Activated associative desorption of C + O → CO from Ru(001) induced by femtosecond laser pulses

Abstract

The femtosecond (fs)-laser-induced associative desorption of CO from a C/O coadsorbate on Ru(001) has been investigated. The recombination of the atomic reactants is found to originate predominantly from oxidation of isolated ‘reactive’ carbon atoms, whereas oxidation of surface carbon with carbon–carbon bonds is not observed. Due to the excess of oxygen atoms (C coverage in the few-percent range) the Cads + Oads → COgas formation exhibits first-order kinetics. For both excitation wavelengths 400 and 800 nm, a strongly nonlinear fluence (F) dependence of the CO desorption yield Y is observed with exponents n≈4 in a power law parametrization Y∝⟨F⟩n. Furthermore, excitation with 400 nm pulses leads to a significantly higher desorption yield as compared to 800 nm laser light with cross sections and desorption probabilities for 400 and 800 nm excitation of σeff=4.9×10−18 cm2, Pdes=0.17 and σeff=1.1×10−18 cm2, Pdes=0.07, respectively, at an absorbed fluence of ⟨F⟩=170 J m−2. This wavelength dependence is attributed to the shorter optical penetration of 400 nm light in the Ru substrate leading to higher surface temperatures at the same absorbed energy rather than to nonthermalized hot electrons. In addition, two-pulse-correlation measurements show a full-width at half-maximum of ∼ 20 ps excluding a purely electron-driven reaction mechanism, which should exhibit a subpicosecond response time. However, careful qualitative and quantitative analyses based on frictional modelling of the adsorbate–substrate coupling reveals that the C–O association reaction is mediated by both substrate phonons and electrons. The electronic, i.e. nonadiabatic contribution with a coupling constant of ηel=1/500 fs−1 is responsible for the ultrafast activation of the reaction found in the frictional modelling to occur within ∼1 ps after excitation. Similarities to the associative desorption of N2 (isoelectronic with CO) from N/Ru(001), a system for which density-functional calculations exist, can be drawn. Finally, the energy transfer to nuclear degrees of freedom during the C–O association process on the Ru(001) surface has been studied with time-of-flight measurements. The obtained translational energies expressed by Ttrans=⟨Etrans⟩/2kB≈700 K exhibit only a weak dependence on the absorbed laser fluence and are by a factor of ∼3 lower than the calculated surface temperatures present after fs-laser excitation. Possible origins of this discrepancy, such as unequal energy partitioning between the molecular degrees of freedom or nonadiabatic damping, are discussed.