Scattering of aligned H₂ from Si(100)
type: CreativeWork
description: 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 …
abstract: 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 …
author: Reilly, Christopher Scott
language: en
keywords: Gas-surface dynamics, Pump-probe, Alignment, Molecular hydrogen, Silicon, Angular momentum
date published: 2019-12-16
date modified: 2026-07-24T05:01:22Z
provider: University of Texas
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: The University of Texas at Austin

PropertyValue
property ID: Handle
value: https://hdl.handle.net/2152/86463

PropertyValue
property ID: DOI
value: 10.26153/tsw/13414