Nonadiabatic Dynamics at the Gas-Molten Metal Interface: State-to-State Resolved Scattering of NO from Hot Gallium (600-1000 K)

This work reports the first quantum-state-resolved collisional energy-transfer studies of supersonically cooled NO colliding with the surface of hot, molten Ga and detected by laser-induced fluorescence on the A(2)Sigma(+) (nu = 0) <- X-2 Pi(nu = 0, 1) electronic transition band. The studies are performed at both low (Et-inc = 2.0(7) kcal/mol) and hyperthermal 20(2) kcal/mol) collision energies and as a systematic function of the gas-molten metal interfacial temperature (600-1000 K). The results provide evidence for efficient rotational and spin-orbit excitation, the latter of which signals the presence of nonadiabatic surface hopping dynamics. Furthermore, the temperature-dependent studies also yield direct evidence for efficient vibrational excitation of NO at a gas-molten metal interface, in remarkably close agreement with studies of NO scattering from hot molten Au. Of particular dynamical relevance, this vibrationally inelastic scattering efficiency closely follows Arrhenius behavior, with an activation energy (E-a = 1850 (130) cm(-1)) in quantitative agreement with the NO(nu = 1 4-- 0) energy spacing of 1876 cm(-1). This behavior provides confirmation for significant contributions from a nonadiabatic excitation mechanism, whereby a continuum of thermally populated electron-hole pair states in the molten Ga metal facilitates resonant energy transfer from the metal to the NO. This is also entirely consistent with models proposed by Tully, Wodtke, and co-workers for the inverse scattering process, namely, efficient multiquantum relaxation of NO(nu) by collisions with single-crystal Au(111), postulated to occur via transient electron transfer from the metal surface to form a temporary NO- ((3)Sigma(+)) anion. In further support of this model, we present high-level ab initio calculations at the CASSCF/AVnZ (n = 3, 4) level for the simplest Ga-NO cluster, yielding direct evidence for significant electron transfer from Ga to NO as a function of Ga-N interatomic distance.

Automated summary

This study presents the first quantum-state-resolved collisional energy-transfer investigations of supersonically cooled NO colliding with hot, molten Ga surfaces. Efficient rotational and spin-orbit excitation, as well as vibrational excitation of NO at the gas-molten metal interface, were observed. The results provide evidence for nonadiabatic surface hopping dynamics and significant contributions from a nonadiabatic excitation mechanism involving resonant energy transfer.