Steric Effects in the Chemisorption of Vibrationally Excited Methane on Nickel

In this thesis, steric effects in the dissociative chemisorption of quantum state-prepared methane on single crystal surfaces of nickel (Ni(100) and Ni(110)) are detected and quantified for the first time. Exploiting a new, continuous-wave, high-power, single-mode infrared optical parametric oscillator, I produced an intense, quantum state-prepared molecular beam by rapid adiabatic passage. During the infrared excitation of the antisymmetric (ν3) stretch of CH4 or the C-H (ν1) stretch of CD3H by linearly polarized radiation, the angular momentum and vibrational transition dipole moment of methane is aligned in the laboratory frame. The excited, aligned molecular beam is used to probe the stereodynamics of the chemisorption reaction of vibrationally excited methane. For the reaction on Ni(100), an increase in state-prepared methane reactivity of nearly 60% is observed when the laser polarization direction is changed from normal to parallel to the surface. The dependence of the alignment effect on the rotational branch used for excitation (P-, Q-, or R-branch) indicates that alignment of the vibrational dipole moment rather than the angular momentum is responsible for the alignment dependent reactivity. Dephasing of the initially prepared alignment due to hyperfine coupling is observed to be on the timescale of 5-15 microseconds which does not preclude the study of alignment dependent reactivity in our experimental setup. Reactivity decreased monotonically from parallel to perpendicular alignment for both methane isotopologues studied. The alignment effect is shown to be independent of incident velocity for CH4(ν3) and decreases with increasing velocity of CD3H(ν1). For the Ni(110) surface, which consists of parallel rows of closely spaced Ni atoms separated by one-layer deep troughs, I probed if the reactivity depends on the methane alignment relative to the direction of the surface rows. The CH4(ν3) reactivity increases a factor of two when the laser polarization direction is changed from normal to parallel to the surface. Alignment of the vibration perpendicular to the surface rows produced a ∼10% higher reactivity than alignment parallel to the surface rows. Steric effects in a chemical reaction reveal detailed information about the reactive potential energy surface, which makes experimental studies of stereochemistry a powerful probe of microscopic chemical dynamics. The results in this thesis demonstrate and quantify specific steric requirements for this benchmark gas-surface reaction and will serve as a stringent test of multi-dimensional dynamics calculations.

Steric Effects in the Chemisorption of Vibrationally Excited Methane on Ni(100)

Rainer Beck, Régis Bisson, Bruce Yoder

Newly available, powerful infrared laser sources enable the preparation of intense molecular beams of quantum-state prepared and aligned molecules for gas/surface reaction dynamics experiments. We present a stereodynamics study of the chemisorption of vibrationally excited methane on the (100) surface of nickel. Using linearly polarized infrared excitation of the C-H stretch modes of two methane isotopologues [CH4(n3) and CD3H(n1)], we aligned methane’s angular momentum and vibrational transition dipole moment in the laboratory frame. An increase in methane reactivity of as much as 60% is observed when the laser polarization is parallel rather than normal to the surface. The dependence of the alignment effect on the rotational branch used for excitation indicates that alignment of the vibrational transition dipole moment of methane is responsible for the steric effect. Potential explanations for the steric effect in terms of an alignment-dependent reaction barrier height or electronically nonadiabatic effects are discussed.

2010

Bond-selective and surface-site-specific dissociation of methane on platinum

Ana Gutiérrez González

I present a molecular beam study of methane dissociation on differ-ent surface sites of several platinum single crystal surfaces (Pt(111), Pt(211), Pt(210), Pt(110)-(2x1)). The experiments were performed in a molecular beam/surface-science apparatus that combines rovibrational state-selective excitation of reactants with detection of the reaction products by reflection absorption infrared spectroscopy (RAIRS), Auger electron spectroscopy (AES) and King and Wells (K&W) technique. First, I explore how the barrier for methane dissociation depends on the Pt surface site (terrace, step, kink, and ridge atoms). For that, I compare the average reaction probabilities of methane on the different crystals using K&W. Moreover, I used the site-specific detection of chemisorbed methyl species by RAIRS to obtain the site-specific reactivity on steps, ridges and terraces. The highest methane reactivity and therefore the lowest barrier was observed for the surface atoms with the lowest coordination. The trans-lational energy dependence of the reactivity shows clear evidence for a direct activated mechanism on all the surface sites studied. The decreasing catalyt-ic activity for Pt atoms with increasing coordination number agrees well with theoretical predictions using first principles quantum theory calculations. Second, I present the role of rovibrational excitation of the incident methane on the chemisorption on different surface sites. For CH4, excitation of one quantum of the antisymmetric C-H stretch vibration (Îœ3) was found to be less efficient than an equivalent amount of translational energy normal to the sur-face for promoting the dissociation on all the surface sites. The vibrational efficacy was seen to be lower for dissociation on the low coordinated sites, in accordance with calculations of transition state geometries. For CH3D, the excitation of the antisymmetric C-H stretch vibration (Îœ4) led to bond selec-tivity on the steps and terraces of the Pt(211) surface. These results show how careful control of the translational energy and rovibrational state of the incident CH3D can be used for site-specific and bond-selective dissociation of methane. As predicted previously by theory but not observed before experi-mentally, the C-H bond selectivity is shown to decrease with increasing translational energy. Third, I explore the effect of surface temperature on the methane dissocia-tion. The sticking coefficient for a direct chemisorption reaction such as CH4 on Pt, might be expected to be independent of surface temperature, because the reaction happens very fast leaving very little time for the molecule to equilibrate with the surface. However, I observed an increase in methane reactivity with increasing surface temperature at low incident CH4 energies. This observation is interpreted by a âloss of coordinationâ from the atoms displaced out of the plane due to their vibrational motion when the surface is heated. The surface-site and quantum-state-specific data presented in this thesis are ideally suitable for testing theoretical models that aim to describe the me-thane-surface reaction at the microscopic level. Moreover, the characteriza-tion of different catalytically active sites for methane dissociation contrib-utes to advancing the understanding of methane reactivity towards real cata-lytic conditions, where surfaces with several different atomic terminations are used.

EPFL2019

Orientation, rotation and solvation of ions in helium nanodroplets

Verónica Oliver Álvarez de Lara

Helium nanodroplets are liquid, finite size, nanoscale helium clusters that have been employed since the 1990s as a matrix for high-resolution spectroscopy of molecules. Spectroscopic studies of ionic species inside helium droplets were not however realised until 2010, even though the potential information about their structure is of special interest since they are fundamental reaction intermediates in biochemistry. Additionally, the interaction of these species with the helium environment can be determined from these experiments, which is interesting from a fundamental point of view. To determine structural information and study product selectivity, we proposed using high electric fields to orient the molecular ions in helium nanodroplets. Pendular state spectroscopy of neutral polar molecules in helium nanodroplets in the presence of electric fields has previously given satisfactory results to achieve molecular orientation, allowing one to distinguish biomolecular isomers. Our first experiments validated the technique for neutral aniline since the polarisation difference ratios, which express the degree of orientation, obtained from the NH2-stretch transitions were well-reproduced by calculations. On the other hand, the measured degree of orientation of cationic aniline is noticeably smaller than calculations show. Even so, the variation of the degree of orientation with electric field lets us conclude that orientation of the ions in helium droplets was observed. The results of several experiments performed to investigate the possible effects influencing the apparent orientation of the cations did not lead to any straightforward explanation. Our study therefore shows that orientation of molecular ions indeed seems possible and leads the way to possible future experiments. To examine the formation of ionic-alkali solvation complexes in helium droplets upon photoionisation of the alkali, sodium, and understand the helium-dopant interaction we have performed ion spectroscopy. We found that several scenarios are possible after ionisation including dissociation, solvation and complete solvent evaporation depending on the ionisation process, droplet size and number of water ligands considered. Direct ionisation of sodium leads to solvation in the droplet, collision with the water molecules and formation of the ionic complexes if the droplet size is sufficiently large. For these conditions, rovibrational spectroscopy of Na(OH2)+ was performed to obtain the A rotational constant in helium droplets and thus determine if the dopant-helium interaction affects the moment of inertia. From the results, we could confirm the latter but could not deduce the extent to which this was the case. This study shows how the spectral information of the solvated ions is useful to understand complexation dynamics in helium droplets and acquire further understanding about the dopant-helium interaction.

EPFL2019

Bond-selective and surface-site-specific dissociation of methane on platinum

Ana Gutiérrez González

EPFL2019

Orientation, rotation and solvation of ions in helium nanodroplets

Verónica Oliver Álvarez de Lara

EPFL2019