AVS 52nd International Symposium
    Surface Science Tuesday Sessions
       Session SS3-TuA

Invited Paper SS3-TuA1
Probing Molecular Motion Induced by Femtosecond Laser Pulses on a Site-by-Site Basis with Scanning Tunneling Microscopy

Tuesday, November 1, 2005, 2:00 pm, Room 206

Session: Ultrafast Surface Dynamics
Presenter: T.F. Heinz, Columbia University
Authors: L. Bartels, University of California, Riverside
F. Wang, Columbia University
D. Moeller, Columbia University
E. Knoesel, Rowan University
T.F. Heinz, Columbia University
Correspondent: Click to Email
The fundamental time scale for nuclear motion and energy flow in surface dynamics lies in the femto- to picosecond range. With the on-going advances in laser technology, many powerful new approaches have been developed to explore the dynamics of elementary surface processes directly in the time domain. In this paper, we present an application of femtosecond laser radiation to examine the dynamics of the elementary process of surface diffusion. To this end, femtosecond laser irradiation is used to excite adsorbed CO molecules on a Cu(110) surface; the ensuing motion of individual molecules across the surface is characterized on a site-to-site basis by in-situ scanning tunneling microscopy (STM). Adsorbate motion both along and perpendicular to the rows of the Cu(110) surface occurs readily, in marked contrast to the behavior seen for equilibrium diffusion processes. In addition, STM permits us to determine the rate of the concurrent laser-induced desorption process. The experimental findings for the efficiency and direction of the molecular motion can be understood as a manifestation of strong coupling of the lateral degrees of freedom of the adsorbed molecules to the substrate electronic excitation produced by the femtosecond laser radiation. The overall efficiency for surface diffusion is modeled using data on the thermal diffusion of CO/Cu(110) in conjunction with a description of coupling of the frustrated translational mode to the electronic and lattice excitation of the substrate. The dominant role of the substrate electronic excitation emerges from this analysis. The experiment represents a successful combination of the capabilities of utlrafast laser radiation for accessing non-equilibrium phenomena and the capabilities of STM for accessing the atomic length scale. Possible future research directions will be discussed.