Scattering of aligned H₂ from Si(100)

Reilly, Christopher Scott
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The field of chemical dynamics seeks to understand the path to reaction. What are the roles played by the different degrees of freedom possessed by the reactants? In surface physics/chemistry the reaction under consideration is often unimolecular, i.e. the adsorption of a molecule onto or its desorption from some surface. In this case the participating degrees of freedom are the translation, vibration, and rotation of the impinging/departing molecule. The studies described in this dissertation focus on the rotational degrees of freedom. Using optical excitation it is possible to both prepare an aligned ensemble of molecules and detect changes in the ensemble’s alignment induced by scattering from the surface. From these changes information is obtained about the nature of torques applied to a molecule along the path to reaction. The particular system studied is a hydrogen molecule (H₂) scattering from asingle crystal silicon surface oriented in the (100) direction. In contrast with the metal surfaces typically studied in gas-surface dynamics, the silicon surface exhibits highly directional (covalent) bonding, which may be expected to give rise to strong coupling between the surface and an impinging molecule’s angular momentum. Our experiment measures the likelihood of a reorientingcollision, where the magnitude of the molecule’s angular momentum is preserved but its direction changed. An initial laser pulse transforms a beam of H₂ molecules from a supersonic molecular beam originating as an unaligned ensemble in the j= 1 rotational state to analigned ensemble the j= 3 state. The excited molecules are aligned in a plane de-termined by the polarization of the exciting laser radiation so that ensembles can be prepared with their bonds lying preferentially parallel (helicoptering) and perpendicular (cartwheeling) to the surface plane. The rotationally excited molecules are then allowed to scatter from the Si(100) surface, and the alignment of the scattered j=3 molecules is resolved by measuring modulation in the ionization yield with the polarization of the ionizing radiation from a second laser pulse. By comparing the results of this procedure obtained with the ionizing laser running parallel to the Si(100) dimers to those obtained with the laser running at 45°, we can discriminate between changes to alignment originating from corrugation in the molecule-surface potential with the bond’s polar (θ) and azimuthal angle (φ) relative to the surface normal. The results of the experiment indicate substantial but not complete reduction in alignment of the scattered molecules. Quantifying the alignment using the cylindrically symmetric component of the quadrupole moment of the ensemble’s bond angle probability distribution, we find alignment survival ratios ranging between 50-70%, with our measurements indicating better survival (60-70%) for the cartwheeling molecules than the helicopters (50-60%). Futher, measurements at different azimuthal surface orientations of the scattered alignment of molecules impinging with cartwheeling alignment yield a complete determination of the scattered bond angle distribution.The results indicate a weak corrugation in the molecule-surface potential with the bond angle’s azimuthal coordinate. Quantum mechanical scattering calculations performed by the author using a model potential developed by Brenig and Pehlke [Prog. Surf. Sci.,83, 263 (2008)] are also presented. The model is found to predict qualitatively different alignment survivals than are observed in our measurements, though in both experiment and theory the degree realignment is found to be substantial.