| Hydrogen molecules interact with an atomically clean silicon surface in a complex manner. The goal of this experiment is to understand the mechanisms and dynamics of this interaction. Silicon is a reactive semiconductor that is used in the fabrication of many electronic devices such as computers and cellular telephones. Silicon surfaces are reactive because of the presence of dangling bonds (unpaired electrons) at the surface. Terminating the dangling bonds with hydrogen atoms passivates the surface. Both passivation and activation (desorption of hydrogen) are important steps in silicon processing during device fabrication. As diatomic hydrogen molecules dissociate and adsorb to the silicon surface, they have to overcome an energy barrier. When the hydrogen molecules desorb from a fully saturated silicon surface, on the other hand, no energy barrier exists, seemingly violating the principle of microscopic reversibility. Therefore, our challenge is to reveal the microscopic pathways and dynamics by which hydrogen interacts with silicon. This interaction can be studied by Resonance Enhanced Multiphoton Ionization (REMPI) spectroscopy, an effective technique to measure the distribution of rotational and vibrational energy levels and the velocity among the hydrogen molecules during desorption from the silicon surface. We will use two-photon excitation to ionize and detect hydrogen molecules in a state-resolved fashion. After the hydrogen molecules are ionized, the ions will be attracted to a multi-channel plate (MCP) detector by an applied electric field. Our ionization scheme requires vacuum ultraviolet (VUV) photons with an energy in excess of 10 eV, which will be generated by combining the energy of three laser photons into a single VUV photon in a process called resonant four-wave mixing (FWM) in mercury vapor. Spectral data (i.e., the wavelength dependence of the ion signal) will be used to determine the desorbing molecules' internal (rotational and vibrational) state distributions, while time-of-flight data will be give clues about their velocity distributions. The focus of my RISE project was to modify and assemble an existing REMPI detector, perfecting its design and construction, and to test its performance. The completed detector will be used to elucidate the dynamics of hydrogen molecules desorbing from the silicon surface. |
