Picosecond time-resolved adsorbate response to substrate heating: Spectroscopy and dynamics of CO/Cu(100)

Abstract

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm⁻¹ (ν₁) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that ν₁ is anharmonically coupled to the frustrated translation near 32 cm⁻¹ (ν₄), so that transient shifts in ν₁ indicate population changes in ν₄. The ν₁ response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a ≊2 ps time constant to an intermediate value, which then decays on a ≊200 ps time scale. The data agree well with a model whereby ν₄ couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds.