Hailemariam Ambaye

Translational to rotational energy transfer in molecule-surface collisions

A theoretical approach that combines classical mechanics for treating translational and rotational degrees of freedom and quantum mechanics for describing the excitation of internal molecular modes is applied to the scattering of diatomic molecules from metal surfaces. Calculations are carried out for determining the extent of energy transfer to the rotational degrees of freedom of the projectile molecule. For the case of observed spectra of intensity versus final rotational energy, quantitative agreement with available experimental data for the scattering of NO and N(2) from close packed metal surfaces is obtained. It is shown that such measurements can be used to determine the average rotational energy of the incident molecular beam. Measurements of the exchange of energy between translational and rotational degrees of freedom upon collision are also described by calculations for these same systems.

Atomic and molecular collisions with surfaces: comparisons of Ar and N2 scattering from Ru(0001)

Recently reported molecular beam studies of Ar and N2 scattering from Ru(0001) at thermal and hyperthermal energies exhibited a number of characteristics that are unusual in comparison to other systems for which molecular beam experiments have been carried out under similar conditions. For both systems the measured energy losses were unusually small. In the case of the Ar measurements some of the angular distributions exhibited an anomalous shoulder feature in addition to a broad peak near the specular direction, and quantum mechanical diffraction was observed under conditions for which it was not expected. These measurements are analysed and compared to calculations with a mixed quantum-classical scattering theory. This theory uses classical mechanics to describe the translational and rotational degrees of freedom while internal molecular vibrational modes are treated quantum mechanically. Many of the unusual features observed in the measurements are explained, but only upon using an effective surface mass of 2.3 Ru atomic masses, which implies collective effects in the Ru crystal. The large effective mass, because it leads to substantially larger Debye–Waller factors, explains and confirms the observations of diffraction features. It also leads to the interesting conclusion that Ru is a metal for which molecular beam scattering measurements in the purely quantum mechanical regime, where diffraction and single-phonon creation are dominant, should be possible not only with He atoms, but with many other atomic and molecular species with larger masses.

Molecule scattering from insulator and metal surfaces

Calculations are carried out and compared with data for the scattering of CH4 molecules from a LiF(001) surface and for O2 scattering from Al(111). The theory is a mixed classical-quantum formalism that includes energy and momentum transfers between the surface and projectile for translational and rotational motions as well as internal mode excitation of the projectile molecule. The translational and rotational degrees of freedom couple most strongly to multiphonon excitations of the surface and are treated with classical dynamics. Internal vibrational excitations of the molecules are treated with a semiclassical formalism with extension to arbitrary numbers of modes and arbitrary quantum numbers. Calculations show good agreement for the dependence on incident translational energy, incident beam angle and surface temperature when compared with data for energy-resolved intensity spectra and angular distributions.

Scattering of O2 from Al(111)

Experimental results are presented for the scattering of well-defined beams of molecular oxygen incident on clean Al(111). The data consist of scattered angular distributions measured as a function of incident angle, and for fixed incident angle, the dependence on surface temperature of the angular distributions. The measurements are interpreted in terms of a scattering theory that treats the exchange of energy between the translational and rotational motions of the molecule and the phonons of the surface using classical dynamics. The dependence of the measured angular distributions on incident beam angle and temperature is well explained by the theory. Rotational excitation and quantum excitation of the O(2) internal stretching mode are briefly discussed.

Calculations of scattering of N2 molecules from Ru(0001)

Recent experimental measurements of state resolved scattering of nitrogen molecules from a Ru(0001) surface are discussed in comparison with a mixed quantum-classical theory that has given reasonable explanations for similar data on other systems. Acceptable agreement between data and calculations is obtained, but only upon assuming an effective mass of the surface equal to 2.3 times the mass of a single Ru atom.