Quantum interference observed in state-resolved molecule-surface scattering

While the dynamics of collisions between a molecule and a solid surface
are ultimately quantum mechanical, decohering effects owing to the large
number of interacting degrees of freedom typically obscure the wave-like
nature of these events. However, a partial decoupling of internal
molecular motion from external degrees of freedom can permit striking
interference effects to be manifest despite significant momentum exchange
between the molecule and the bath of surface vibrations. We report
state-prepared and state-resolved measurements of methane scattering from
a gold surface which demonstrate total destructive interference between
molecular states related by a reflection symmetry operation. High contrast
interference effects prevail for all processes investigated, including
vibrationally excited and vibrationally inelastic collisions. The results
demonstrate the uniquely quantum mechanical effect of discrete symmetries
in molecular collision dynamics.