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Water dissociation on transition-metal catalysts is an important step in steam reforming and the water-gas shift reaction. To probe the effect of translational and vibrational activation on this important heterogeneous reaction, we performed state-resolved gas/surface reactivity measurements for the dissociative chemisorption of D2O on Ni(111), using molecular beam techniques. The reaction occurs via a direct pathway, because both the translational and vibrational energies promote the dissociation. The experimentally measured initial sticking probabilities were used to calibrate a first-principles potential energy surface based on density functional theory. Quantum dynamical calculations on the scaled potential energy surface reproduced the experimental results semiquantitatively. The larger increase of the dissociation probability by vibrational excitation than by translation per unit of energy is consistent with a late barrier along the O-D stretch reaction coordinate.

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4 on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4 in three different vibrational symmetry components, A1, E, and F2, of the 2ν3 antisymmetric stretch overtone vibration as well as in the ν1 + ν3 symmetric plus antisymmetric C–H stretch combination band of F2 symmetry. The quantum state specific dissociation probability S0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0 for the most reactive state ν1 + ν3 is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1 state approach the surface, the vibrational amplitude becomes localized on the reacting C–H bond, making them very reactive. This behavior results from the weakening of the reacting C–H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C–H stretches. Similarly, we find that overtone normal mode states with more ν1 character are more reactive: S0(2ν1) > S0(ν1 + ν3) > S0(2ν3). The 2ν3 eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1 and ν1 + ν3 normal modes, being of A1 and F2 symmetry, can contribute to the 2ν3-A1 and 2ν3-F2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1 character due to symmetry.

In this thesis, I describe the design and first applications of a new molecular-beam/surface-science ultra-high vacuum apparatus. The new machine combines rovibrational state-selective infrared pumping of gas-phase reactants with reflection absorption infrared spectroscopy (RAIRS) detection of surface-bound reaction products for quantum-state resolved studies of chemical reactions occurring at the gas/surface interface. RAIRS enables the online detection of adsorbates formed by chemisorption reactions of reactants prepared in a molecular beam that are incident on a single crystal metal surface with well-defined translational energy and angle of incidence. For state-resolved reactivity studies, the incident reactants can be prepared in selected rovibrational states using infrared pumping by a tunable, continuous-wave optical parametric oscillator. Our RAIRS setup achieves sub-monolayer detection sensitivity (2×10-4 ML CO and 1×10-2 ML CH3) within a 35 seconds acquisition time which enables us to record uptake curves of chemisorption products in real time during a molecular beam exposure. I obtained assigned RAIRS spectra of the nascent dissociation products of CH4, CH3D, CH2D2, CHD3 and CD4 on Pt(111) at 150 K; the assignments for the partially deuterated methane species are reported for the first time. I used the capability of distinguishing different surface species by RAIRS together with the quantum state-specific preparation of gas-phase reagents to probe for vibrational bond-selectivity in the dissociative chemisorption of CH3D, CH2D2 and CHD3 on Pt(111). For the three partially deuterated methane isotopologues, I observed that incident kinetic and/or thermal vibrational energy produce a nearly statistical distribution of C-H and C-D bond cleavage products. In contrast, activation by a single quantum of C-H stretch normal mode excitation leads to bond-selective dissociation via C-H bond cleavage. I also explored the capability of RAIRS to quantify the state-resolved reactivity in a gas/surface reaction. Using RAIRS, I measured the laser-off sticking coefficient S0 and the state-resolved S0 for the vibrationally excited 3(v=1, J=2) state of CH4 on Pt(111) at 150 K for different normal incident kinetic energies in the range of 21.6 - 67.3 kJ/mol. The increase of S0(laser-off) at Ts=150 K with kinetic energy is observed I to be steeper than what previously measured at higher surface temperatures Ts=578-800 K, demonstrating the effect of surface phonon excitation on the dissociative chemisorption of methane on Pt(111). I also report an intriguing dependence of the saturation coverage of the methyl products on the reactivity of the incident CH4. Furthermore, isotope effects for methane dissociation on Pt(111) were studied by comparing the S0(laser-off) of CH4, CH3D, CH2D2, CHD3 and CD4 at the same total incident energy (~70 kJ/mol), showing a decrease in S0 with increasing deuteration. Furthermore, the RAIRS results enabled a preliminary comparison of the relative vibrational state-specific reactivity of CH4, CH3D, CH2D2 and CHD3, each containing a single quantum of C-H stretch excitation but delocalized over increasing number of C-H bonds. Finally, RAIRS was also used to detect the transient physisorption of CH4 on Pt(111) and to exam the role of vibrational energy in the physisorption of CH4 on Pt(111) at 77 K. The results showed that one quantum of C-H stretch (v3) excitation has no measurable effect on the physisorption of methane on Pt(111), in sharp contrast to the vibrationally bond-selective chemisorption. The results presented in this thesis demonstrate that RAIRS is a powerful detection method for state-resolved gas/surface reaction dynamics studies which will be applicable to the study of many other molecule/surface systems.

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 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.