Femtosecond wavepacket dynamics of Cs adsorbates on Pt(111): Coverage and temperature dependences

Femtosecond time-resolved second harmonic generation (TRSHG) has been used to observe vibrational wavepacket dynamics at a Cs-covered Pt(111) surface. The creation and dephasing of vibrational coherence are monitored via the intensity modulations in the second harmonic of probe pulses as a function of pump-probe delay. The oscillatory signals are found in TRSHG signals upon the excitations at 580 and $800\mathrm{nm}$, which are the manifestation of nuclear wavepacket dynamics on the surface. The Cs-coverage dependence studied in detail indicates that the wavepacket dynamics of $\mathrm{Cs}-\mathrm{Pt}$ stretching modes and Pt surface phonon modes are responsible for the TRSHG signals. The cos-like initial phase of the oscillatory signals and the coverage dependence of the initial amplitude suggest that the vibrational coherence is associated with the resonant excitation between Cs-derived states in the quantum well of the Cs overlayer. The rate of $\mathrm{Cs}-\mathrm{Pt}$ vibrational dephasing increases with the surface temperature. This behavior cannot be accounted for by the increasing contribution from the hot bands of the $\mathrm{Cs}-\mathrm{Pt}$ stretching mode. Instead, pure dephasing caused by anharmonic coupling between $\mathrm{Cs}-\mathrm{Pt}$ stretching and parallel modes in the Cs overlayer is likely the dominant mechanism for the vibrational dephasing.