Quantum State Resolved 3D Velocity Map Imaging of Surface-Scattered Molecules: Incident Energy Effects in HCl + Self-Assembled Monolayer Collisions

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• 作者：Carl H. Hoffman ; David J. Nesbitt
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：August 4, 2016
• 年：2016
• 卷：120
• 期：30
• 页码：16687-16698
• 全文大小：581K
• 年卷期：0
• ISSN：1932-7455

文摘

Thermal and hyperthermal HCl (v = 0, J = 0) collision dynamics at the surface of methyl-terminated self-assembled monolayers (SAMs) are probed by state-selective ionization followed by velocity-map imaging (VMI) to yield a full 2π steradian map of final 3D velocity distributions (v_x, v_y, v_z) as a function of rovibrational (v, J) quantum state. “DC slicing” of the scattered HCl flux normal to the surface (v_z) provides a powerful tool for eliminating incident beam contamination, as well as access to fully correlated, 3D flux weighted rovibrational quantum state + translational scattering dynamics in unprecedented detail. At low collision energies (E_inc ≈ 0.7(1) kcal/mol), the scattering dynamics are completely dominated by trapping-desorption (TD) events, for which both external (i.e., translational) and internal (i.e., rovibrational) degrees of freedom quantitatively track the SAM surface temperature (T_S). Hyperthermal scattering data at high collision energies (E_inc ≈ 17(1) kcal/mol) provide direct evidence for growth of an additional nonequilibrium, impulsive scattering (IS) channel, with a strong forward scattering propensity broadly distributed around the specular angle. The competition between linear and angular momentum transfer for such a rapidly rotating hydride species (B_HCl ≈ 10 cm^–1) is investigated in the IS channel, which reveals strong retention of translational energy with only modest rotational excitation (κ_trans ≈ 48(7)%, κ_rot ≈ 6(2)%) and in clear contrast with studies of more slowly tumbling species (B_CO2 ≈ 0.4 cm^–1) such as CO₂ (κ_trans ≈ 6(2)%, κ_rot ≈ 20(4)%). Most importantly, the combination of (i) full 2π steradian angular data with (ii) full quantum state resolution permits a model free deconstruction of the experimental velocity map images into TD and IS components, which provides striking, independent confirmation of the hyperthermal yet Boltzmann-like nature of both the (i) IS quantum state and the (ii) out-of-plane momentum distributions. In summary, this novel combination of VMI with quantum state resolved scattering techniques provides powerful synergistic opportunities for correlated investigation of quantum state resolved reactive and inelastic energy transfer dynamics at gas–liquid-like interfaces with chemically “tunable” surface moieties.