Valence-shell electrons, photoelectron spectroscopy

Extensive discussion on the ionization potentials of 1,2,5-thiadiazole and its derivatives can be found in CHEC(1984) and CHEC-II(1996) <1984CHEC(6)513, 1996CHEC-II(4)355>. Hel photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, S2s, Cls and Nls edges, and Sis synchrotron radiation photoabsorption spectroscopy were used to probe the occupied and unoccupied valence levels of benzothiadiazole 2 <1991MI165>. 

The electron, upon excitation, is ejected from an inner shell into vacuum and the energy of the free electron is then measured. This technique is called X-ray photoelectron spectroscopy. If the electron is ejected from the valence band by ultraviolet radiation, the technique is called ultraviolet photoelectron spectroscopy. Excitation energies not greater than those provided by ultraviolet radiation are necessary for electron excitation from the valence band or for electrons from the valence shell of adsorbed molecules. 

By application of valence shell electron energy loss spectroscopy, inner shell electron energy loss spectroscopy, magnetic circular dichroism spectroscopy, as well as photoelectron and electron transmission spectroscopy it can be deduced that the ji-electron system of borazine resembles that of benzene [8]. Table 4/13 presents the Rydberg term value matrix and Table 4/14 the energies and assignments from the electron energy loss spectrum (EELS) [8]. 

Valence shell UV photoelectron spectroscopy has allowed certain trends to be identified. Ionization energies follow in increasing order of energy (i) d" electrons, i.e. metal-centered non-bonding electrons, (ii) nitrogen lone pair electrons or ligand-centered MOs, (iii) M—N Ji-bonding MOs and (iv) M—N o MOs.19 98... 

When UV photons are used, the available energy provides only the possibility of studying the outer electron shells. Therefore UPS (Ultraviolet Photoelectron Spectroscopy) studies the valence band structures of materials. 

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

In addition to using X-rays to irradiate a surface, ultraviolet light may be used as the source for photoelectron spectroscopy (PES). This technique, known as ultraviolet photoelectron spectroscopy (UPS, Figure 7.38), is usually carried out using two He lines (Hel at 21.2 eV and Hell at 40.8 eV), or a synchrotron source. This technique is often referred to as soft PES, since the low photon energy is not sufficient to excite the inner-shell electrons, but rather results in photoelectron emission from valence band electrons - useful to characterize surface species based on their bonding motifs. It should be noted that both UPS and XPS are often performed in tandem with an Ar" " source, allowing for chemical analysis of the sample at depths of < 1 J,m below the surface. 

The main peaks in X-ray Photoelectron Spectroscopy (XPS) for molecules appear because of the photoionization of core electrons. In addition, satellite peaks on the high binding energy side of the main peak have often been observed. These peaks are generally referred to as shakeup satellite peaks. In the sudden approximation, the shakeup process which accompanies photoionization can be considered as a two-step process. First, a core electron is emitted as a photoelectron, creating an inner shell vacancy. In the next step, electron(s) in the same molecule transfer from valence orbital(s) to unoccupied orbital(s) with relaxation of orbital energies. It is important to study these satellites in order to understand the valence and excited states of molecules (1). 

A technique called photoelectron spectroscopy is used to measure the ionization energy of atoms. A sample is irradiated with UV light, and electrons are ejected from the valence shell. The kinetic energies of the ejected electrons are measured. Since the energy of the UV photon and the kinetic energy of the ejected electron are known, we can write... 

Core-electron ionization spectra contain the information not only about inner-core electrons but also about valence electrons and chemical bonds. Extensive experimental studies have measured the core-electron binding energies (CEBE) of numerous molecules [112,113] and the recent development of X-ray photoelectron spectroscopy (XPS) has enabled the detailed analysis of the satellites accompanied by the inner-shell ionization. 

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