Events
Final Defense: Andrew Hutchison
Monday, April 18, 2022, 03:00pm
Andrew Hutchison, UT-Austin
"Inelastic Scattering of N₂ from Si(100)"
Abstract: Gas-surface scattering of N₂ from Si(100) is characterized between 290K and 1200K at several incident kinetic energies using REMPI spectroscopy. The N₂/Si(100) system should exhibit strong single-collision energy exchange between N₂ molecules and individual Si surface atoms. This scattering regime is difficult to treat in gas-surface interaction models, so it is not well understood what behavior should be expected. Scattered translational, rotational, and vibrational excitation is quantified.
We confirm a previous assessment that trapping appears to be weak for surface temperatures above 300K. The therefore-direct rotational and translational scattering kinetics exhibit surprising scaling with surface temperature and incident energy, which is qualitatively different from the single-crystal metal targets typically used in similar molecular beam scattering experiments. It seems that N₂ is coupling to surface degrees of freedom in a highly structured way. One possible explanation involves coupling to specific optical surface phonon modes, but confirming this will require additional measurements to directly test for large surface parallel momentum transfers, which the data in this work is not able to estimate. Rotational scattering exhibits a scaling change near the known metal-insulator transition at 850K, suggesting that inelastic N₂ scattering may have some use as a surface structure probe.
No vibrational population was observed in surface scattering. However, based on the precision of this zero signal, the scattering probability into v=1 is assigned an upper bound of 0.4%, and the efficiency of direct translation to vibration coupling is assigned an upper bound of 40%.
Abstract: Gas-surface scattering of N₂ from Si(100) is characterized between 290K and 1200K at several incident kinetic energies using REMPI spectroscopy. The N₂/Si(100) system should exhibit strong single-collision energy exchange between N₂ molecules and individual Si surface atoms. This scattering regime is difficult to treat in gas-surface interaction models, so it is not well understood what behavior should be expected. Scattered translational, rotational, and vibrational excitation is quantified.
We confirm a previous assessment that trapping appears to be weak for surface temperatures above 300K. The therefore-direct rotational and translational scattering kinetics exhibit surprising scaling with surface temperature and incident energy, which is qualitatively different from the single-crystal metal targets typically used in similar molecular beam scattering experiments. It seems that N₂ is coupling to surface degrees of freedom in a highly structured way. One possible explanation involves coupling to specific optical surface phonon modes, but confirming this will require additional measurements to directly test for large surface parallel momentum transfers, which the data in this work is not able to estimate. Rotational scattering exhibits a scaling change near the known metal-insulator transition at 850K, suggesting that inelastic N₂ scattering may have some use as a surface structure probe.
No vibrational population was observed in surface scattering. However, based on the precision of this zero signal, the scattering probability into v=1 is assigned an upper bound of 0.4%, and the efficiency of direct translation to vibration coupling is assigned an upper bound of 40%.
Location: PMA 5.120 and Zoom (Meeting ID: 963 0323 9956 Passcode: 12bucklemy)
