The road to hot electron photochemistry at surfaces: a personal recollection.

A very important part of contemporary fs-laser surface photochemistry (SPC) is based on a proposed mechanism in which a laser pulse incident upon an adsorbate-covered surface photoexcites substrate electrons which in turn inelastically scatter from atoms and molecules (chemists may call them "reactants") in or on the surface. The present narrative outlines my own very personal SPC saga that began with early exposure to the wonders of and fascination with inelastic resonant electron scattering from gas phase atoms and molecules that dominated the Atomic and Electron Physics activities at NBS (now NIST) in 1968 when I arrived. How this lead to a fundamental understanding of important aspects of SPC is the focus of this essay.

Transition from direct tunneling to field emission in metal-molecule-metal junctions.

Current-voltage measurements of metal-molecule-metal junctions formed from pi-conjugated thiols exhibit an inflection point on a plot of ln(I/V(2)) vs 1/V, consistent with a change in transport mechanism from direct tunneling to field emission. The transition voltage was found to scale linearly with the offset in energy between the Au Fermi level and the highest occupied molecular orbital as determined by ultraviolet photoelectron spectroscopy. Asymmetric voltage drops at the two metal-molecule interfaces cause the transition voltage to be dependent on bias polarity.

Excited states at surfaces: Fano profiles in STM spectroscopy of adsorbates.

The Fano-Anderson model for a discrete state embedded within a continuum is revisited within the context of excitation and decay processes which lead to some manifestations of Fano lineshape profiles. The phenomenon of resonance tunneling between an STM tip and a metal surface upon which there are isolated adsorbed atoms is discussed and the relationship between the spectroscopic signature of such systems and that of the Fano profile is taken up. Recent experimental studies of Kondo systems of magnetic adsorbates such as Co and Ce adsorbed on noble metal (111) surfaces have motivated this work.