Observation of quantum interference in state-resolved methane-surface scattering

The close-packed gold surface is so inert to methane activation that a molecule impinging on the surface is turned around before it can resolve atomic corrugation. The resulting “flatness” of molecule-surface interaction potential gives rise to an essentially perfect decoupling throughout the collision process of a certain subset of the molecule’s rotational degrees of freedom from the decohering influence of surface vibrations. While for all but the lightest molecular species the collisional generation and absorption of surface vibrations (i.e. phonons) typically obscures the ultimately quantum mechanical nature of the scattering process, this special decoupling in the methane-gold interaction permits observation of striking interference effects in the distribution of quantum states populated in the scattering event. This effect is characterized by a scattering selection rule enforcing a conservation of the reflection parity (Fig. 1a) of the molecular wavefunction. Moreover, this effect is unique to molecules with some minimal amount of internal structure and is thus absent for simple closed shell diatomics which have typically been the focus of attention in the field of gas-surface dynamics.

Using laser-based quantum state preparation and detection [1], we report the first observation of wavefunction reflection parity conservation in molecule–surface scattering [2]. Preparing molecules in the J = 0 rotational state yields a pure (odd) reflection parity. Probing the scattered molecules reveals an almost complete absence of population in rovibrational states of opposite parity (Fig. 1b). In contrast, preparation in nonzero J states produces a mixed incident parity, and no such absence is observed. Reflection parity conservation is observed in vibrationally elastic collisions for molecules prepared both in the ground and the nu3 excited state, with the latter showing parity purity ratios approaching 100:1. Strong parity conservation also appears in the relatively rare ν₃ to ν₁ vibrational relaxation events, providing insight into their microscopic mechanism. Finally, we probe rotational alignment following J_o = 0 to J’ ≠ 0 excitation via the polarization-sensitive optical response of the scattered molecules. The observed response systematically tracks the surface normal (Fig. 1c), consistent with an auxiliary M_J = 0 to 0 selection rule.

References

[1] Reilly et al., 214202, 158, J. Chem. Phys., (2023)

[2] C. S. Reilly, D. J. Auerbach, L. Zhang, H. Guo, and R. D. Beck, Science 387, 962 (2025)