Coherent quantum scattering of CH4 from Ni(111)

Adsorbate dissociation due to heteromolecular electronic energy transfer from fluorobenzene thin films

Study of the near-UV photodissociation dynamics for monolayer (ML) quantities of CH3I on thin films of a series of fluorobenzenes and benzene (1-25 ML) grown on a Cu(100) substrate finds that in addition to gas-phase-like neutral photodissociation, CH3I dissociation can be enhanced via electronic energy transfer to the CH3I following photoabsorption in several of the thin films studied. Distinct CH3 photofragment kinetic energy distributions are found for CH3I photodissociation on C6H5F, 1,4-C6H4F2 and C6H6 thin films, and distinguished from neutral photodissociation pathways using polarized incident light. The effective photodissociation cross section for CH3I on these thin films is increased as compared to that for the higher F-count fluorobenzene thin films due to the additional photodissociation pathway available. Quenching by the metal substrate of the photoexcitation via this new pathway suggests a significantly longer timescale for excitation than that of neutral CH3I photodissociation. The observations support a mechanism in which neutral photoexcitation in the thin film (i.e. an exciton) is transported to the interface with CH3I, and transfers the electronic excitation to the CH3I which then dissociates. The unimodal CH3 photofragment distribution and observed kinetic energies on the fluorobenzene thin films suggest that the dissociation occurs via the 3Q1 excited state of CH3I.

Surface-induced vibrational energy redistribution in methane/surface scattering depends on catalytic activity

Recent state-to-state experiments of methane scattering from Ni(111) and graphene-covered Ni(111) combined with quantum mechanical simulations suggest an intriguing correlation between the surface-induced vibrational energy redistribution (SIVR) during the molecule/surface scattering event and the catalytic activity for methane dissociation of the target surface (Werdecker, Phys. Rev. Res., 2020, 2, 043251). Herein, we report new quantum state and angle-resolved measurements for methane scattering from Ni(111) and Au(111) probing the extent of ν 3 → ν 1 antisymmetric-to-symmetric conversion of methane stretching motion for two surfaces with different catalytic activities. Consistent with the expectations, the extent of SIVR occurring on the more catalytically active Ni(111) surface, as measured by the ν 1 : ν 3 scattered population ratio, is found to be several times stronger than that on the more inert Au(111) surface. We also present additional insights on the rovibrational scattering dynamics contained in the angle- and state-resolved data. The results together highlight the power of state-resolved scattering measurements as a tool for investigating methane-surface interactions.

Stopping molecular rotation using coherent ultra-low-energy magnetic manipulations

Rotational motion lies at the heart of intermolecular, molecule-surface chemistry and cold molecule science, motivating the development of methods to excite and de-excite rotations. Existing schemes involve perturbing the molecules with photons or electrons which supply or remove energy comparable to the rotational level spacing. Here, we study the possibility of de-exciting the molecular rotation of a D₂ molecule, from J = 2 to the non-rotating J = 0 state, without using an energy-matched perturbation. We show that passing the beam through a 1 m long magnetic field, which splits the rotational projection states by only 10⁻¹²eV, can change the probability that a molecule-surface collision will stop a molecule from rotating and lose rotational energy which is 9 orders larger than that of the magnetic manipulation. Calculations confirm that different rotational orientations have different de-excitation probabilities but underestimate rotational flips (∆m_J\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ne$$\end{document}≠0), highlighting the importance of the results as a sensitive benchmark for further developing theoretical models of molecule-surface interactions.

Manipulating the rotational motions of molecules may provide a tool for controlling chemical processes. Here the authors demonstrate that the rotation of a D₂ molecule can be stopped, upon collision with a metal surface, by a magnetic field that affects the rotational levels to a much smaller extent than the energy difference upon de-excitation.

Time-of-flight measurements of the low-energy scattering of CH4 from Ir(111)

We have measured high-resolution time-of-flight (TOF) spectra of methane scattered from an Ir(111) surface at an incident energy of 81 meV. The angular distributions of scattered CH₄ reveal the presence of a sharp and intense specular peak in addition to sharp features corresponding to rotationally inelastic diffraction (RID) peaks along the two main symmetry directions of Ir(111). TOF spectra have been recorded at several RID positions for the two high-symmetry directions. The data show that the scattering dynamics of CH₄ is more complex than the one reported for H₂/D₂, where energy losses in TOF correspond to the expected excitation/deexcitation RID energy transitions. For CH₄, this is the case only for RID peaks showing up far from the specular peak, whereas those appearing close to the specular position present different behaviors, depending on the incident direction. The results are compared with Ne scattering TOF data, which allows to assess the relevance of multiphonon scattering in the energy-exchange process. Finally, we report experimental evidence of selective adsorption resonances detected with CH₄ beams. This will allow characterizing the CH₄-metal surface physisorption well by measuring angular distributions with CH₄ beams.

Contrasting mechanisms for photodissociation of methyl halides adsorbed on thin films of C6H6 and C6F6

The mechanisms for photodissociation of methyl halides (CH3X, X = Cl, Br, I) have been studied for these molecules when adsorbed on thin films of C6H6 or C6F6 on copper single crystals, using time-of-flight spectroscopy with 248 nm and 193 nm light. For CH3Cl and CH3Br monolayers adsorbed on C6H6, two photodissociation pathways can be identified - neutral photodissociation similar to the gas-phase, and a dissociative electron attachment (DEA) pathway due to photoelectrons from the metal. The same methyl halides adsorbed on a C6F6 thin film display only neutral photodissociation, with the DEA pathway entirely absent due to intermolecular quenching via a LUMO-derived electronic band in the C6F6 thin film. For CH3I adsorbed on a C6F6 thin film, illumination with 248 nm light results in CH3 photofragments departing due to neutral photodissociation via the A-band absorption. When CH3I monolayers on C6H6 thin films are illuminated at the same wavelength, additional new photodissociation pathways are observed that are due to absorption in the molecular film with energy transfer leading to dissociation of the CH3I molecules adsorbed on top. The proposed mechanism for this photodissociation is via a charge-transfer complex for the C6H6 layer and adsorbed CH3I.

Fundamental mechanisms for molecular energy conversion and chemical reactions at surfaces

The dream of theoretical surface chemistry is to predict the outcome of reactions in order to find the ideal catalyst for a certain application. Having a working ab initio theory in hand would not only enable these predictions but also provide insights into the mechanisms of surface reactions. The development of theoretical models can be assisted by experimental studies providing benchmark data. Though for some reactions a quantitative agreement between experimental observations and theoretical calculations has been achieved, theoretical surface chemistry is in general still far away from gaining predictive power. Here we review recent experimental developments towards the understanding of surface reactions. It is demonstrated how quantum-state resolved scattering experiments on reactive and nonreactive systems can be used to test front-running theoretical approaches. Two challenges for describing dynamics at surfaces are addressed: nonadiabaticity in diatomic molecule surface scattering and the increasing system size when observing and describing the dynamics of polyatomic molecules at surfaces. Finally recent experimental studies on reactive systems are presented. It is shown how elementary steps in a complex surface reaction can be revealed experimentally.

Diffraction of CH4 from a Metal Surface

Diffraction with matter waves has been reported since the beginning of quantum mechanics. In free space, diffraction effects have been observed even with objects as large as C₆₀ molecules. However, in scattering from a solid surface, pure elastic diffraction features have never been observed with molecules larger than D₂. Here we report the observation of pure molecular diffraction for CH₄ scattered off of an Ir(111) surface. These results prove that quantum coherence is preserved, despite the small separation between rotational levels and the interaction with surface phonons. Density functional theory calculations of the potential energy surface provide some clues to understand the larger corrugation sampled by CH₄ molecules in comparison to Ne atoms. Our results show that isotope separation of polyatomic molecules may be possible using gas-surface diffraction.

Low-energy methane scattering from Pt(111)

We have measured the temperature dependence of angular distributions of CH₄ from Pt(111) at an incident energy of 109 meV. A broad angular distribution has been observed along the two main symmetry directions, whereby the peak center shifts from the supra-specular position to the sub-specular position when the surface temperature increases from 120 K to 800 K. Different widths have been measured for the scattering patterns along the [ 1¯01 ] and the [ 2¯11 ] azimuthal directions. Based on calculations performed within the binary collision model, these differences have been ascribed to different corrugations of the CH₄-Pt(111) interaction potential along the two high-symmetry directions. This corrugation has been estimated from the model calculations to amount ∼0.03 Å, a factor of three larger than the one measured with helium diffraction.

Vibrational Energy Redistribution in a Gas-Surface Encounter: State-to-State Scattering of CH_{4} from Ni(111)

The fate of vibrational energy in the collision of methane (CH_{4}) in its antisymmetric C-H stretch vibration (ν_{3}) with a Ni(111) surface has been studied in a state-to-state scattering experiment. Laser excitation in the incident molecular beam prepared the J=1 rotational state of ν_{3}, and a bolometer in combination with selective laser excitation detected the scattered methane. The rovibrationally resolved scattering distributions reveal very efficient vibrational energy redistribution from ν_{3} to the symmetric C-H stretch vibration (ν_{1}). The branching ratio ν_{1}/ν_{3} is near 0.4 and insensitive to changes in incident kinetic energy in the range from 100 to 370 meV. State-resolved angular distributions and measurements of the residual Doppler linewidths prove that the scattering is direct. The observed vibrationally inelastic scattering provides direct experimental evidence for surface-induced vibrational energy redistribution.

Dissociative chemisorption of methane on Ni(111) using a chemically accurate fifteen dimensional potential energy surface

A new chemically accurate potential energy surface for the dissociative chemisorption of methane on the rigid Ni(111) surface.