An axis-specific rotational rainbow in the direct scatter of formaldehyde from Au(111) and its influence on trapping probability

Abstract

The conversion of translational to rotational motion often plays a major role in the trapping of small molecules at surfaces, a crucial first step for a wide variety chemical processes that occur at gas-surface interfaces. However, to date most quantum-state resolved surface scattering experiments have been performed on diatomic molecules, and little detailed information is available about how the structure of nonlinear polyatomic molecules influences the mechanisms for energy exchange with surfaces. In the current work, we employ a new rotationally resolved 1 + 1' resonance-enhanced multiphoton ionization (REMPI) scheme to measure the rotational distribution in formaldehyde molecules directly scattered from the Au(111) surface at incidence kinetic energies in the range 0.3-1.2 eV. The results indicate a pronounced propensity to excite a-axis rotation (twirling) rather than b- or c-axis rotation (tumbling or cartwheeling), and are consistent with a rotational rainbow scattering model. Classical trajectory calculations suggest that the effect arises-to zeroth order-from the three-dimensional shape of the molecule (steric effects). Analysis suggests that the high degree of rotational excitation has a substantial influence on the trapping probability of formaldehyde at incidence translational energies above 0.5 eV.

Quantum State and Doppler-Resolved Scattering of Thermal/Hyperthermal DCl at the Gas–Liquid Interface: Support for a Simple “Lever Arm” Model of the Energy-Transfer Dynamics

“…The resulting histogram from such a calculation is shown in Figure a, which reveals a highly peaked rotational distribution with a pronounced maximum near J ≈ 5. This peaking is of course simply an example of a “rotational rainbow” structure, both predicted and observed in crossed molecular beam and gas–single-crystal scattering studies. − This sharp rainbow peak is the result of a broad extremum in J (θ) at around J ≈ 5, cos­(θ) ≈ −0.1, which is the essential ingredient for a classical rainbow (see Figure b). − Interestingly, this rainbow feature clearly identifies that the reduced levels of DCl rotational excitation observed experimentally arise from the higher probability of colliding “Cl versus D end first,” which results in correspondingly smaller impact parameters for rotationally exciting the DCl subunit. However, it is also important to note that Figure b predicts dramatic enhancement of the rotational excitation efficiency for nonisotropic sampling of the D atom orientation prior to the collisional event, as could be achieved for a light dipolar hydride such as DCl by strongly charged species or molecular ions in the interfacial region …”

Chemical dynamics from the gas‐phase to surfaces

“…Classical trajectory and quantum wavepacket calculations based on empirical potentials provide support for this interpretation 229,230 and rotational rainbows have since been seen many times 229,231–237 . Since then, it was possible to see an N‐side and an O‐side rainbow for NO scattering using oriented beams of NO 238 and to witness a rainbow in formaldehyde, where the rotation about the CO bond axis exhibits a rainbow 239 …”

Bridging the Gap between Direct Dynamics and Globally Accurate Reactive Potential Energy Surfaces Using Neural Networks

“…Similar stateresolved scattering measurements for polyatomic molecules have been reported at gas−liquid interfaces 48,49 and were very recently available on metal surfaces. 50,51 In our calculations, we prepared trajectories with the initial rovibrational states using the standard normal-mode sampling and extracted the final vibrational states by normal-mode analysis with an improved one Gaussian Binning (1GB) method, 52−54 which requires many more trajectories for a converged population of each product quantum state than those for the overall energy transfer displayed in Figure 2a. We present results in Figure 3 for the incident CO 2 (000, J = 0) and CO 2 (001, J = 1) at E i = 0.78 eV, where (ν 1 ν 2 ν 3 , J) correspond to the quantum numbers of the symmetric stretching, bending, asymmetric stretching normal modes, and rotation, respectively.…”