Ultraviolet electronic structure probed

Figure 3.8 Electronic structure probed by ultraviolet photoemission spectroscopy and important parameters in discussing organic devices. Here, binding energy (Eb) refers to the...

Ultraviolet photoelectron spectroscopy (UPES) probes the valence band in the solid or the molecular orbitals in adsorbed species. The technique has no direct analytical potential but gives information on the local geometry at an adsorption site and on electronic structure. [35]... 

This technique is not a direct structural probe, but has been used as an experimental method to augment theoretical calculations on the bonding in cluster systems, including a number of alkyne-substituted complexes (389-391). The basis of the technique is that photons in the vacuum ultraviolet region of the spectrum, whose energy is about 10 eV, interact with molecules in the gas phase to cause either promotion of electrons from one bound state to another or their ejection as free electrons. Photoelectron spectroscopy is only concerned with processes that liberate electrons, either by direct ionization [Eq. (15)],... 

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. 

Direct correlations between the results of MO calculations and those of experiments (on electronic structure) have until recently been plagued by a number of problems related to the experimental state of the art The data obtained from visible and ultraviolet absorption spectra arise from transitions between occupied and unoccupied levels, and hence represent a convoluted density of states. In addition, valence electronic transitions in solids in the range of 10-40 eV are accessible experimentally only under limited and difficult conditions deeper spectral regions require varied experimental techniques to probe them. 

In this work, the chemical and electronic stnictuFcs of polymer/oligomcr thin films arc analyz l as a metal overlayer is gradually deposited onto their surfaces. For this purpose, we have used X-ray Photocicetron SpcctroscO (XPS) and Ultraviolet Photocicetron Spectroscopy (UPS). XPS probes the binding energies of the core levels to determine the nature of the surface dicmical species and allows one to follow their evolution during the deposition process. UPS is a sensitive probe of the density of valence electronic states, which directly represent the electronic structure of (he polymer. 

Electrons of various energies, as well as photons in the x-ray or ultraviolet energy ranges, are used to probe the electronic structure. The spectroscopic... 

Reflection anisotropy spectroscopy (RAS) probes the electronic structure of surfaces and interfaces using visible and near-ultraviolet photons. From its origins in the 1980s as an in situ real-time monitor of semiconductor growth processes, RAS has evolved into a technique that has been applied to surfaces in UHV, surfaces under high pressure of ambient gas and solid/liquid interfaces in the field of electrochemistry, together with more spedaUst applications such as liquid crystal devices. (Note that the technique was also known as reflection difference spectroscopy (RDS) in the early years.) Most optical probes are not surface sensitive... 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-93]. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pump-probe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon... 

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron-sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 

The significance of vibrational optical activity becomes apparent when it is compared with conventional electronic optical activity in the form of optical rotatory dispersion (ORD) and circular dichroism (CD) of visible and near-ultraviolet radiation. These conventional techniques have proved most valuable in stereochemistry, but since the electronic transition frequencies of most structural units in a molecule occur in inaccessible regions of the far-ultraviolet, they are restricted to probing chromophores and their immediate intramolecular environments. On the other hand, a vibrational spectrum contains bands from most parts of a molecule, so the measurement of vibrational optical activity should provide much more information. 

Thin film science and technology is the deposition and characterization of layered structnres, typically less than a micron in thickness, which are tailored from the atomic scale upwards to achieve desired functional properties. Deposition is the synthesis and processing of thin films under controlled conditions of chemical processing. Chemical vapor deposition (CVD) and gas-phase molecular beam epitaxy (MBE) are two processes that allow control of the composition and structure of the films. Characterization is the instrumentation that use electrons, X-ray, and ion beams to probe the properties of the film. Epitaxial films of semiconductors are used for their electronic properties to emit light in the infrared (IR) and the ultraviolet rays. 

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