Adsorbate induced electronic states

B.I. Lundqvist, H. Hjelmberg, and O. Gunnarsson Adsorbate-Induced Electronic States in Photoemission and the Electronic Properties of Surfaces, Eds. B. Feuerbacher, B. Fitton, and R.F. Willis (Wiley, New York, 1978). 

Lundqvist, B. I., Hjelmberg, H., Gunnarsson, O. Adsorbate induced electronic states. In Photoemission and the electronic properties of surfaces. Feuerbacher, B., Fitton, B., Willis, R. F. (eds.). New York John Wiley-Interscience 1978... 

Fig. 13. Schematic illustration of the IET dissociation process, (a) Inelastic electrons are injected into the molecule through the adsorbate-induced resonance state, (b) The energy required for a dissociation can be supplied by single- or multiple-excitation processes.

In the case of ordered adlayers, the dispersion of adsorbate-induced valence states can be determined tvith UPS by restricting the angular acceptance of the energy analyzer. From data obtained by this so-called angular resolved ultraviolet photoelectron spectroscopy (ARUPS) method the electronic band-structure can be determined, from which in turn important information on the adsorbate-adsorbate interaction can be extracted [92WEI]. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

Fig. 6-68. Surface states created by oovsdently adsorbed particles on semiconductor electrodes BL = bonding level in adsorption = electron donor level D ABL = antibonding level in adsorption = electron acceptor level A W. = probability density of adsorption-induced surface state.

The procedure is to extract by deconvolution from the measured emission probabilities the surface density of occupied states for energies between the Fermi level and about 10 eV below that, and to predict the relative atomic positions from that information about the electronic structure of the surface for example, adsorbate-induced peaks will occur, that depend on the adsorbate and its position, as in UPS. This technique is primarily sensitive to the outermost atoms of the surface, in particular adsorbates, since the emitted electrons originate from those regions only. The difficulties in deconvoluting and interpreting the density-of-states information have limited the use of INS. 

As discussed in Section 4.4.1, 02 molecularly adsorbs onto Pt(lll) into two molec-ularly adsorbed states an 02 state centered at the bridge site and an 02 state centered at the fee hollow site. When an STM tip is positioned over an isolated 02 adsorbate, tunneling electrons from the tip induce dissociation of only that molecule without affecting the surrounding [153,326]. The instant of dissociation is readily... 

Figure 3.44. Dissociation of 02 adsorbed on Pt(lll) by inelastic tunneling of electrons from a STM tip. (a) Schematic ID PES for chemisorbed Of dissociation and illustrating different types of excitations that can lead to dissociation, (b) Schematic picture of inelastic electron tunneling to an adsorbate-induced resonance with density of states pa inducing vibrational excitation (1) competing with non-adiabatic vibrational de-excitation that creates e-h pairs in the substrate (2). (c) Dissociation rate Rd for 0 as a function of tunneling current I at the three tip bias voltages labeled in the figure. Solid lines are fits of Rd a IN to the experiments with N = 0.8, 1.8, and 3.2 for tip biases of 0.4, 0.3, and 0.2 V, respectively and correspond to the three excitation conditions in (a). Dashed lines are results of a theoretical model incorporating the physics in (a) and (b) and a single fit parameter. From Ref. [153].

Introducing ionized groups onto the outer surface of a dendrimer induces the adsorption of counterionized functional molecules. A porphyrin dendrimer with anionic groups on the outer surface has been shown to adsorb the electron accepter methylviologen (MV +) (Fig. 3.15). Photoexcitation of the central porphyrin induces electron transfer to the adsorbed methylviolo-gens. As the size of the dendrimer was increased, electron back-transfer was effectively suppressed and a long-lived electron separation state was achieved. 

The characteristic feature of the electronic structure of II adsorbed on transition metals is a II-induced bonding state pulled off the bottom... 

Laser-induced reaction has been widely used to stimulate gas-surface interaction. Lasers are also used to probe molecular dynamics in heterogeneous systems as well. In the applied area, the laser photochemical techniques are successfully applied to produce well defined microstructures and new materials for microelectronic devices (1). Enhanced adsorption and chemical reaction on surfaces can be achieved by a photoexcitation of gaseous molecules, adsorbed species as well as solid substrates. The modes of the excitation include vibrational and electronic states of the gaseous species and of the adsorbates surface complexes. Both a single and a multiple photon absorption may be involved in the excitation process. 

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