Quantum Control of Nuclear Motion at a Metal Surface ² H. Petek,* H. Nagano, M. J. Weida, and S. Ogawa AdVanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-0395, Japan ReceiVed: March 30, 2000 The possibility of quantum control of surface photochemical reactions is demonstrated for the system of Cs/Cu(111). Coherent nuclear wave packet motion is induced by resonant dipole excitation with 3.08 eV light from an occupied surface state intrinsic to the Cu(111) surface to an unoccupied Cs antibonding state for a Cs atom coverage of 0.1 monolayer. Interferometric time-resolved two-photon photoemission measurements show that the polarization induced between the two states decays in ∼25 fs. This makes it possible to control the position and momentum of the excited state wave packet through the phase of the excitation field. Quantum control is demonstrated for excitation with phase-related <20 fs pulse pairs or single chirped laser pulses. The phase dependence of two-photon photoemission spectra demonstrates the control of desorptive motion of Cs atoms. I. Introduction A longstanding goal in the field of chemistry, and a central theme of research in the Moore group, is the laser activation of nonthermal, bond-specific chemical reactions. The advent of lasers brought promise of relatively cheap and bright mono- chromatic radiation that could potentially induce selective chemistry otherwise not possible by conventional means. Much research has focused on activation and reaction of specific bonds through vibrational excitation. 1,2 A more recent approach is to perform specific chemical transformations by actively manipu- lating the quantum mechanical properties of molecules with light. 3,4 The major hurdle to achieving practical laser control of chemical reactions is the transfer of energy among the various nuclear degrees of freedom of a molecule on a subpicosecond time scale. 5,6 Nevertheless, in the past few years, quantum control schemes have been proposed and achieved in molecules of increasing complexity. 7-11 The aim of the present contribution is to demonstrate the coherent manipulation of atomic motion at a metal surface. Inducing nonthermal and selective chemistry by electronic excitation of adsorbed molecules on surfaces is important in many practical applications ranging from semiconductor device fabrication to catalysis. 12,13 When a molecule is adsorbed on a metal, its photochemistry is greatly altered from the gas phase through electronic interaction with the surface. The role of the surface is 2-fold: (i) hybridization between the adsorbate and substrate orbitals results in substantially different optical absorp- tion spectra from those of free molecules; and (ii) strong interaction with the substrate causes electronic quenching of the adsorbate typically on ,10 fs time scale, which effectively competes with the thermalization of the electronic excitation. Thus, compared with the gas phase, molecules on surfaces can have substantially different, nonthermal chemistry, albeit with minuscule quantum yields. 12,14 A prime example is methane physisorbed on the Pt(111) surface, where thermal activation only induces desorption at 70 K, while excitation with 6.4 eV photons leads to efficient scission of the C-H bond. Since gas- phase CH 4 does not appreciably absorb light below 8.5 eV, it is remarkable that this process occurs with high selectivity and significantly lower photon energy on a metal. 15 Although the ultrafast dissipation of the electronic excitation on metals favors observation of nonthermal chemistry, it is the bane of actively manipulating surface processes by quantum control techniques. The understanding of surface photochemistry requires direct probing of the electron-hole (e-h) pair creation and relaxation at adsorbate covered surfaces. Such studies have been greatly advanced by the development of femtosecond time-resolved two-photon photoemission (TR-2PP) techniques. 16 In analogy to multiphoton ionization techniques in the arsenal of gas-phase spectroscopy and dynamics, 6,17 TR-2PP measures the energy and momentum-resolved photoemission current following the excitation of a metal with a pump-probe laser pulse sequence. Since the two-photon excitation process can be coherent or sequential, TR-2PP experiments can distinguish between the phase relaxation of the coherent polarizations excited in the sample and the intermediate state population decay. The quasi- elastic electron-phonon (e-p) scattering mainly contributes to the phase relaxation, while the inelastic electron-electron (e-e) scattering also induces population decay. Thus, for occupied and unoccupied states that lie within several eV from the Fermi level E F , TR-2PP can often supplant the traditional approach of estimating electronic relaxation rates from the photoemission line widths. 18 Some of the most important findings of TR-2PP experiments are phase and energy relaxation rates of surface states. The image and crystal potentials at the surface-vacuum interface confine the surface electrons in naturally occurring quantum wells for certain ranges of energies and momenta defined by the projected band gaps in the metal band structure. Quantum confinement attenuates the surface state interaction with the bulk, resulting in decoherence times of <1-100 fs, and lifetimes in the range of <5-1000 fs. 19-23 Of particular interest are the phase relaxation times, which define the time scales for the coherent manipulation of the induced polarizations through the phase of the excitation light. Here, the optical phase is used to control the nuclear motion of Cs atoms above a Cu(111) surface. ² Part of the special issue “C. Bradley Moore Festschrift”. * Corresponding author. Current address: Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260. E-mail: petek@pitt.edu. 10234 J. Phys. Chem. A 2000, 104, 10234-10239 10.1021/jp001218a CCC: $19.00 © 2000 American Chemical Society Published on Web 08/04/2000