[Show abstract][Hide abstract] ABSTRACT: We present an analysis method of normal incidence x-ray standing wave (NIXSW) data that allows detailed adsorption geometries of complex molecules to be retriev ed. This method (Fourier vector analysis) is based on the comparison of both the coherence and phase of NIXSW data to NIXSW simulations of different molecular geometries as the relevant internal degrees of freedom are tuned. We introduce this analysis method using the prototypical molecular switch azobenzene (AB) adsorbed on the Ag(111) surface as a model system. The application of the Fourier vector analysis to AB/Ag(111) provides, on the one hand, detailed adsorption geometries including dihedral angles, and on the other hand, insights into the dynamics of molecules and their bonding to the metal substrate. This analysis scheme is generally applicable to any adsorbate, it is necessary for mole cules with potentially large distortions, and will be particularly valuable for molecules whose distortion on adsorption can be mapped on a limited number of internal degrees o f freedom.
[Show abstract][Hide abstract] ABSTRACT: The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of nonlocal collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial.
Physical Review B 07/2013; 88(3):035421. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We use time-resolved two-photon photoemission to study two molecular photoswitches at the Au(111) surface, namely azobenzene and its derivative tetra-tert-butyl-azobenzene (TBA). Electronic states located at the substrate–adsorbate interface are found to be a sensitive probe for the photoisomerization of TBA. In contrast to TBA, azobenzene loses its switching ability at the Au(111) surface. Besides the different adsorption geometries of both molecules, we partly attribute the quenching in the case of azobenzene to a shift of the highest occupied molecular orbital (HOMO) with respect to the gold d-bands, which renders the hole transfer involved in the photoisomerization mechanism of TBA inefficient.
New Journal of Physics 04/2012; 14(4). · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Time- and angle-resolved two-photon photoemission (2PPE) spectroscopies have been used to investigated the electronic structure, electron dynamics and localization at the interface between tetra-tert-butyl imine (TBI) and Au(111). At a TBI coverage of one monolayer (ML), the two highest occupied molecular orbitals, HOMO and HOMO-1, are observed at an energy of −1.9 and −2.6 eV below the Fermi level (EF), respectively, and coincide with the d-band features of the Au substrate. In the unoccupied electronic structure, the lowest unoccupied molecular orbital (LUMO) has been observed at 1.6 eV with respect to EF. In addition, two delocalized states that arise from the modified image potential at the TBI/metal interface have been identified. Their binding energies depend strongly on the adsorption structure of the TBI adlayer, which is coverage dependent in the submonolayer (≤1 ML) regime. Thus the binding energy of the lower interface state (IS) shifts from 3.5 eV at 1.0 ML to 4.0 eV at 0.5 ML, which is accompanied by a pronounced decrease in its lifetime from 100 fs to below 10 fs. This is a result of differences in the wave function overlap with electronic states of the Au(111) substrate at different binding energies. This study shows that in order to fully understand the electronic structure of organic adsorbates at metal surfaces, not only adsorbate- and substrate-induced electronic states have to be considered but also ISs, which are the result of a potential formed by the interaction between the adsorbate and the substrate.
New Journal of Physics 12/2010; · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We examine the influence of the adsorption geometry and electronic coupling between azobenzene-based molecular switches and metal surfaces on their photoisomerization ability. Using the normal-incidence X-ray standing wave technique and large-scale density functional theory (DFT) calculations we determine adsorption geometries for azobenzene and 3,3´,5,5´-tetra-tert-butyl-azobenzene (TBA) adsorbed on Ag(111). Comparing the experimental determined and calculated vertical bonding distances between the photochemically active diazo (-N=N-) moiety of both molecules, reveals that the photoisomerization ability is rather insensitive on the adsorption height, as the N-Ag adsorption distance in TBA/Ag(111) is only 0.14 Å (0.13 Å for the calculated value) larger than the corresponding value for azobenzene. Our DFT calculations predict also similar adsorption heights of the diazo-bridge for azobenzene and TBA adsorbed on Au(111) even though TBA undergoes a photoinduced isomerization while these process is suppressed in azobenzene/Au(111). The photoisomerization ability of TBA/Au(111) and its suppression for azobenzene on Au(111), Ag(111) as well as for TBA on Ag(111) thus demonstrate that a purely geometrical argumentation explaining the isomerization properties fails. Thus the electronic structure of the complete adsorbate/substrate complex has to be taken into account in order to control molecular functionality at surfaces.
Chemical Physics Letters 10/2010; · 1.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy have been employed to follow the reversible trans to cis isomerization of tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). For one monolayer the molecules adopt an adsorption geometry characteristic of the trans-TBA isomer. The azo-bridge (N = N) is aligned nearly parallel to the surface and the phenyl rings exhibit a planar orientation with a small tilt angle <or=4 degrees with respect to the surface normal. Illumination of the molecular layer at 455 nm triggers the trans to cis isomerization which is associated with a pronounced change of the geometrical and electronic structure. The N1s to pi* transition of the central azo-bridge shifts by 0.45 +/- 0.05 eV to higher photon energy and the transition dipole moment (TDM) is tilted by 59 +/- 5 degrees with respect to the surface normal. The pi-system of one phenyl ring is tilted by about 30 degrees with respect to the surface normal, while the second ring plane is oriented nearly perpendicular to the surface. This reorientation is supported by a shift and broadening of the C-H resonances associated with the tert-butyl legs of the molecule. These findings support a configuration of the photo-switched TBA molecule on Au(111) which is comparable to the cis-isomer of the free molecule. In the photo-stationary state 53 +/- 5% of the TBA molecules are switched to the cis configuration. Thermal activation induces the back reaction to trans-TBA.
Physical Chemistry Chemical Physics 05/2010; 12(17):4488-97. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We employ normal-incidence x-ray standing wave and temperature programed desorption spectroscopy to derive the adsorption geometry and energetics of the prototypical molecular switch azobenzene at Ag(111). This allows us to assess the accuracy of semiempirical correction schemes as a computationally efficient means to overcome the deficiency of semilocal density-functional theory with respect to long-range van der Waals (vdW) interactions. The obtained agreement underscores the significant improvement provided by the account of vdW interactions, with remaining differences mainly attributed to the neglect of electronic screening at the metallic surface.
[Show abstract][Hide abstract] ABSTRACT: High resolution electron energy loss spectroscopy and two-photon photoemission was employed to derive the adsorption geometry, electronic structure, and the photoisomerization ability of the molecular switch tetra-tert-butyl-stilbene (TBS) on Au(111). The results are compared with the azobenzene analogue, tetra-tert-butyl-azobenzene (TBA), adsorbed on Au(111). TBS was found to adsorb on Au(111) in a planar (trans) configuration similar to TBA. The energetic positions of several TBS-induced electronic states were determined, and in comparison to TBA, the higher occupied molecular states (e.g., the highest occupied molecular orbital, HOMO) are located at similar energetic positions. While surface-bound TBA can be switched with light between its trans and cis configurations, in TBS this switching ability is lost. In TBA on Au(111), the trans → cis isomerization is driven by a substrate-mediated charge transfer process, whereby photogenerated hot holes in the Au d band lead to transient positive ion formation (transfer of the holes to the TBA HOMO level). Even though the energetic positions of the HOMOs in TBA and TBS are almost identical and thus a charge transfer should be feasible, this reaction pathway is obviously not efficient to induce the trans → cis isomerization in TBS on Au(111). Quantum chemical calculations of the potential energy surfaces for the free molecules support this conclusion. They show that cation formation facilitates the isomerization for TBA much more pronounced than for TBS due to the larger gradients at the Franck−Condon point and the much smaller barriers on the potential energy surface in the case of the TBA.
Journal of Physical Chemistry C - J PHYS CHEM C. 12/2009; 114(2).
[Show abstract][Hide abstract] ABSTRACT: Two-photon photoemission spectroscopy is employed to elucidate the electronic structure and the excitation mechanism in the photoinduced isomerization of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). Our results demonstrate that the optical excitation and the mechanism of molecular switching at a metal surface is completely different compared to the corresponding process for the free molecule. In contrast to direct (intramolecular) excitation operative in the isomerization in the liquid phase, the conformational change in the surface-bound TBA is driven by a substrate-mediated charge transfer process. We find that photoexcitation above a threshold hnu approximately 2.2 eV leads to hole formation in the Au d-band followed by a hole transfer to the highest occupied molecular orbital of TBA. This transiently formed positive ion resonance subsequently results in a conformational change. The photon energy dependent photoisomerization cross section exhibit an unusual shape for a photochemical reaction of an adsorbate on a metal surface. It shows a thresholdlike behavior below hnu approximately 2.2 eV and above hnu approximately 4.4 eV. These thresholds correspond to the minimum energy required to create single or multiple hot holes in the Au d-bands, respectively. This study provides important new insights into the use of light to control the structure and function of molecular switches in direct contact with metal electrodes.
The Journal of Chemical Physics 11/2008; 129(16):164102. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two-photon photoemission (2PPE) spectroscopy is employed to quantify the photochemically and thermally induced trans cis isomerization of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on an Au(111) surface. The isomerization of TBA is accompanied by sig-nificant changes in the electronic structure, namely differ-ent energetic positions of the lowest unoccupied molecu-lar orbital of both isomers and the appearance of an unoc-cupied final state for cis-TBA. A quantitative analysis of these effects allows the calculation of cross sections for the reversible isomerization and determination of the ratio between both isomers in the photostationary state, where 55 ± 5% of the molecules are switched to cis-TBA. The cross section for the photoinduced trans → cis isomeriza-tion is 3.3 ± 0.5 × 10 −22 cm 2 , while for the back reaction, a value of 2.7 ± 0.5 × 10 −22 cm 2 is obtained. Furthermore a pronounced reduction of the activation energy by a factor of four compared to the free molecule is found for the ther-mally activated cis → trans isomerization of the surface-adsorbed TBA. This demonstrates that the potential energy landscape of the adsorbed TBA is remarkably different from the liquid phase.
Applied Physics A 11/2008; 9320:253-260. · 1.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two-photon photoemission spectroscopy is employed to analyze reversible changes in the electronic structure of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(1 1 1), which are induced by UV-light and thermal activation. Cycles of illumination and annealing steps confirm the reversibility of the switching process, which we assign to a trans/cis-isomerization of TBA molecules in direct contact with the Au(1 1 1) surface. Pronounced changes in the photoelectron spectra due to UV-light exposure allow to calculate an effective cross section of r eff % 8 · 10 À22 cm À2 at 4.14 eV and r eff % 4 · 10 À21 cm À2 at 4.4 eV, respectively, for the trans-to cis-isomerization.
Chemical Physics Letters 08/2007; 444(s 1–3). · 1.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Occupied and unoccupied electronic states in tetra-tert-butyl-azobenzene (TBA) absorbed on Ag(111) have been investigated by one-photon and two-photon photoemission spectroscopy.
These measurements allow the quantitative determination of energetic positions of the highest occupied (HOMO) and the lowest
unoccupied molecular orbital (LUMO) as well as the n=1 image potential state. The assignment of the electronic states are
supported by quantum chemical calculations. Experimentally a HOMO–LUMO gap of 2.85eV is observed, whereas the gap obtained
from the calculated molecular orbital energies is 0.92eV larger. This discrepancy can be explained by image charge screening.
Furthermore, two unoccupied final states located 0.18 and 0.43eV above the vacuum level, respectively, have been identified.
Applied Physics A 08/2007; 88(3):465-472. · 1.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The adsorption of 1,3,5,7-cyclooctatetraene (COT) on Ru(001) is studied by temperature-programmed desorption (TPD), work function measurements, as well as time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. The TPD data show that COT films grow at 115K in a metastable phase when the coverage is increased from the chemisorbed monolayer to the bulk-like molecular multilayer structure. The metastable states desorb at a temperature which is ≈9K lower than the desorption temperature of the stable multilayer. At 165±2K, they undergo an irreversible and thermally activated transformation into the stable multilayer phase. This transition is accompanied by a pronounced increase in the total 2PPE yield by more than one order of magnitude as well as the appearance of image potential states. The image states have binding energies of −0.70eV and −0.24eV for the n=1 and n=2 states, respectively, and a lifetime of 20±5fs for both states. Their appearance is interpreted as an indication of island formation in the stable multilayer regime. 2PPE spectroscopy of the image potential states provides a sensitive probe of structural transitions in the adsorbate layers.