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In this work, the state-resolved reactivity of methane excited to different C-H stretch vibrations have been measured on a Ni(100) surface. Two kinds of experiments have been performed. In the first series of experiments, we have measured the reactivity of dideutero methane (CD2H2) excited in two different C-H stretch vibrational states which are n...
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Some factors, such as pressure and temperature, affect the rate of chemical reactions. In addition, the activation energy barrier must be overcome for the reaction to be initiated. It can be preferred to overcome this barrier by using catalysts and preheating. The catalyst ensures that it obtains the energy to react quickly by transferring it to the reactants. Similarly, the translational, vibrational, and rotational energy levels of reactants can be increased by preheating. According to the kinetic molecular theory of gases, preheating increases the kinetic energies of the gases and the speed of their collision, so the reaction takes place faster. This study theoretically investigates possible reactions of methane that can occur with the effect of only vibrational energy levels. The vibrational excitation of the molecules affects the reaction rates, and the activation barrier is overcome with lower energies. Using laser-based techniques makes the excitation of well-defined vibrational modes possible. This study investigated inelastic collisions of a methane molecule with well-characterized energy levels in infrared spectroscopy with some gases and the vibrational energy transfers that occur in these collisions. The methane molecule is the simplest form of a molecular structure consisting of more than three atoms of hydrogen atoms, which play an essential role in combustion chemistry. It shows that C⸺H stretch excitation increases the reaction rate of methane (CH4) molecules.
Our experiment is designed to probe steric effects in a gas-surface reaction. In this chapter, I present results for the study of CH4(ν3 = 1) and CD3H(ν1 = 1) reactivity on Ni(100). We exploit the linear polarization of the excitation laser to align the excited molecule’s angular momentum \( ( {\overrightarrow {J} } ) \) and vibrational dipole moment \( \left( {\overrightarrow {\mu }_{21} } \right) \) to probe the dynamical stereochemistry of vibrationally excited methane chemisorption on a Ni(100) surface. An explanation of how the alignments are produced and quantified is given in Chap. 3 of this thesis.
Figure 2.1 shows the overview of our experimental setup. There are three main parts: a continuous molecular beam source, an ultra high vacuum chamber and a continuous infrared excitation setup. I discuss the details of the experimental setup used during this thesis work in the next sections. Attention is focused on the components of the experimental setup that were developed during my thesis work.
The dynamics of H(2)O adsorption on Pt{110}-(1 x 2) is studied using supersonic molecular beam and temperature programed desorption techniques. The sticking probabilities are measured using the King and Wells method at a surface temperature of 165 K. The absolute initial sticking probability s(0) of H(2)O is 0.54+/-0.03 for an incident kinetic energy of 27 kJmol. However, an unusual molecular beam flux dependence on s(0) is also found. At low water coverage (theta<1), the sticking probability is independent of coverage due either to diffusion in an extrinsic precursor state formed above bilayer islands or to incorporation into the islands. We define theta=1 as the water coverage when the dissociative sticking probability of D(2) on a surface predosed with water has dropped to zero. The slow falling H(2)O sticking probability at theta>1 results from compression of the bilayer and the formation of multilayers. Temperature programed desorption of water shows fractional order kinetics consistent with hydrogen-bonded islands on the surface. A remarkable dependence of the initial sticking probability on the translational (1-27 kJ/mol) and internal energies of water is observed: s(0) is found to be essentially a step function of translational energy, increasing fivefold at a threshold energy of 5 kJ/mol. The threshold migrates to higher energies with increasing nozzle temperature (300-700 K). We conclude that both rotational state and rotational alignment of the water molecules in the seeded supersonic expansion are implicated in dictating the adsorption process.
