Vibrational Energy Redistribution in a Gas-Surface Encounter: State-to-State Scattering of CH4 from Ni(111)

The fate of vibrational energy in the collision of methane (CH4) in its antisymmetric C-H stretch vibration (ν3) 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 1⁄4 1 rotational state of ν3, 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 ν3 to the symmetric C-H stretch vibration (ν1). The branching ratio ν1=ν3 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.

State-to-state scattering of CH₄ from Ni(111) and Gr/Ni(111)

Maarten Eduard van Reijzen

In this thesis, I describe the design and implementation of the first state-to-state surface scattering experiments for methane from clean single crystalline surfaces in ultrahigh vacuum. The experiments use infrared laser pumping to prepare the incident CH4 in a specific rovibrationally excited quantum state and detect the scattered methane molecules with quantum state resolution using a cryogenic bolometer in combination with infrared laser tagging. To demonstrate the capabilities of this method, I present data on state-to-state scattering of CH4 from a Ni(111) and a graphene covered Ni(111) surface including the rotational and vibrational state distributions of the scattered molecules. For scattering of CH4 in its vibrational ground state, we observe significant rotational excitation. For incident CH4 with 9 kJ/mol of kinetic energy incident at 68° from the surface normal onto a Ni(111) surface at 673 K, rotational levels for J up to 10 are populated for the scattered CH4 corresponding to a rotational temperature of 162 K. The angular distribution of the scattered methane peaks near the specular angle. Both the angular distribution and the large difference between the rotational temperature of scattered CH4 and the temperature of the surface are evidence for a direct scattering process. Incident CH4 is prepared with one quantum of antisymmetric C-H stretch vibration in the v3=1 ,J=3 state by infrared pumping. The rotational and vibrational state distributions of the scattered methane are recorded by the bolometer detector using infrared laser tagging. For scattering from a clean Ni(111) surface, we observe that 40% of the scattered CH4 stays in the v3 state and therefore scatters vibrationally elastically. Vibrational relaxation during the scattering process is observed to populate the v1 state (15%) as well as the 2v2 (2%) vibrational state. No population above the detection limit was found in other vibrationally excited states at or below the energy of the v3 state (~3000 cm^(-1)). 8% of the scattered CH4 was detected in v=0. The fast energy dissipation of 0.37 eV may indicate the participation of electron-hole pair excitation in the scattering of CH4(v3,J=3) from the Ni(111) surface. To investigate the influence of the electronic structure of the surface on the energy transfer during scattering a layer of graphene was grown on the Ni(111) through chemical vapour deposition. The hybridization of the graphene and nickel electronic structures changes the metallic nature of the Ni(111) surface to that of an insulator for Gr/Ni(111). Scattering CH4(v3,J=3) from Gr/Ni(111) results in stronger rotational excitation but weaker vibrational energy transfer. 62% of incident CH4(v3,J=3) remained in the v3 state and no population could be detected in other vibrationally excited states. Relaxation to v=0 was reduced from 8% to 3% for scattering from Gr/Ni(111). State-to-state scattering experiments provide detailed information on the energy transfer processes in surface scattering. The bolometer infrared laser tagging (BILT) technique implemented in this thesis is shown to be a highly sensitive, quantum state resolved detection method that is generally applicable to polyatomic molecules with infared active vibrational modes. In contrast to resonance enhanced multiphoton ionization, BILT does not require a long lived excited electronic state which makes it ideally suited for state resolved scattering experiments of methane.

EPFL2016

State-to-state surface scattering of methane studied by bolometric infrared laser tagging detection

Bo-Jung Chen

State-to-state molecule/surface scattering experiments prepare the incident molecules in a specific quantum state and measure the quantum state distribution of the scattered molecules. The comparison of state resolved experiments with theory can serve as stringent tests of the molecule/surface interaction potential and of the scattering dynamics. The overall motivation is to develop a predictive understanding of the molecule/surface interactions and reactions needed for the understanding and optimization of heterogeneous catalysis. In this thesis, I describe the design characterization, and first applications of a new apparatus, dedicated to performing state-to-state surfaces scattering using the bolometric infrared laser tagging detection (BILT). An important advantage of the BILT detection method over other state-resolved detection techniques such as resonant multi-photon ionization (REMPI), is its applicability to any molecule with an infrared active vibrational mode and a rotationally resolved vibrational gas phase spectrum. For example, BILT detection can be used to detect important molecules such as methane and carbon dioxide for which REMPI detection is not possible. Our new state-to-state scattering machine features a liquid helium-cooled bolometer detector installed on a rotatable lid allowing independent variation of the incident and the scattering angle. With this capability, one can study energy transfer such as the conversion of translational to rotational or vibrational energy as well as vibrational energy redistribution for molecules colliding with a well-defined single crystal surface at a wide variety of scattering geometry. To demonstrate the capabilities of the BILT machine, I studied the rotationally inelastic scattering of CH4 from Ni(111). The results show that rotational excitation depends not only on the kinetic energy along the surface normal but also on the parallel component although with lower efficiency. The extent of rotational excitation is found to increase with increasing surface temperature. The conversion efficiency appears to be higher for low velocity component normal to the surface. The observations indicate a mechanism of rotational excitation by phonon annihilation with the probability related to the relative velocity of the incoming molecules and surface atoms. Using the experimentally determined formula which takes into account the conversion efficiency of the normal, the parallel component of incident kinetic energy, and the surface thermal energy to the rotational energy, the mechanism of rotational excitation of CH4 scattering from Ni(111) is quantitatively unraveled. Besides rotational inelastic scattering, I report very distinct behavior of vibrationally inelastic scattering of CH4 from clean Ni(111) and Gr-Ni(111). Vibrational energy transfer to the surface dramatically changes when a clean Ni(111) surface is covered with a single layer of graphene. Theoretical calculations based upon reaction path Hamiltonian suggest that the probability of the vibrational energy transfer is related to the catalytic activity of the surface impact sites. Therefore, by monitoring the fate of the initially prepared vibrational energy, the state-to-state surface scattering technique can potentially serve as a probe of the catalytic activity of surfaces.

EPFL2022

Bond-Selective and Mode-Specific Dissociation of CH3D and CH2D2 on Pt(111)

Rainer Beck, Phil Morten Hundt, Hirokazu Ueta, Maarten Eduard van Reijzen

Infrared laser excitation of partially deuterated methanes (CH3D and CH2D2) in a molecular beam is used to control their dissociative chemisorption on a Pt(111) single crystal and to determine the quantum state-resolved dissociation probabilities. The exclusive detection of C-H cleavage products adsorbed on the pt(111) surface by infrared absorption reflection spectroscopy indicates strong bond selectivity for both methane isotopologues upon C-H stretch excitation. Furthermore, the dissociative chemisorption of both methane isotopologues is observed to be mode-specific. Excitation-of symmetric C-H stretch modes produces a stronger reactivity increase than excitation of Me antisymmetric C-H stretch modes, whereas bend overtone excitation has a weaker effect on reactivity. The observed mode specificity and bond selectivity are rationalized by the sudden vector projection model in terms of the overlap of the reactant's normal mode vectors with the reaction coordinate at the transition state.