Excitation mechanism and ultrafast vibrational wavepacket dynamics of alkali-metal atoms on Pt(1Â 1Â 1)

We have investigated experimentally and theoretically the mechanism for the coherent excitation of stretching vibrational motions of alkali-metal atoms (Cs and K) on a Pt(1 1 1) surface and the electronic states involved in this excitation process. The coherent motions were initiated by ultrashort optical pulses and their dephasing processes were monitored by time-resolved second harmonic generation (TR-SHG). TR-SHG traces showed the oscillating modulations of 2.3-2.4 THz for Cs and 4.5-5.2 THz for K-covered surfaces, which are characteristic to the stretching vibrations of the alkali-metal atoms. The first-principles calculations on Cs/Pt(1 1 1) based on the density functional theory revealed the strong hybridization between the 6s and 6p orbitals of Cs when Cs is adsorbed on Pt(1 1 1). The stretching frequencies estimated from the potential energy curve along this coordinate were in reasonable agreement with the observed ones. Moreover, the calculations on the Pt(1 1 1)-(2 × 2)Cs indicate that there is a Cs-induced unoccupied band located in the energy range of 1-2 eV above the Fermi level. The electron density of this band is localized at the vacuum side of Cs and shows the anti-bonding feature with respect to the Cs-Pt bond. Thus, this band is a possible candidate responsible for the coherent excitation of the stretching mode via the impulsive resonant Raman process.