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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.

In this work, I present the results of my studies on the state-resolved reactivity of silane (SiH4) on the Si(100)-(2x1) surface. The results demonstrate a co-existence of both a direct and a precursor mediated mechanisms for the dissociative chemisorptions of SiH4 on Si(100)-(2x1). Above an incident kinetic energy of 34 kJ/mol, where SiH4 chemisorption occurs via the direct mechanism, I find that both translation and vibrational energy activate the chemisorptions. Vibrational energy is slightly less effective than translation energy in promoting the reaction since adding 51.6 kJ/mol of vibrational energy leads to the same reactivity increases as adding 47 kJ/mol of translational energy. Vibrational excitation leads to at most a factor of 4 increase in reactivity of SiH4 on Si(100)-(2x1). While translational activation was reported before [Jones et. al, Chem. Phys. Lett., 1994, 229, 401], this is the first time vibrational activation is observed for SiH4 chemisorption on Si(100)-(2x1). I also investigate mode-specific reactivity by comparing the reactivity of SiH4 excited to two different nearly isoenergetic vibrational states, |2000> and |1100>. The results show that the reactivity of SiH4 excited to the |2000> state is consistently higher than that of the |1100> state over the range of incident kinetic energy studied. This mode-specific reactivity of SiH4 on Si(100)-(2x1) is rationalized in terms of different vibrational amplitude of the |2000> and the |1100> states: the former contains two quanta of Si-H stretch in a single Si-H bond, whereas the latter contains one quantum of Si-H stretch in each of two Si-H bonds. Below 34 kJ/mol, where SiH4 chemisorption on Si(100)x(2x1) occurs via the precursor-mediated mechanism, the effect of internal vibrational energy on the reactivity is found for the first time. While increasing translational energy decreases SiH4 reactivity, vibrational energy increases its reactivity. After the incident SiH4 molecule is trapped in the weakly bound precursor state, its vibrational energy appears to be conserved long enough that it can still promotes the chemisorption reaction once the trapped SiH4 encounters a reactive site. The chemisorption of SiH4 is, however, also weakly activated with vibrational energy in the precursor-mediated mechanism, in which a reactivity enhancement of less than a factor of 2 is observed for vibrationally excited SiH4. The use of isotopic selective excitation in deposition experiments to control the isotopic abundance of Si is not successful due to the fact that SiH4 chemisorption on Si(100)-(2x1) is weakly activated with vibrational energy. With a reactivity enhancement of at most a factor of 4, the relative isotope abundance of 28Si is expected to increase by only 0.6% in the laser excitation experiment. Such a small change in the natural isotopic abundance could not be detected reliably in the limited analysis time by our SIMS setup. However, this experiment can be applied to other system, where vibration excitation is more efficient to promote the reaction. Furthermore, vibrational activation of precursor-mediated gas-surface reaction discovered in this work will open up a new possibility to control gas-surface reaction of physisorbed species on surfaces.

In this thesis, I report state-resolved measurements of the chemisorption probability of CH4 on Pt(111) and Pt(110)-(1×2) for several rovibrationally excited states (2ν3, ν1+ν4, and 2ν2+ν4) in addition to the ground state. Measurements of the state resolved reactivity as function of the incident translational energy lead to state-resolved reactivity curves for each of the states under study. The relative efficacy of activating the dissociation reaction is obtained for each excited state by comparing the increase in reactivity observed upon excitation of a particular state to the effect of increasing the translational energy of CH4 in the ground state. The results provide clear evidence for mode specific reactivity with the highest efficacy for the stretch-bend combination (ν1+ν4), followed by the stretch overtone (2ν3) and the bend overtone state (2ν2+ν4). The results demonstrate that vibrational activation of CH4/Pt chemisorption process does not simply scale with the total internal energy of the incident CH4 molecule, which is a central assumption of the PC-MURT statistical model for dissociative chemisorption reactions developed by the group of Harrison [Ukraintsev et al., Chem. Phys., 1994. 101(2): p. 1564]. On the contrary, the qualitative predictions of the vibrationally adiabatic model proposed by Halonen et al. [J. Chem. Phys., 2001. 115(12): p. 5611] are in good agreement with our results. The higher efficacy of the ν1+ν4 state can also be rationalized by observing that, at the transition state, the breaking C-H bond is both stretched and bent from its equilibrium geometry, therefore I suggest that this state might have a significant projection on the reaction coordinate [Psofogiannakis et al., J. Phys. Chem. B, 2006. 110 : p. 24593 ; Anghel et al., Phys. Rev. B, 2005. 71 : p. 4]. Comparison between the state-resolved reactivity for CH4(2ν3) on Pt(111) and Ni(111) is used to obtain information about differences in barrier height and transition state location for the dissociation on the two different metals [Bisson et al., J. Phys. Chem., 2007. 111: p. 12679]. Finally, for the more corrugated Pt(110)-(1×2) surface, I determined the state-resolved sticking coefficients for different polar and azimuthal angles of incidence. Comparison between the reaction probability for incidence parallel and perpendicular to the missing rows of this surface shows shadowing effects that are consistent with predominant reactivity of the top layer Pt atoms.