding Energies for the Metals 22Ti to 30Zn

Accurate core level binding energies for the L and M shells in the metals 22Ti to 30Zn have been measured by means of ESCA. Comparisons with previously reported data in many cases show discrepancies of several eV. The spin-orbit splittings of the 2p subshells have been measured with high accuracy and are compared with theoretical values.

Interaction of rhodium with hydroxylated alumina model substrates

In order to investigate how metal growth and metal-oxide interaction depend on the chemical properties of oxide surfaces, we describe a modification procedure which allows the introduction of surface hydroxyl groups on a well-ordered Al2O3 film on NiAl(110). The modification — based on deposition of metallic Al and subsequent water exposure — is characterized using LEED spot-profile analysis (SPA-LEED) and high-resolution photoelectron spectroscopy (PES). Upon Al deposition, small aggregates are formed, which are oxidized completely in the final preparation step as verified via PES. The presence of OH-groups is supported by the appearance of additional Al 2p and O 1s surface features. The origin of oxide core and valence level binding energy shifts induced by the modification procedure is discussed. Growth and metal-substrate interaction of Rh deposited onto the hydroxylated Al2O3 film is compared to Rh growth on the non-modified oxide surface. It is shown that at 300 K nucleation preferentially occurs on modified oxide areas (SPA-LEED). Photoelectron spectroscopy of both oxide and rhodium core levels points to a direct chemical interaction between the metal and surface hydroxyl groups.

Beamline I511 at MAX II, capabilities and performance

The new undulator beamline I511 at MAX-lab, now under commissioning, has been optimized for X-ray emission and photoelectron spectroscopies. Using an SX-700 high flux monochromator the accessible photon energy range is from 90 eV to about 1500 eV. The per

Electron shake-up and correlation satellites and continuum shake-off distributions in X-Ray photoelectron spectra of the rare gas atoms

Abstract The rare gases H, He, Ne, Ar, Kr and Xe have been studied in the gas phase at low pressure by means of high resolution photoelectron Spectroscopy. Monochromatized Al Kα radiation at 90° was used to excite the photoelectrons. Extended energy regions of all valence electron spectra and the most interesting core electron spectra have been recorded. The spectra contain a large number of correlation satellites and shake-up as well as double shake-up satellites which originate from unknown highly excited, singly ionized states. The energies and intensities of these discrete satellites have been determined and several states are identified. The assignments rely partly on results from relativistic MCSCF calculations, and partly on comparisons with optical data and recent Auger electron data. Relativistic MCSCF energies for a large number of excited states in Kr + and Xe + are reported. Extended shake-off continua in both the valence region and in the core electron regions have been recorded. The shake-off continua of Ne and Ar reveal several Fano-like resonance states. The resonances occur when shake-up photoionization channels, involving inner valence electron excitation or multiple valence excitation, have the same energies and symmetries as photoionization channels for outer valence electron shake-off. A theoretical model for this type of resonance is presented and some numerical results are obtained and compared with experimentally determined Fano parameters for the most prominent resonance in the Nels shake-off spectrum.

Lifetime Broadening and CI-Resonances Observed in ESCA

The electron spectrum of the M2 and M3 levels in bromine and krypton have been studied by high resolution ESCA. The M1M2,3M2,3 super Coster-Kronig transitions become energetically forbidden for Zless than sim-36 (Kr) and recent calculations therefore predict a decrease in the M2 and M3 natural linewidths around krypton in the periodic system. The experiment shows that the 3p3/2 linewidth is smaller in Kr than in Br. It is, however, also found that this decrease in linewidth is followed by configuration interaction (CI) between 3p2P and 3d2nl*2P states. Several descrete CI resonances are observed in the Kr 3p spectrum. Such resonances are also studied in the N shell for the elements from Te (Z = 52) to Ba (Z = 56). For these elements the effect is found to be much larger due to the strong collective character of 4d-nf* excitations.