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

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.

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

Jörn Werdecker

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.

EPFL2018

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.

2018

Experimental studies of the equilibrium and out-of-equilibrium electronic structure of non-symmorphic topological materials

Gianmarco Gatti

Symmetry and topology are fundamental properties of nature. Mathematics provides us with a general framework to understand these concepts. On one side, symmetry describes the invariance properties of an object for specific transformations. On the other side, topology classifies objects under continuous deformations. Two objects with different topologies cannot be deformed one into each other without creating or annihilating a singularity, sometimes referred to as 'node'. These concepts gradually found applications in physics, namely in the description of the electronic properties of solids, which is the focus of this thesis. Symmetry and topology protect special nodes in the band structure of a crystal, where several states are degenerate in energy. Close to a node, the electron wave function obeys the Dirac or Weyl Hamiltonians, which were formally introduced to describe fermions in high-energy particle physics. These exotic fermions exhibit unique optical and transport properties, fully manifesting their quantum nature. Symmetry guides us in the search for which classes of materials may host topological nodes. Recently the attention of the scientific community has been attracted by crystals with non-symmorphic symmetries (screw axes and glide planes), as promising candidates for topological phases. This thesis focuses on two families of non-symmorphic crystals, whose topological properties have been investigated by combining conventional angle-resolved photoelectron spectroscopy (ARPES) with state-of-the-art spin-resolved (spARPES) and time-resolved ARPES (trARPES). The experimental results are supported by calculations carried out in collaboration with theory groups. ZrSiTe and ZrSiSe belong to the same class of non-trivial semimetals. They host Dirac electrons with large mobility. The results of my spARPES experiments clarify that spin-orbit interaction (SOI) not only removes the topological nodes in ZrSiTe, but also induces a 'hidden' spin polarization of its bulk electronic states, otherwise forbidden by the inversion symmetry of the lattice. Moreover, trARPES data provide evidence that the electron-electron interaction is only partially screened in the metallic state of ZrSiSe. As a consequence, the band velocity is enhanced, at odds with general expectations. More importantly, I show that this band renormalization can be controlled at the ultrafast time scale (namely fs scale, with 1 fs=10^{-15} s) via intense optical excitation, paving the way for engineering the band structure of Dirac semimetals. Tellurium is a chiral semiconductor, with a small and direct band gap. The low-symmetry of its lattice and the simple chemical composition make it the ideal case to study the interplay between symmetry and topology. With ARPES I determine several Weyl nodes in its electronic structure. By means of spARPES, I demonstrate that in their surrounding the spin exhibits a hedgehog configuration. This observation is new and it highlights the connection between spin-dependent and topology-related properties in Te. Finally, I illustrate promising preliminary results based on trARPES that explore the appealing possibility of optically controlling the topology of the electronic structure of Te upon excitation of coherent phonons.