Femtosecond Surface Vibrational Spectroscopy of CO Adsorbed on Ru(001) during Desorption

Abstract

Using time-resolved sum-frequency generation spectroscopy, the C-O stretch vibration of carbon monoxide adsorbed on a single-crystal Ru(001) surface is investigated during femtosecond near-IR laser excitation leading to desorption. A large transient redshift, a broadening of the resonance, and a strong decrease in intensity are observed. These originate from coupling of the C-O stretch to low-frequency modes, especially the frustrated rotation, that are highly excited in the desorption process. PACS numbers: 68.35.Ja, 33.70.Jg, 78.47. + p, 82.20.Rp The dynamics of the interaction between molecules and metal surfaces is of fundamental importance in surface science, since these determine key physical and chemical properties-essential in, e.g., catalysis-such as energy transfer and molecular reactivity [1] [6] at surfaces. These experiments were limited to temperatures below those at which desorption of the adsorbate occurs. However, when one is interested in surface reactions, the more relevant situation occurs at higher temperatures, where higher-lying vibrational modes get thermally occupied that play an important role in the surface chemistry.We present here femtosecond time-resolved vibrational sum-frequency generation (fs-SFG) spectra of CO molecules adsorbed on a Ru(001) surface, taken while a significant number (ϳ50%) of these molecules is desorbing due to fs laser excitation. With this fs-SFG method, snapshots of the (C-O) stretch vibration can be taken, under conditions where desorption is occurring (at lattice temperatures transiently exceeding the desorption temperature by over 500 K), shedding new light on the dynamics of the desorption process and the coupling of vibrational modes at the surface.The experiments were carried out in an ultrahigh vacuum chamber (base pressure 1 3 10 210 mbar) equipped with standard surface science tools. Our commercial laser system produces 800 nm, 110 fs pulses of 4.5 mJ͞pulse at 400 Hz, chopped down to 20 Hz for these experiments. One-third of the energy is used to excite the surface ("pump" pulse, typical absorbed fluence 55 J͞m 2 [7]), and the remaining energy is used to pump an optical parametric generator/amplifier (OPG͞OPA) providing tunable (l 2 10 mm, bandwidth ϳ150 cm 21 ) IR pulses with an energy of typically 10 mJ and a duration of 150 fs. The portion of the 800 nm pulse which is not converted into IR in the OPG͞OPA-process is spectrally narrowed down to 4 cm 21 , and is used in the SFG experiments as the visible (VIS) up-conversion pulse. An extensive description of the complete experimental setup as well as the surface cleaning, preparation, and characterization procedures can be found in Ref. [8].The surface sensitivity of SFG relies on the fact that the second-order nonlinear susceptibility is nonvanishing at interfaces [9]. This enables the generation of an electric field E SFG out of two incidents fields E VIS and E IR , where energyhv and parallel momentum k k must be con-For frequencies v IR within the IR bandwidth resonant with the vibrational tra...

Photoinduced Surface Dynamics of CO Adsorbed on a Platinum Electrode

“…The low-frequency shift and the broadening of the CO spectra at a delay of 0 ps suggest that the low-frequency modes of adsorbed CO, that is, stretching, frustrated rotation, and frustrated translation modes of Pt-CO, were thermally excited by pump pulses, as reported by Bonn et al 32 Thus, it is concluded that the transient site migration of adsorbed CO on the Pt surface was caused by a transient rise in the surface temperature of Pt induced by pump pulses.…”

Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)

“…2(c). The temporal evolution of the spectroscopic features resembles the transient responses observed with vibrational spectroscopy for CO that were interpreted as a heating of the molecule leading to diffusion [1] or desorption [2][3][4] largely driven by frustrated rotations, that typically couple to the electron system on a subpicosecond time scale [1,27]. The apparent stronger interaction evidenced by the increasingd intensity and the redshift with intensity lowering of the 2~Ã indicates that the majority of CO molecules moves away from the on-top site towards more highly coordinated bridge and hollow sites [8,22].…”

“…This technique has been successfully utilized, e.g., to monitor transient excitation of external adsorbate vibrational modes that couple to diffusion [1] and to desorption reactions [2][3][4].…”

“…In particular, excitation of frustrated rotations was observed during the fs-laser-induced desorption process of CO from Ru(0001) [2]. Here we build on those findings and directly study the transient evolution of the electronic structure.…”

Generation of ultra-short hydrogen atom pulses by bunch-compression photolysis

“…Generally produced by mechanically actuated choppers or valves, pulse durations are limited to a few ms. Just as ultrashort light and electron pulses have become central tools for a wide variety of dynamical studies in physics, chemistry and biology [5][6][7][8][9][10][11] , non-mechanical approaches to produce much shorter pulses of neutral atoms and molecules offer fascinating new experimental possibilities. Timing experiments of all kinds initiated by bimolecular collisions become possible, for example, direct lifetime measurements of bimolecular collision complexes (the lifetimes of collision complexes can be indirectly inferred from scattering angular distributions.…”