Electron Excitation at Surfaces

When an electron in an atom or molecule is excited into an unoccupied bound state, it leaves an electron vacancy, or hole, behind. The electron-hole pair thus created exhibits a Coulomb attraction that is modified by the screening of all the other electrons in the system. The same phenomenon occurs when an electron is excited—by light (or electron beam) of appropriate energy—into a bound state above the Fermi level in an semiconductor. The electron-hole pair created in this circumstance is called an exciton, and its attractive Coulomb interaction is screened by the static dielectric constant of the solid. There is a finite probability that the exciton may migrate from atom to atom (or molecule to molecule) through the solid before deexcitation, the destruction of the electron-hole pair by recombination, occurs. Exciton 

In a metal, the superposition of many electron-hole pairs leads to a wave-like disturbance of the charge density at the surface. This disturbance is called the surface plasmon. Its frequency is related to the bulk plasma frequency co/, as co, = oj/,/V2. The existence of both surface and bulk plasma excitation was detected under conditions of electron-beam or photon excitation, and their corresponding energies are in the range of 5-20 eV (8-32 x 10 J). 

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Liebsch A 1997 Electronic Excitations at Metal Surfaces (New York Plenum)... 

The ejection of atoms or molecules from the surface of solid in response to primary electronic excitation is referred to as electronically stimulated desorption (ESD) or desorption induced by electronic transitions (DIET). Localization of electronic excitations at the surface of RGS induces DIET of atoms both in excited and in ground states, excimers and ions. Most authors (see e.g. Refs. [8,11,23,30] and references therein) discuss their results on DIET from RGS in terms of three different desorption mechanisms namely (i) M-STE-induced desorption of ground-state atoms (ii) "cavity-ejection" (CE) mechanism of desorption of excited atoms and excimers induced by exciton self-trapping at surface and (iii) "dissociative recombination" (DR) mechanism of desorption of excimers induced by dissociative recombination of trapped holes with electrons. 

ESD induced by electrons extracted at the tip of a scanning tunnelling microscope (STM) has recently applied to the study of the H-Si and Cl-GaAs(l 10) systems by Shen and Avouris [98]. A feature of the STM is the capability to provide an intense electron density (lO A/cm ) of atomic dimensions directly at the surface atoms. Under these conditions, desorption can also proceed with multiple-vibrational excitation. A combination of ESD and STM holds promise as a tool for atomistic DIET, also as a means to asses the influence of electron irradiation at surfaces [99]. In ESDIAD an interesting aspect is the determination of the trajectory of the ion on leaving the surface, which will be discussed later in relation to the TiOa surface. 

The pump-probe pulses are obtained by splitting a femtosecond pulse into two equal pulses for one-color experiments, or by frequency converting a part of the output to the ultraviolet region for bichromatic measurements. The relative time delay of the two pulses is adjusted by a computer-controlled stepping motor. Petek and coworkers have developed interferometric time-resolved 2PPE spectroscopy in which the delay time of the pulses is controlled by a piezo stage with a resolution of 50 attoseconds [14]. This set-up made it possible to probe decoherence times of electronic excitations at solid surfaces. 

A. Liebsch, Phys. Rev. Lett.. 1993, 71, 145 A. Liebsch, Electronic Excitations at Metal Surfaces, Plenum Press, New York, 1997. 

The situation is much more complicated for a vibrational wavepacket launched from a r" 0 initial eigenstate (see Fig. 9.7). However, a simple picture based on the classical Franck-Condon (stationary phase) principle (see Section 5.1.1) captures the essential details of the wavepacket produced at to on the electronically excited potential surface. First, there is the limiting case of an excitation pulse sufficiently short that an exact replica of the electronic ground state vibrational eigenstate, (R v" / 0), is created at to on the excited potential surface,... 

The enhancement of the electromagnetic fields at the surface of an internally reflecting crystal arise via the excitation of surface plasmon polaritons (SPP) [30] by the incident IR light. These SPPs are collective electronic excitations at metal surfaces and, theoretically, provide a sensitive probe of the optical properties of the interfacial region via resonance with the in-... 

Class B clusters are also weakly bound, but their constituent molecular units have large density of states with low vibrational frequencies which approach those of the van der Waals modes. Thus, these clusters are much more likely to decay via rapid IVR followed by a statistical dissociation. At least this is their behavior on the electronically excited potential surface. Studies in the infra-red have not been carried out. Among the well studied systems are p-difluorobenzene—X (Tiller et al., 1989 Hye-Keun et al., 1988 Butz et al., 1986) and stilbene—X dimers, where A is a rare gas (Semmes et al., 1990 Khundkar et al., 1983 DeHaan et al., 1989). 

Such a nonadiabatic reaction pathway appears to be rather improbable since the quenching of electronic excitations at metal surfaces is usually much faster than the timescale for nuclear motion, and with the02/Li system,theprobability for exoelectron emission is indeed <10 e/incident O2 molecule. The competition between nuclear and electronic motion is nicely reflected by the exponential increase of the electron )deld with the velocity of the impinging molecules as shown in Fig. 4.5 for the system O2 -I- Cs [11]. Note that a velocity of 2 x 10 m/s is equivalent to a distance of 0.2 nm in 100 fs, just in agreement with the timescales for electronic relaxation. 

As an alternative to the diode device, Hasselbrink et al. [27] proposed a metal-insulator-metal (MIM) layer system. Electrons excited at the outer metal surface may pass through the conduction band of an insulating oxide layer to a second metal electrode. 

Irradiation of adsorbate-covered surfaces with higher energy photons (typically up to 6.4 eV) with lower intensities opens the possibility of direct valence excitation. Since the lifetimes of electronic excitations at metal surfaces are much shorter than those for nuclear motion, photochemical reactions appear rather improbable. Surprisingly, however, the cross sections determined for photodesorption were found to be comparable to those found for reactions with free molecules, mainly because the short lifetime of the excited state is compensated by a much larger cross section for absorption of the light [32,62-64]. This process takes place in the near-surface region of the metal (within about 10 nm), where relaxation of the photoexcited electrons leads to rapid establishment of a transient energy distribution. As depicted in Fig. 4.11, these hot electrons may scatter at the surface or are resonantly attached to an empty level of the adsorbate. 

Gergen B, Nienhaus H, Weinberg WH et al (2001) Chemically induced electronic excitations at metal surfaces. Science 294 2521-2523... 

White JD, Chen J, Matsiev D et al (2005) Conversion of large-amplitude vibration to electron excitation at a metal surface. Nature 433 503-505... 

The probability that an electron excited at depth z below the surface can reach the surface without inelastic scattering is given by... 

Metal nanoparticles embedded in a dielectric are known to exhibit interesting optical properties resulting from collective electronic excitations at the interfece between the metal and the dielectric matrix, leading to surface plasmon... 

Note that the specular reflection geometry used here implies that the electron momentum ik before and after interaction with the surface is approximately conserved. At electron loss energies above 2 eV, electronic excitations at the surface have to be considered. 

Excitation of the surface is done by photons, and the dipole selection rales can also be apphed to the first step in two-photon photoemission. The experimental parameter space is expanded compared to regular photoemission by the option to choose different photon energies and polarizations for the two photons employed. For short photon pulses generated by femtosecond lasers, the time delay between the two photons is an additional experimental parameter that allows the time-resolved sampling of the population in the excited state. This last feature in particular is unique to two-photon photoemission and permits the detailed investigation of the electron dynamics at surfaces, which is the topic of Chapter 6. 

For two Bom-Oppenlieimer surfaces (the ground state and a single electronic excited state), the total photodissociation cross section for the system to absorb a photon of energy ai, given that it is initially at a state x) with energy can be shown, by simple application of second-order perturbation theory, to be [89]... 

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