Nuclear wavepacket dynamics of surface adsorbates

　The dynamics of surface-adsorbate vibrational modes are of considerable importance because these modes govern behaviors such as adsorption, diffusion, and desorption. We have found that the optical excitation and relaxation dynamics of these modes can be studied by time-resolved second harmonic (SH) generation and have applied this technique to a single layer of alkali atoms on metal surfaces. The alkali-substrate stretching modes are impulsively excited by ultrashort laser pulses and the subsequent coherent nuclear motions are probed via intensity modulation of the SH of the time-delayed probe pulses. This enables us to elucidate the electron-phonon coupling of surface adsorbates and the relaxation dynamics of the adsorbate-substrate modes. Direct observation of the nuclear wavepackets of adsorbates will open up new areas of laser control of surface photochemistry.



Fig.1 (Left) A schematic drawing of experimental scheme of the time-resolved second harmonic generation (TRSHG). A pump pulse impulsively excites the coherent nuclear motion of alkali adsorbates and subsequent nuclear displacement is probed via intensity modulation in the second harmonic of time-delayed probe pulse. (Right) A typical trace of TRSHG from Cs covered Pt(111).

references

1. "Impulsive excitation of a vibrational mode of Cs on Pt(111)", K. Watanabe, N. Takagi and Y. Matsumoto, Chemical Physics Letters, 366, 606-610 (2002).

2. "Direct Time-Domain Observation of Ultrafast Dephasing in Adsorbate-Substrate Vibration under the Influence of a Hot Electron Bath: Cs adatoms on Pt(111)", K. Watanabe, N. Takagi and Y. Matsumoto, Phys Rev Lett., 92, 57401 (4 pages) (2004).

3. "Femtosecond wavepacket dynamics of Cs adsorbates on Pt(111): coverage and temperature dependences", K. Watanabe, N. Takagi and Y. Matsumoto, Phys Rev B, 71, 085414 (2005).

4. "Mode selective excitation of coherent surface phonons on alkali-covered metal surfaces", K. Watanabe, N. Takagi and Y. Matsumoto, Phys. Chem. Chem. Phys., 7, 2697-2700 (2005).

5. "Excitation mechanism and ultrafast vibrational wavepacket dynamics of alkali-metal atoms on Pt(111)", Y. Matsumoto, K. Watanabe and N. Takagi, Surf Sci., 593, 110-115 (2005).

6. "Coherent Surface Phonon Dynamics at K-covered Pt(111) Surfaces Investigated by Time-Resolved Second Harmonic Generation", M. Fuyuki, K. Watanabe, and Y. Matsumoto, Phys. Rev. B., 74, 195412 (6 pages) (2006).

7. "Electron-phonon coupling at an atomically defined interface: Na quantum well on Cu(111)", M. Fuyuki, K. Watanabe, D. Ino, H. Petek, and Y. Matsumoto, Phys. Rev. B, 76, 115427 (4 pages) (2007).

8. "Ultrafast electron and lattice dynamics at potassium-covered Cu(111) surfaces", K. Watanabe, K. Inoue, I. F. Nakai, M. Fuyuki, and Y. Matsumoto, Phys. Rev. B, 80, 075404 (10 pages) (2009).

9."Adsorbate-localized versus substrate-mediated excitation mechanisms for generation of coherent Cs-Cu stretching vibration at Cu(111)", K. Watanabe, Y. Matsumoto, T. Yasuike, and K. Nobusada, J. Phys. Chem. A, 115, 9528-9535 (2011).

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