Vibrationally and Rotationally Inelastic Scattering of Methane from Ni(111)

This thesis presents a quantum-state-resolved molecular beam study of the non-reactive scattering of methane (CH4) from a Ni(111) surface. It is one of the first experimental investigations in which the internal quantum state distribution of a polyatomic molecule is measured after surface scattering.

The quantum state populations of scattered CH4 were probed by selective rovibrational excitation using a high-power continuous-wave (cw) infrared (IR) laser in combination with a cryogenic bolometer. This technique is introduced as Bolometric detection with Infrared Laser Tagging (BILT) and its experimental realization is described in detail. Example data illustrates the capabilities and the performance of the method. Scattering experiments were conducted in a near-specular geometry at grazing incidence 65°) and exit angles (70°). The surface temperature was in all cases 673 K. Two aspects of the scattering dynamics of CH4 at Ni(111) were investigated.

First, the fate of initial vibrational energy in the gas-surface encounter between CH4 and Ni(111) was studied in a state-to-state scattering experiment. Here, incident CH4 was prepared with one quantum of the anti-symmetric C-H stretch vibration (v3) and in rotational state J=1 by coherent IR pumping. The results include the first observation of vibrational energy redistribution in the direct scattering of a molecule from a surface. Specifically, a portion of the CH4 molecules, which were initially in the v3 state, were detected in the symmetric C-H stretch state (v1) after scattering. The probability for this vibrationally inelastic process is about 40% compared to the vibrationally elastic process in which CH4 remains in the initially prepared v3 state. This branching ratio is insensitive to changes in incidence kinetic energy in the range 100-370 meV. Rotational excitation is in all cases significant, where molecules that underwent v3-to-v1 conversion carry away an increased amount of rotational energy. The results are discussed in the context of previously observed mode-specific reactivity in this gas-surface system.

Second, the rotational excitation of scattered CH4 in its vibrational ground state was investigated. The scattering is likewise direct and the final rotational state distributions are non-Boltzmann, revealing a propensity for scattering into low-J states. Extended analysis of the rotational-state-resolved angular distributions and the Doppler-broadened absorption profiles suggest that, at low incidence kinetic energies, rotational excitation is dominated by energy transfer from the surface, i.e. phonon annihilation. This conclusion is supported by classical scattering simulations, which recover the rotational excitation at low incidence kinetic energies. However, they strongly overestimate the efficiency of translational-to-rotational energy transfer.

The highly detailed scattering data obtained in this work can serve as stringent test of multi-dimensional dynamical models of this prototypical gas-surface reaction, thereby paving the way toward a predictive understanding of heterogeneous catalysis. This work also proves that BILT detection using state-of-the-art IR light sources is sufficiently sensitive to enable state-to-state surface scattering experiments on polyatomic molecules, opening the possibility to study their dynamics at surfaces with unprecedented detail.

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

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

Rainer Beck, Bo-Jung Chen, Maarten Eduard van Reijzen, Jörn Werdecker

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.