Coherent surface phonon dynamics at K-covered Pt(111) surfaces investigated by time-resolved second harmonic generation

We have investigated coherently excited surface phonons at K-covered Pt(111) surfaces by using femtosecond time-resolved second harmonic generation spectroscopy. The frequency of the K-Pt stretching phonon mode depends on the superstructure of K: 5.0-5.3 and 4.5-4.8 THz for (2x2) and (root 3x root 3)R30 degrees superstructures, respectively. In addition to the stretching mode, three surface phonon modes are simultaneously observed when the (root 3x root 3)R30 degrees superstructure is formed. The decay time of the K-Pt stretching mode becomes shorter and its frequency redshifts as the absorbed fluence of the pump pulse increases. This is in stark contrast to the Pt surface phonon modes whose frequencies are independent of fluence. The fluence dependence of the K-Pt stretching mode is interpreted to be due to anharmonic coupling between the K-Pt stretching and lateral modes.