F Martín

Density functional theory study of H and H2 interacting with NiAl(110)

We present results of extensive density functional theory (DFT) calculations for H and H2 interacting with NiAl(110). Continuous representations of the full dimensional potential energy surface (PES) for the H/NiAl(110) and H2/NiAl(110) systems are obtained by interpolation of the DFT results using the corrugation reducing procedure. We find a minimum activation energy barrier of approximately 300 meV for dissociative adsorption of H2, which is consistent with the energy threshold obtained in molecular beam experiments for H2 (nu=0). We explain vibrational enhancement observed in experiments as the consequence of vibrational softening in the entrance channel over the most reactive surface site. The H2/NiAl(110) PES shows a high surface site selectivity: for energies up to 0.1 eV above threshold, H2 adsorption can only take place around top-Ni sites (within a circle of radius approximately 0.3 A). A strong energetic corrugation is observed: energy barriers for dissociation vary by more than 1 eV between the most and the least reactive sites. In contrast, geometric corrugation is much less pronounced and comparable to that of low index single metal surfaces like Cu or Pt.

Angular distribution of H2 molecules scattered from the Pd(111) surface

We study the scattering of H2 (v=0,J=0) molecules by the Pd(111) surface using classical trajectory methods. We show that the variation of the reflectivity with incidence angle can be explained with the combination of two processes: “dynamic trapping” and “direct dissociation” that verify total and normal energy scaling, respectively. The presence of the dissociation channel barely affects the angular distribution of scattered molecules. In the patterns of final angular distributions, the main difference, with respect to atom scattering, is a strong momentum transfer from motion normal to the surface toward molecular rotation.

Adsorption and scattering of H2 and D2 by NiAl(110).

We present quasiclassical dynamics calculations of H2 and D2 scattering by the NiAl(110) surface using a recently proposed six-dimensional potential-energy surface (PES) obtained from density-functional theory calculations. The results for dissociative adsorption confirm several experimental predictions using (rotationally hot) D2 beams, namely, the existence of a dissociation barrier, the small isotopic effect, the importance of vibrational enhancement, and the existence of normal energy scaling. The latter conclusion shows that normal energy scaling is not necessarily associated with weak corrugated surfaces. The results for rotationally elastic and inelastic diffractions are also in reasonable agreement with experiment, but they show that many more diffractive transitions are responsible for the observed structures than previously assumed. This points to the validity of the PES recently proposed [P. Riviere, H. F. Busnengo, and F. Martin, J. Chem. Phys. 121, 751 (2004)] to describe dissociative adsorption as well as rotationally elastic and inelastic diffractions in the H2NiAl(110) system.

A classical dynamics method for H2 diffraction from metal surfaces.

We present a discretization method that allows one to interpret measurements on diffraction of diatomic molecules from solid surfaces using six-dimensional (6D) classical trajectory calculations. It has been applied to the D2NiAl(110) and H2Pd(111) systems (which are models for activated and nonactivated dissociative chemisorption, respectively) using realistic potential energy surfaces obtained from first principles. Comparisons with experimental results and 6D quantum dynamical calculations show that, in general, the method is able to predict the relative intensity of the most important diffraction peaks. We therefore conclude that classical mechanics can be an efficient guide for experimentalists in the search for the most significant diffraction channels.

Magic and hot giant fullerenes formed inside ion irradiated weakly bound C60 clusters

We find that the most stable fullerene isomers, C(70)-C(94), form efficiently in close-to central collisions between keV atomic ions and weakly bound clusters of more than 15 C(60)-molecules. We observe extraordinarily high yields of C(70) and marked preferences for C(78) and C(84). Larger even-size carbon molecules, C(96)-C(180), follow a smooth log-normal (statistical) intensity distribution. Measurements of kinetic energies indicate that C(70)-C(94) mainly are formed by coalescence reactions between small carbon molecules and C(60), while C(n) with n≥96 are due to self-assembly (of small molecules) and shrinking hot giant fullerenes.

Experimental evidence of dynamic trapping in the scattering of H2 from Pd(110)

We have performed H2(D2) diffraction experiments on a Pd(110) surface using two different high-sensitivity set-ups. We have found that, although the total reflectivity of Pd(110) is comparable to that observed in other reactive systems, the corresponding H2(D2) diffraction patterns are quite different: no diffraction peak, including the specular one, is observed on Pd(110). This unexpected result is the consequence of dynamic trapping. Such interpretation is supported by classical dynamics calculations based on accurate ab initio potential energy surfaces.

Fragmentation in collisions of Na9+ clusters with Cs atoms

We have evaluated charge transfer, excitation and fragmentation cross sections in Na9+ + Cs collisions using a molecular close-coupling formalism and a postcollisional rate-equation model. The calculated charge transfer cross sections are in good agreement with recent experimental measurements below v = 0.04 au. We show that the relative abundance of the different fragments depend critically on the cluster temperature and the spectrometer time-of-flight window.

Two-photon double ionization of H2 at 30 eV using exterior complex scaling

Calculations of fully differential cross sections for two-photon double ionization of the hydrogen molecule with photons of 30 eV are reported. The results have been obtained by using the method of exterior complex scaling, which allows one to construct essentially exact wavefunctions that describe the double continuum on a large, but finite, volume. The calculated cross sections are compared with those previously obtained by Colgan et al (J. Phys. B: At. Mol. Opt. Phys. 41 121002), and discrepancies are found for specific molecular orientations and electron ejection directions.

Theoretical study of interference effects in single electron ionization of N2 molecules by proton impact

Single ionization of diatomic molecules due to the impact of fast proton beams is theoretically studied. The main interest is focussed on the analysis of the possible existence of interference patterns in the corresponding electron spectra due to coherent emission from the vicinities of the molecular centres. The case of N2 targets is considered. A fully coplanar geometry in which the emitted electron, the incident particle and the molecule are all in the same plane is considered. The dynamics of the reaction is described within the continuum distorted wave-eikonal initial state model, and in the exit channel a two-effective centre approximation is employed. It is shown that fingerprints of coherent electron emission appear in the corresponding angular distributions. Also, double differential cross sections as a function of the energy and of the angle of the ionized electron, obtained by averaging the angular spectra over all molecular orientations, are calculated. Electron emission in opposite phases is found for the two inner 1sσg- and 1sσ*u-orbitals, according with their gerade and ungerade character.

The role of autoionizing states in two-photon dissociative ionization of H2 by xuv ultrashort laser pulses

A theoretical study of two-photon ionization of H2 by low-intensity ultrashort xuv laser pulses is reported. The method is based on the solution of the time-dependent Schrodinger equation in a basis of stationary molecular vibronic states which include all electronic and vibrational degrees of freedom. In contrast with previous work, the doubly excited states, which also contribute to the ionization probabilities through autoionization, are explicitly included. We have found that, just below the one-photon ionization threshold, molecular autoionization leads to an enhancement of dissociative ionization, whose corresponding probability can be an order of magnitude larger than that of the nondissociative ionization process, and even larger than the corresponding dissociative probability in the one-photon absorption region. This result suggests that multicoincidence experiments, in which the orientation of the molecule with respect to the polarization axis is determined, might be easier to perform in the two-photon absorption regime than in the one-photon absorption regime. Electron angular distributions in the same range of photon energies are also reported.