We present new experimental and theoretical results for reactive scattering of dihydrogen from Cu(100). In the new experiments, the associative desorption of H 2 is studied in a velocity resolved and final rovibrational state selected manner, using time-of-flight techniques in combination with resonance-enhanced multi-photon ionization laser detection. Average desorption energies and rotational quadrupole alignment parameters were obtained in this way for a number of (v = 0, 1) rotational states, v being the vibrational quantum number. Results of quantum dynamics calculations based on a potential energy surface computed with a specific reaction parameter (SRP) density functional, which was derived earlier for dihydrogen interacting with Cu (111), are compared with the results of the new experiments and with the results of previous molecular beam experiments on sticking of H 2 and on rovibrationally elastic and inelastic scattering of H 2 and D 2 from Cu(100). The calculations use the Born-Oppenheimer and static surface approximations. With the functional derived semi-empirically for dihydrogen + Cu(111), a chemically accurate description is obtained of the molecular beam experiments on sticking of H 2 on Cu(100), and a highly accurate description is obtained of rovibrationally elastic and inelastic scattering of D 2 from Cu(100) and of the orientational dependence of the reaction of (v = 1, j = 2 − 4) H 2 on Cu(100). This suggests that a SRP density functional derived for H 2 interacting with a specific low index face of a metal will yield accurate results for H 2 reactively scattering from another low index face of the same metal, and that it may also yield accurate results for H 2 interacting with a defected (e.g., stepped) surface of that same metal, in a system of catalytic interest. However, the description that was obtained of the average desorption energies, of rovibrationally elastic and inelastic scattering of H 2 from Cu(100), and of the orientational dependence of reaction of (v = 0, j = 3 − 5, 8) H 2 on Cu(100) compares less well with the available experiments. More research is needed to establish whether more accurate SRP-density functional theory dynamics results can be obtained for these observables if surface atom motion is added to the dynamical model. The experimentally and theoretically found dependence of the rotational quadrupole alignment parameter on the rotational quantum number provides evidence for rotational enhancement of reaction at low translational energies.

Abstract:The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two timedelayed partial beams directly on a CCD camera. The partial beams are obtained by wave front beam splitting in an autocorrelator operating at photon energies from hν = 30 to 200 eV. At zero delay a visibility of (0.63 ± 0.04) is measured. The delay of one partial beam reveals a coherence time of 6 fs at 23.9 nm. The visibility further displays a non-monotonic decay, which can be rationalized by the presence of multiple pulse structure. ©2008 Optical Society of America

Energy transfer to different degrees of freedom during the femtosecond-laser-induced recombinative desorption of D 2 from a deuterium-covered Ru͑0001͒ surface ͑D ads +D ads /Ru→ D 2,gas +Ru͒ has been investigated. ͑1+1Ј͒-resonance-enhanced multiphoton photoionization ͑REMPI͒ and time-of-flight ͑TOF͒ measurements are utilized to provide information on the internal and external ͑translational͒ energy content, respectively. Rovibrational population distributions of the reaction product are detected via various B 1 ⌺ u + ← X 1 ⌺ g + Lyman bands using tunable vacuum ultraviolet laser radiation in the resonant excitation step. Rotational quantum state populations in the vibrational ground state and the first excited state are measured yielding average rotational energies of ͗E rot ͘ / k B = 800 and 1500 K, respectively, for an absorbed laser fluence ͗F͘ of 85 J / m 2 . In addition, a mean vibrational energy of the desorbing molecules is extracted which amounts to ͗E vib ͘ / k B = 1200 K. Extensive TOF measurements enable complete energy balancing with ͗E trans ͘ /2k B = 2500 K at ͗F͘ =85 J/m 2 and underline the nonthermal and unequal energy partitioning between the different degrees of freedom within the reaction product. The effects of multidimensional electronic friction between substrate and adsorbate layer and peculiarities of the potential energy landscape governing the D 2 recombination are discussed.

The pulse duration of soft x-ray free-electron laser ͑FEL͒ radiation is directly measured by time-resolved observation of doubly charged helium ions at 51.8 eV. A wave front splitting autocorrelator produces two correlated FEL pulses with a resolution of better than a femtosecond. In the interesting intensity range from 10 13 to 10 16 W / cm 2 direct and sequential double ionization contribute to the ion yield which has significant influence on the correlation width, being a general feature at high photon energies. Here, a duration of L = ͑29Ϯ 5͒ fs is derived for the soft x-ray pulses at FLASH.

We report the first measurement of the preferential steric orientation of D 2 molecules associatively desorbing from a metal surface. The flux of D 2 desorbing from Pd(100) is probed by laser induced fluorescence with linearly polarized tunable vacuum ultraviolet radiation in the B 1 S 1 u ͑y 0 , J 0 , M 0 ͒ √ X 1 S 1 g ͑y 00 , J 00 , M 00 ͒ Lyman bands. In ͑y 00 0͒ an increasing positive alignment with rotational quantum number is observed up to J 00 6, establishing the preferred helicopter motion of the molecules. In ͑y 00 1͒ and also for (y 00 0, J 00 7 and 8) an isotropic J vector distribution is measured. PACS numbers: 68.45.Da, 33.20.Ni, 79.20.Rf, The dynamical processes which govern the associative desorption and its counterpart, dissociative adsorption, of hydrogen on transition metal surfaces are of general interest for the understanding of catalytic reactions on surfaces. Molecular beam studies of the dissociative adsorption behavior on palladium [1,2], tungsten [3,4], and platinum [5] single-crystal surfaces show that the initial sticking probability exhibits a minimum as the kinetic energy of the hydrogen molecules is increased. This behavior is often interpreted as precursor mediated adsorption. Internal state selective studies revealed an enhanced vibrational population for desorption from Pd (100) [6], similar to the desorption of hydrogen from copper single-crystal surfaces [7,8]. This behavior could be reconciled by quantum mechanical calculations showing a late barrier in the dissociative adsorption potential, although with a considerably lower barrier height than in copper [6]. The rotational population distribution in the desorption flux has been measured earlier in our group [9]. It always showed lower rotational temperatures than the surface temperature. An interpretation of this behavior predicts a specific steric orientation of the molecular axis during desorption [10]. Similarly, interpreting the rotational state dependence of the velocity of H 2 and D 2 desorbing from Cu (111) [11], combined with the observed rotationaltranslational coupling, leads to the prediction of a preferential orientation of the molecular rotational axis parallel to the surface normal.Very recently a potential energy surface for the H 2 ͞Pd(100) system has been derived from density functional theory [12]. It shows activated as well as nonactivated pathways for dissociative adsorption, without any molecular precursor potential well. On this potential energy surface, Gross, Wilke, and Scheffler [13] performed the first six-dimensional quantum calculation for the dynamical behavior of a hydrogen/metal system. They could reproduce the main experimentally observed features very well: the decrease of the initial sticking coefficient with kinetic energy and the rotational cooling in desorption. A key element of this potential energy surface and their dynamical calculation is the steering of slow hydrogen molecules onto adsorption paths which do not show a barrier. In this calculation Gross, Wilke, and Scheffler predicted...