Abstract
We have studied CO coordinated to ruthenium tetraphenylporphyrin (RuTPP)/Cu(110) and directly adsorbed to Cu(110), using femtosecond pump-sum frequency probe spectroscopy, to alter the degree of electron-vibration coupling between the metal substrate and CO. We observe the facile femtosecond laser-induced desorption of CO from RuTPP/Cu(110), but not from Cu(110). A change in the vibrational transients, in the first few picoseconds, from a red- to blue-shift of the C–O stretching vibration under photodesorption conditions, was also observed. This drastic change can be explained, if the cause of the C–O frequency redshift of Cu(110) is not the usually-assumed anharmonic coupling to low frequency vibrational modes, but a charge transfer from hot electrons to the CO 2π* state. This antibonding state shifts to higher energies on RuTPP, removing the C–O redshift and, instead, reveals a blueshift, predicted to arise from electron-mediated coupling between the coherently excited internal stretch and low frequency modes in the system.
Highlights
Understanding adsorbate dynamics on surfaces is crucial to many phenomena, ranging from heterogeneous catalysis to sensing devices
Any sub-picosecond response is thought to represent a signature of nonadiabatic dynamics, where energy is directly transferred between hot electrons and the adsorbate vibrational modes, while changes on the tens-of-picoseconds timescale relate to hot phonons and adiabatic dynamics
The typical redshift of the internal stretch disappeared at short delay times and was replaced by a blue shift at a higher pump fluence
Summary
Understanding adsorbate dynamics on surfaces is crucial to many phenomena, ranging from heterogeneous catalysis to sensing devices. Any sub-picosecond response is thought to represent a signature of nonadiabatic dynamics, where energy is directly transferred between hot electrons and the adsorbate vibrational modes, while changes on the tens-of-picoseconds timescale relate to hot phonons and adiabatic dynamics. The majority of studies have focused on the vibrational dynamics of CO, which, so far, have been generally attributed to coupling between hot electrons and the frustrated rotation (FR) mode; for example, on Ru(001) [4] and Pt(111) [5]. The origin of the observed redshift of the C–O stretch, while the surface electrons are hot, is thought to be caused by excitation of the frustrated translational (FT) mode, which is indirectly heated through the frustrated rotation, as modeled by Ueba and Persson [6,7].
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