Li Chen
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.
EPFL2012
