Laser-Excited Hot-Electron Induced Desorption

The quest for femtochemistry provided a guiding motivation for many researchers [1,2] far in advance of the award of the 1999 Nobel Prize in Chemistry to Ahmed Zewail “for his pioneering investigation of fundamental chemical reactions, using ultra short laser flashes, on the time scale on which the reactions actually occur” [3]. The rich array of possibilities that femtochemistry potentially offered within the domain of surface-science-related phenomena was the driving force for a very significant research effort, starting in the mid 1980s, involving core members of the Surface Dynamical Processes Group within the Surface Science Division (SSD). This group was first part of the Center for Chemical Physics, then the Center for Atomic, Molecular, and Optical Physics, and most recently the Chemical Science and Technology Laboratory. The initial desorption results with pulsed laser-induced surface processing reported in 1988 [4] strongly suggested that an entirely new physical mechanism, different from that operating in gas phase femtochemistry, was required to account for the laser-induced molecular processes on solid surfaces. Stimulated by this discovery, a transient quantum wave packet model was conceived, developed, and first reported in the 1990 paper Laserexcited hot-electron induced desorption: a theoretical model applied to NO/Pt(111) [1]. In this model, laser-excited hot electrons produced within the solid are inelastically scattered from the surface chemical system via resonance formation of a temporary negative ion. This picture has become a standard paradigm for modeling, and hence understanding, almost all hot-electron-induced molecular processes at surfaces involving not only laser excitation, but also STM (Scanning Tunneling Microscope)-, tunnel junction-, and electrochemically-produced hot electrons. For this reason among others, the consequences of this coupled theoretical and experimental research are expected to have an active and lasting impact on our understanding and control of many of the most important electron-induced surface processes of chemical significance [5]. The decade of the 1970s was filled with the excitement accompanying the initial development, utilization, and theoretical interpretation of surface-sensitive spectroscopic probes which enabled a quantum mechanical description of clean and composite surfaces. Our ability to determine not only the electronic state of the surface [6], but also the properties characterizing the quantized nuclear motion of the bound constituent atoms, as revealed in vibrational spectroscopies [7], were opening up entirely new vistas in conceptualizing and understanding surface properties and processes. Once the machinery for static surface characterization was in place, it was natural to address next the intellectual and technical challenges presented in surface dynamics. This was understood to mean the observation and control of time-dependent fundamental processes that are the atomic-level (both spatial and temporal) building blocks determining the rates for excitation, decay, growth, aging, chemical reactions, catalysis, etc. Bondselective nonequilibrium placement of energy within a molecular system, subsequent energy flow and redistribution, intentional use of the out-of-equilibrium state, and production of far-from-statistical product distributions and branching ratios became the new goals. In the late 1970s, Rich Cavanagh came to NBS as an NRC Postdoctoral Fellow working with John Yates, initially on infrared and neutron surface vibrational spectroscopy and thermal desorption. About the same time, David King joined the laser chemistry group headed by John Stephenson to study time-dependent, laser-assisted molecular processes in the gas phase. A propitious collaboration was initiated between Cavanagh and King which produced the first-ever quantum-statespecific (translational, vibrational, and rotational) energy distribution measurements of molecules thermally desorbed from metal surfaces (NO/Ru(001)) in which laser excited fluorescence (LEF) techniques were used to deduce the internal state population distributions [8]. In early 1988, new NRC Postdoctoral Fellows Steve Buntin and Lee Richter joined Cavanagh and King to study “fast” pulsed laser-induced desorption, again using LEF to interrogate the desorbed molecules. This combination of laser applications, both as an active participant in the process under study (desorption) and also as a diagnostic tool (energy distributions), was crucial in the development of femtosecond surface chemistry [9,10]. In their “paradigm-establishing” 1988 note in Physical Review Letters [4], convincing evidence was presented which suggested that, contrary to expectations, desorption was caused not by local laser heating of either the substrate lattice or by the direct pumping of the adsorbate-surface bond, but instead by some as yet unknown mechanism involving electrons