Surface Side



Image Courtesy of Science Magazine (see reference 3 below).


Research Summary

We have developed and are applying new experimental and theoretical methods for the study of electron behavior at ultrathin interfaces. The image potential interaction between an electron and a metal surface can bind an excess electron a few Angstroms or nanometers outside the surface in one of a series of Rydberg states. Electrons promoted to these states with an femtosecond UV laser pulse are sensitive to the electrostatic potential in the near-surface region. A second femtosecond laser pulse ejects the surface electron and enables us to use very sensitive
time-of-flight, angle-resolved photoelectron spectroscopy to characterize the energy and momentum of the electron on the surface. By using tunable sub-100 femtosecond (< 10-13s) laser pulses and delaying the time between  pump and probe pulses, the dynamics of the surface electron population are monitored as well.

Successive monolayers of an adsorbate grown on the metal surface affect the binding energies, lifetimes, spatial distributions, and spin of electrons at the interface. We have investigated a wide range of ultrathin, metal-insulator interfaces recently. The two-photon photoemission experiments described above have yielded important and unique information about the growth mechanisms and dielectric properties of ultrathin metal-insulator interfaces. Lifetime measurements on the excited image potential electrons yield direct information about the penetration of the wave function across the insulating layer and into the metal surface. Current efforts aim at the problems of surface magnetism and the unique properties of nanometer-scale semiconductor layers. Our ability to measure lifetimes on the order of 50 fs while maintaining good energy resolution permits us to study some basic problems related to magnetic interfaces electron localization dynamics. Another important application of these techniques is the study the nature of the electronic states at interfaces between metals and conducting polymers. In general these methods enable us to study the dynamics of electrons interacting with condensed matter in 2-D and how the properties change as the system evolves to 3-D.

Experimental Setup



ss_setup
 

Our experimental apparatus for ultrafast two-photon photoemission represents the state-of-the-art in surface science and ultrafast lasers. With this apparatus, we can study the ultrafast dynamics of electrons at interfaces. The sample studied is a metal single crystal, with a layer of different material adsorbed on it, forming a heterojunction. An excited electronic population is created when the sample absorbs the pump photon from the laser. A second, probe photon is absorbed sometime later, and ejects an electron into the vacuum chamber, where its energy is determined by time-of-flight. By varying the delay of the probe photon with respect to the pump photon, we can map out the time-dependent behavior of the electron.

The laser apparatus consists of a multiline Ar+ Ion Laser (Coherent Innova 400) pumping two solid-state lasers: a Modelocked Femtosecond Ti:Sapphire Oscillator (Coherent Mira 900), and a Ti:Sapphire Regenerative Amplifier (Coherent RegA 9000). The short (200 fs), amplified, 800 nm (near IR) pulses emitted from the RegA 9000 are "used" by the Optical Parametric Amplifier (Coherent OPA 9000) to generate tunable, 100 fs pulses in the range of wavelengths 470-700 nm (most of the visible light range).

One experimental configuration involves using a UV photon as a "pump" to place an electron into an excited state on the surface, followed by a second "probe" pulse in the visible, which supplies the electron with sufficient energy to escape into the vacuum, where it is detected by time-of-flight. In this configuration, the visible (600 nm) photons are supplied by the OPA, and the UV photons are produced by Second Harmonic Generation which occurs when high-intensity visible light is focussed into certain types of "nonlinear" crystals.

The delay between pump and probe photons is produced by moving mirrors on a precision translation stage. Since light travels at 3X10-6 meters/second, a change of one micron in the optical path delays the light pulse by 3.3 femtoseconds.

Publications
  1. "Time- and angle-resolved two-photon photoemission studies of electron localization and solvation at interfaces", P. Szymanski, S. Garrett-Roe, and C.B. Harris, Prog. Surf. Sci. 78, p. 1 (2005).
  2. "Dynamics of an Electron at a Metal / Polar Interface", P. T. Snee, S. Garrett-Roe, and C. B. Harris, J. Phys. Chem. B, 107, p. 13608 (2003).
  3. "Direct Observation of Two-Dimensional Electron Solvation at Alcohol/Ag(111) Interfaces," S. H. Lui, A.D. Miller, K.J. Gaffney, P. Szymanski, S. Garrett-Roe, I. Bezel, and C.B. Harris, J. Phys. Chem. B 106, p. 12908 (2002).
  4. "Electron Solvation at a Metal / Polar Interface", P. T. Snee, S. Garrett-Roe, and C. B. Harris, J. Phys. Chem. B, 107, p. 13608 (2003).
  5. "Electron Solvation in Two Dimensions," Miller A. D., Bezel, I., Gaffney, K. J., Garrett-Roe S., Liu, S. H., Szymanski, P., Harris C.B. Science, 297, p.1163-1166, (2002).
  6. "Evolution of a Two-Dimensional Band Structure at a Self-Assembling Interface," Miller A. D., Gaffney K.J., Liu S. H., Szymanski P., Garrett-Roe S., Wong C. M., Harris C.B. J. Phys. Chem A., 106, p.7636, (2002).
  7. "Femtosecond dynamics of electrons photoinjected into organic semiconductors at aromatic-metal interfaces," Gaffney K.J., Miller A.D., Liu S.H., Harris C.B. J. Phys. Chem B., 105, 38, p.9031-9039, (2001).
  8. "The adsorbate electron affinity dependence of femtosecond electron dynamics at dielectric/metal interfaces." Gaffney K.J., Liu S.H., Miller A.D., Szymanski P., Harris C.B. Journal of the Chinese Chemical Society, 47, p.759-763, (2000).
  9. "Femtosecond studies of electron dynamics at interfaces." Ge N.H., Wong C.M., Harris C.B. Acc. Chem. Res., 33, p.111-118, (2000).
  10. "Femtosecond electron dynamics at the benzene/Ag(111) interface." Gaffney K.J., Wong C.M., Liu S.H., Miller A.D., McNeill J.D., Harris C.B. Chem. Phys., 251, p.99-110, (2000).
  11. "Femtosecond studies of electron dynamics at dielectric-metal interfaces." Wong C.M., McNeill J.D., Gaffney K.J., Ge N.H., Miller A.D., Liu S.H., Harris C.B. J. Phys. Chem. B., 103, p.282-292, (1999).
  12. "Femtosecond dynamics of electron localization at interfaces." Ge N.H., Wong C.M., Lingle R.L., McNeill J.D., Gaffney K.J., Harris C.B. Science, 279, p.202-205, (1998).
  13. "Dynamics and spatial distribution of electrons in quantum wells at interfaces determined by femtosecond photoemission spectroscopy." McNeill J.D., Lingle R.L., Ge N.H., Wong C.M., Jordan R.E., Harris C.B. Phys. Rev. Lett., 79, p.4645-4648, (1997).
  14. "Femtosecond dynamics of electrons on surfaces and at interfaces." Harris C.B., Ge N.H., Lingle R.L., McNeill J.D., Wong C.M. Ann. Rev. Phys. Chem., 48, p.711-744, (1997).
  15. "Interfacial quantum well states of Xe and Kr adsorbed on Ag(111)." McNeil J.D., Lingle R.L., Jordan R.E., Padowitz D.F., Harris C.B. J. Chem. Phys., 105, p.3883-3891, (1996).
  16. "Femtosecond studies of electron tunneling at metal-dielectric interfaces." Lingle R.L., Ge N.H., Jordan R.E., McNeill J.D., Harris C.B. Chem. Phys., 208, p.297-298, (1996).
  17. "Femtosecond studies of electron tunneling at metal-dielectric interfaces." Lingle R.L., Ge N.H., Jordan R.E., McNeill J.D., Harris C.B. Chem. Phys., 205, p.191-203, (1996).
  18. "2-Dimensional Localization if Electrons at Interface." Lingle R.L., Padowitz D.F., Jordan R.E., McNeill J.D., Harris C.B. Phys. Rev. Lett., 72, p.2243-2246, (1994).

Home    ·    Group Members    ·    Surface Side    ·    Liquid Side    ·    Group Meeting Schedule
Publications    ·    Links    ·    Past Members    ·    C.B. Harris    ·    College of Chemistry    ·    LBL