Ultraviolet photoelectron spectroscopy measuring

Kim, J.S., et al. 2000. Kelvin probe and ultraviolet photoelectron spectroscopy measurements of indium tin oxide workfunction A comparison. Synth Met 111-112 311-314. 

Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

Figure Bl.19.4. (a) Local conductance STS measurements at specific points within the Si(l 11)-(7 x 7) unit cell (symbols) and averaged over whole cell, (b) Equivalent data obtained by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). (Taken from [19], figure 2.)...

Ultraviolet photoelectron spectroscopy allows the determination of ionization potentials. For thiazole the first experimental measurement using this technique was preformed by Salmona et al. (189) who later studied various alkyl and functional derivatives in the 2-position (190,191). Substitution of an hydrogen atom by an alkyl group destabilizes the first ionization potential, the perturbation being constant for tso-propyl and heavier substituents. Introduction in the 2-position of an amino group strongly destabilizes the first band and only slightly the second. 

The low BE region of XPS spectra (<20 — 30 eV) represents delocalized electronic states involved in bonding interactions [7]. Although UV radiation interacts more strongly (greater cross-section because of the similarity of its energy with the ionization threshold) with these states to produce photoelectrons, the valence band spectra measured by ultraviolet photoelectron spectroscopy (UPS) can be complicated to interpret [1], Moreover, there has always been the concern that valence band spectra obtained from UPS are not representative of the bulk solid because it is believed that low KE photoelectrons have a short IMFP compared to high KE photoelectrons and are therefore more surface-sensitive [1], Despite their weaker intensities, valence band spectra are often obtained by XPS instead of UPS because they provide... 

Laachir, A., Perrischon, V., Badri, A., Lamotte, J., Catherine, E., Lavalley, J.C., El Fallah, J., Hilaire, L., Le Normand, F., Quemere, E., Sauvion, G.N., and Touret, O. 1991. Reduction of Ce02 by hydrogen. Magnetic susceptibility and Fourier-transform infrared, ultraviolet and x-ray photoelectron spectroscopy measurements. J. Chem. Soc. Faraday Trans. 87 1601-10. 

Proehl H, Toerker M, Sellam F, Fritz T, Leo K (2001) Comparison of ultraviolet photoelectron spectroscopy and scanning tunneling spectroscopy measurements on highly ordered ultrathin films of hexa-pm -hexabcnzocoroncnc on Au(lll). Phys Rev B 63 205409/ 1-205409/6... 

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. 

Before the advent of ultraviolet photoelectron spectroscopy and ESCA, experimental evidence on the energy density was mainly available from static magnetic susceptibility and specific heat measurements (134). These provide information on the density of states at the Fermi level and it is impossible to base any conclusions on such experimental information with regard to the shapes of the d-bands in the alloys. It is currently believed that there is very little transfer of d-electrons between the atoms. If an increase in the number of d-electrons on a particular atom does occur, it is due to transfer of electrons from the s,p-band to the lower d-band. This is, of course, related to the difference in electronegativity of the alloying components (135a, 135b). 

Among the methods that have found the most widespread application in the study of radical cations, ultraviolet photoelectron spectroscopy (UV-PES) has a special place, because it provides a wealth of detailed information concerning the orbital energies of organic molecules [57], In this experiment, a substrate is ionized by ultraviolet radiation with photons of known energy (Ehv), e.g. the He(I), line (21.21 eV), and the kinetic energy (Ekin) of the emitted electrons is measured. The vertical ionization potential (Iv) can then be calculated from Ehv and Ekin (Eq. 13). 

Well-monochromatized X-ray sources and good detectors now even allow measurements of the shift in photoelectron energy caused by the valence electrons this valence state spectroscopy was done (1965-1990) with an ultraviolet energy source in place of the X-ray source (UPS, or ultraviolet photoelectron spectroscopy) now XPS instruments can provide the same valence state chemical information by using X-ray excitation. 

The development of gas phase ultraviolet photoelectron spectroscopy (UPS) has proceeded rapidly in the last twelve years. Turner first showed that the process of photoionisation could be used to generate a photoelectron spectrum giving a direct measurement of atomic or molecular ionisation potentials (7). The early results of the Oxford group led to the acceptance of two simple guide lines in the interpretation of such spectra. Firstly the electron kinetic energies were considered to satisfy a modified Einstein relation ... 

XPS spectra were recorded using unmonochromatized Mg K radiation (1253.6 eV), and an unmonochromatized He-resonance lamp was used for ultraviolet photoelectron spectroscopy (UPS). XPS spectra were taken with an analyzer resolution of 0.2 eV, and the net resolution measured as the full width at half-maximum (FWHM) of Au 4f(7/2) was 0.9 eV. The spectrometer is of our own construction and is, e.g., designed to provide optimum angle-dependent XPS or XPS(0) (12,l4). For high 5-values, the photoelectrons leave the sample surface near the grazing angel, and due to the limited escape depth of the electrons, this is a "surface sensitive" mode. In the "bulk sensitive" mode, for low 0-values, the photoelectrons exit near the surface normal, and hence more information from the "bulk" of the sample is obtained (15). 

The electronic and geometric structures of poly[3-(4-octylphenyl)thiophene] (POPT) 109 (Figure 22) have been studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively). Thermochromic effects, and new charge-induced states generated by potassium doping, have been observed by direct UPS measurement <1996SM(76)263>. 

It is essential to have selective experimental and theoretical tools that would allow us to disentangle the different parts of the electronic structure that are important for the formation of the surface chemical bond. The most common way to measure the occupied electronic structure is with valence band photoemission, also denoted as Ultraviolet Photoelectron Spectroscopy (UPS), where the overall electronic structure is probed through ionization of the valence electrons [5]. However, in order to describe the electronic structure around a specific adsorbate, it is necessary to enhance the local information. X-ray Emission Spectroscopy (XES) provides such a method to study the local electronic properties centered around one atomic site [3,6,7]. This is particularly important when investigating complex systems such as molecular adsorbates with many different atomic sites. 

For photoelectron spectroscopy organic layers (neat films and mixtures) with a nominal thickness of 25 nm were deposited on 100 nm thick gold films which were thermally evaporated onto oxidised Si wafers. The electronic properties of the films were characterised using X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) by employing monochromated A1 Ka radiation (hv = 1486.7 eV) for measurement of the core levels as well as ultraviolet radiation [He I (hv = 21.2 eV) and He II (hv = 40.8 eV)] for an analysis of the occupied states near the Fermi level. For a measurement of the secondary electron cut-off to determine the sample work function exactly, an additional bias (-2 V and -5 V) was applied to the sample. 

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