Scattering HCl Molecules from Au(111) and Ag(111) Surfaces

In this thesis, I present an experimental molecular beam surface scattering study of dynamical processes involved when HCl molecules are scattered from Au(111) and Ag(111) surfaces. I investigated vibrational excitation, translational inelasticity and dissociative adsorption in combination with associative desorption. The experiments were conducted in an ultra-high vacuum (UHV) molecular beam/surface scattering apparatus equipped with a pulsed nanosecond infrared (IR) laser source for vibrational state manipulation and pulsed nanosecond ultraviolet (UV) lasers for quantum-state-resolved detection of molecules via resonance enhanced multi-photon ionization (REMPI) before and after the collisions. For HCl/Au(111), I found surface temperature dependent vibrational excitation probabilities (VEPs) from vibrational state v = 0 -> 1 to be in the range of 10^-5 - 10^-3 for incidence energies of Ei = 0.67 - 0.99 eV, which is low compared to other molecule-surface systems. On the other hand, VEPs for the v = 1 -> 2 transition were substantially higher at 10^-3 - 10^-2 for comparable incidence energies. In both cases, excitation probabilities could be divided into electronically adiabatic and nonadiabatic contributions where the latter exponentially depended on the surface temperature Ts. This combination of adiabatic and nonadiabatic vibrational excitation has so far been uniquely observed for HCl scattered from Au(111) and Ag(111) surfaces. Extracting Ei and Ts independent interaction coefficients, I found the nonadiabatic excitation to be stronger than the adiabatic one by a factor of 67 for the v = 0 -> 1 and 24 for the v = 1 -> 2 channel. In comparison, on Ag(111) only the excitation from the vibrational ground state could be observed. With VEPs in the range of 10^-4 - 10^-3 being slightly higher than on gold, this difference was interpreted as enhanced nonadiabatic interactions on silver. Relatively low barriers to dissociation on both metal surfaces (e. g., compared to NO/Au(111)) might have led to the higher v = 1 -> 2 VEPs on Au(111). Further, the absence of higher excitation channels (v = 2 -> 3 on gold, v = 1 -> 2 on silver) might also be explained by the enhanced dissociation of HCl molecules incident in excited vibrational states. Dissociation probabilities of HCl on metal surfaces were measured by AUGER electron spectroscopy after controlled molecular beam dosage monitored by a quadrupole mass spectrometer. Even though the dissociation probabilities on Ag(111) were predicted to be higher than on Au(111), I could not determine them within the current technical limitations. Additionally, the actual dissociation probabilities I determined on Au(111) employing AES remained below 0.06 - 0.17, which is lower than those predicted by a series of computational studies by a factor of 2 - 6. On the other hand, translational energy losses calculated in the latest AIMDEF study match the experimental results for v = 1 -> 1 and v = 1 -> 2 quite accurately. In sum, the results of this thesis, which all indicate that HCl exhibits both electronically adiabatic and nonadiabatic interactions with metal surfaces during scatter- ing events, can serve as benchmark data to test and improve theoretical descriptions of gas-surface interactions, especially in those cases were nonadiabaticity plays an important role.

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

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

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

Jörn Werdecker

EPFL2018

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

Maarten Eduard van Reijzen

EPFL2016