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Notes: We have investigated the electron paramagnetic resonance (EPR) of self-assembled stearic acid films adsorbed on an Al2O3-film. Doping the film with spin labels at different positions of the alkyl chain in order to make the films accessible for EPR spectroscopy provides an opportunity to investigate the rotational motion of the molecule along the alkyl chain. The temperature dependent EPR spectra show a strong dependence of the rotational motion of the molecules with variation of the location of the spin label along the chain. We study the rotational motion by means of the EPR line shape analysis. © 1998 American Institute of Physics.

Notes: In order to interpret the experimental results of the state resolved UV-laser-induced desorption of NO from NiO(100) (rotational and vibrational populations, velocity distributions of the desorbing NO molecules, etc.), we have performed ab initio complete active space self-consistent field (CASSCF) and configuration interaction (CI) calculations for the interaction potential between NO and the NiO(100) surface in the electronic ground state and for those excited states which are involved in the desorption process. The NiO(100)–NO distance and the tilt angle between the NO axis and the surface normal have been varied. A cluster model containing a NiO8−5-cluster embedded in a Madelung potential has been used for representing the NiO(100) surface. The excited states which are important for the desorption process, are charge transfer states of the substrate–adsorbate system, in which one electron is transferred from the surface into the NO-2π-orbital. The potential curves of these excited charge transfer states show deep minima (4 eV–5 eV) at surface/NO distances which are smaller than that in the ground state. The angular dependence of these potentials behaves similar as in the ground state. A semiempirical correction to the calculated excitation energies has been added which makes use of the bulk polarization of NiO. With this correction the charge transfer states are considerably stabilized. The lowest excitation energy amounts to about 4 eV which is in reasonable agreement with the onset of the laser desorption observed experimentally at about 3.5 eV. The density of the NO−-like states is rather high, so that probably several excited states are involved in the desorption process. The potential energy curves for all of these states are quite similar, but the transitions from the ground state into different excited charge transfer states show strongly differing oscillator strengths, which are also strongly dependent on the surface/NO distance. This fact is important for the dynamics of the deexcitation process in the sense of a selection criterion for the states involved. The magnitude of the oscillator strengths is large in comparison with the excitation of NO in the gas phase, which might be an indication for the possibility of optical excitation processes. One dimensional wave packet calculations on two potential energy curves using fixed lifetimes for the excited state in each calculation have been performed and enable us to estimate the mean lifetime of the excited state to be 15 fs≤τ≤25 fs. This implies that the dynamics of the system is dominated by the attractive part of the excited state potential. © 1996 American Institute of Physics.

Notes: We have studied the UV-laser-induced desorption of NO adsorbed on an epitaxial film of NiO(111) grown on Ni(111). The desorbing molecules were detected state selectively via a resonance enhanced ionization technique [REMPI(1+1)] using the A 2Σ(v'=0,1,2)←X 2Π(v‘=0,1,2) transition as intermediate state. Our results are compared with our experiments on NO desorption from NiO(100). The similarities and differences of the results due to the different surface structure of the polar NiO(111) and the non polar NiO(100) are discussed. For both surfaces we observe bimodal velocity flux distributions independent of the rovibrational state. Due to a rotational temperature of about 400 K and a vibrational temperature of 1800 K thermal processes can be ruled out. The wavelength dependence of the desorption cross section strongly correlates with the electronic structure of the NiO indicating a surface mediated excitation process. The spin orientation in the NO molecules influences the life time of the excited state depending on the magnetic property of the NiO surface.

Notes: A design of an electron gun system is presented whose stray light emission is reduced by about three orders of magnitude compared to a regular low-energy electron diffraction gun. This is achieved by a combination of a BaO cathode run at rather low temperature and a 30° tandem parallel-plate analyzer used as an optical baffle. The system provides a high beam current of several microampers at 50 eV beam energy. The system can be used down to ∼10 eV. © 1999 American Institute of Physics.

Notes: We describe a newly developed ultrahigh vacuum (UHV) experiment which combines molecular beam techniques and in situ surface spectroscopy. It has been specifically designed to study the reaction kinetics and dynamics on complex model catalysts. The UHV system contains: (a) a preparation compartment providing the experimental techniques which are required to prepare and characterize single-crystal based model catalysts such as ordered oxide surfaces or oxide supported metal particles; and (b) the actual scattering chamber, where up to three molecular beams can be crossed on the sample surface. Two beams are produced by newly developed differentially pumped sources based on multichannel arrays. The latter are capable of providing high intensity and purity beams and can be modulated by means of a vacuum-motor driven and computer-controlled chopper. The third beam is generated in a continuous or pulsed supersonic expansion and is modulated via a variable duty-cycle chopper. Angular and time-resolved measurements of desorbing and scattered molecules are performed with a rotatable doubly differentially pumped quadrupole mass spectrometer with a liquid-nitrogen cooled ionizer housing. Time-resolved but angle-integrated measurements are realized with a second nondifferentially pumped quadrupole mass spectrometer. In situ measurements of adsorbed species under reaction conditions are performed by means of an adapted vacuum Fourier transform infrared spectrometer. The spectrometer provides the possibility of time-resolved measurements and can be synchronized with any of the beam sources. This contribution provides a general overview of the system and a description of all new components and their interplay. We also present test data for all components employing simple adsorption/desorption and reaction systems. © 2000 American Institute of Physics.

Notes: In recent experiments, the rotational alignment of the laser induced nonthermal desorption of CO adsorbed on an epitaxially grown film of Cr2O3(0001) has been studied [Beauport, Al-Shamery, and Freund, Chem. Phys. Lett. 256, 641 (1996)]. At low-rotational quantum numbers J, the molecules desorb like helicopters (J-vector perpendicular to the surface) while at high J-values cartwheel motion is preferred (J-vector parallel to the surface). These stereodynamic effects and the experimental state resolved velocity distributions of the desorbing species are simulated by means of an exact time-dependent wave packet method in three dimensions. As a basis for this quantum-mechanical treatment of the CO desorption ab initio potential energy surfaces (PES) were used. The PES for the electronic ground state of the CO–Cr2O3(0001) system has been calculated previously by Pykavy et al. [Surf. Sci. 479, 11 (2001)] in an embedded cluster approach. As the intermediate state, generated by the laser irradiation, an internal CO excited state (5σ→2π*) was considered. The PES of this a 3Π-like state of CO adsorbed on Cr2O3(0001) was calculated at the ab initio CASSCF-level. Our key findings in the subsequent wave packet calculations are (1) a high-dimensional treatment of the photodesorption process is very important in this system, essentially the angular coordinates, very often neglected in similar studies, are responsible for a "successful" desorption event; (2) the change from the strongly tilted equilibrium geometry in the electronic ground state to the preferred upright position in the electronically excited state after laser irradiation is essential for the mechanistic picture of the desorption process; (3) the experimental phenomemon of rotational alignment can only be explained if the topologies of the PES of both the electronically excited and the ground state are accounted for; (4) the lifetime of the CO*-intermediate is in the order of 10 fs; (5) the molecule–surface vibrations in the electronic ground state do not much influence the asymptotic results at the experimental temperature of T=100 K. However, the inclusion of excited levels of the hindered rotation helps to gain insight into the desorption mechanism on a microscopic level. © 2002 American Institute of Physics.

Notes: After UV-laser-induced desorption we observe bimodal velocity distributions independent of internal vibrational excitation [up to v=2 (4%)] applying resonance-enhanced multiphoton ionization techniques. Both contributing desorption channels are of nonthermal origin. We introduce a model where the two desorption channels are correlated with the rupture of the molecule surface bond of the librating molecule either on the way toward or away from the surface. We have performed trajectory calculations to simulate the desorption processes. The calculated momentum distributions of the desorbing molecules show either one or two maxima, depending on lifetime, in agreement with experimental results. The vibrational distribution of the desorbing molecules can be reproduced by assuming transition into a state that is characterized by an altered N–O bond length as it is found, for example, in NO−. The model calculations both for velocity distributions and vibrational excitations result in similar lifetimes of the excited state, even though the translational and the vibrational degree of freedom of the desorbing molecules are decoupled.