Time-resolved XPS

Keywords XPS differential charging peak separation time-resolved XPS 1. Introduction... 

Figure 9. 200 time-resolved XPS spectra recorded with 10 ms resolution of the Si2p-Au4f-Cu3p region of gold(core) /silica (shell) nanoparticles deposited on a copper tape. 

If a potentiostat is used in electrochemical corrosion studies, it will consume these electrons and there will be no need for a redox system and electric conductivity of the passive layer. But if cations of lower valency are formed within the film first and will be oxidized later, electronic conduction of the layer is necessary to transport electrons to the metal surface. Time-resolved XPS studies have shown an initial formation of lower valency cations which are oxidized at a later stage of the development of the passive layer. Similarily, Fe(II) will be oxidized to Fe(III) when the potential is increased. These details have been examined, e.g., on iron in alkaline solutions, as already discussed in Sec. 1.4.2.1. These layers are only several nanometers thick, and the migration of cations follows an exponential cur-rent/potential relationship due to the pres-... 

Time-resolved XPS investigations after potentiostatic transients show first the formation of Co(OH)2 during the first seconds even in the range of secondary passivity, which changes later partially to Co(III) oxide and a spinel-type Co(III) oxide. Here again, one observes the lower valent hydroxide, which changes to the stable chemical structure of the passive layer with time. For E = 0.25 V in 1M NaOH, no reduction is found for the passive film formed at E = 0.6. For E < 0.2 V, the reduction to Co(OH)2 is observed, which... 

All analytical methods that use some part of the electromagnetic spectrum have evolved into many highly specialized ways of extracting information. The interaction of X-rays with matter represents an excellent example of this diversity. In addition to straightforward X-ray absorption, diffraction, and fluorescence, there is a whole host of other techniques that are either directly X-ray-related or come about as a secondary result of X-ray interaction with matter, such as X-ray photoemission spectroscopy (XPS), surface-extended X-ray absorption fine structure (SEXAFS) spectroscopy, Auger electron spectroscopy (AES), and time-resolved X-ray diffraction techniques, to name only a few [1,2]. 

The dipole-dipole interactions of the fluorophore in the electronic excited state with the surrounding groups of atoms in the protein molecule or with solvent molecules give rise to considerable shifts of the fluorescence spectra during the relaxation process. These spectral shifts may be observed directly by time-resolved spectroscopic methods. They may be also studied by steady-state spectroscopic methods, but in this case additional data must be obtained by varying factors that affect the ratio between tf and xp. 

However, it was the microviscosity, rather than the bulk viscosity which was probed by using picosecond time-resolved fluorescence methods, since the value of C was constant within a homologous series of solvents but was different for monofunctional and difunctional alcohols. The probe molecule has a natural (radiative) lifetime, Xp of 4.6ns, so the reduction in lifetime results from the competition between fluorescence ( p 2xl0 s ) and the high rate of internal conversion (A ic lO" s ). This was attributed to rotation of the N-tolyl group of the dye, which then has to sweep through the solvent and hence probes the local environment. 

Figure 2a compares the time-resolved Stokes shift of the normal sequence and the abasic sequence. For ease of comparison, the data is shifted to overlap the sequences at early times. In the first nanosecond, the Stokes shifts from both sequences overlap almost perfectly. This results suggests that there is not a large scale collapse of the normal DNA structure at the abasic site. However after 1 ns, the abasic sequence has additional dynamics beyond those of the normal sequence. The fit of the abasic sequence has the same logarithmic component of the normal sequence fit, but with an additional exponential term for the fast rise in the Stokes shift after 1 ns S (r) = S (,-(-4)log o(r//o)-i-4(I xp(-r/r)), with an exponential time constant r of 25 ns. 

Fig. 4.24 Camphoric acid-derived PAD salts studied for gelation and AuCs formation a synthesis of AuCs over a gel bed 5.0 wt% DMF gel of G52 (n = 14) b time-resolved UV-vis spectra of the solution above the gel bed c, d FEG-TEM of wine-red and colorless solution, respectively e, f XPS of wine-red and colorless solution, respectively. Reprinted with permission from Ref. [96]. Copyright 2014 American Chemical Society...

In our recent work, [40] we addressed the synthesis of ZnO by using a facile and reproducible wet-synthesis route based on the controlled hydrolysis and condensation of zinc acetylacetonate as precursor in 4 different solvents (1,2 propanediol, water, ethanol and glycerol) as dispersing media. Irrespective of the nature of the solvent. X-ray Diffraction (XRD) analysis shows the formation of crystalline hexagonal ZnO. Indeed, different particles sizes and very different morphologies were obtained. The composition of the obtained ZnO was determined by elemental analysis. X-ray Photoelectron Spectroscopy (XPS), FT-IR and Thermogravimetric Analysis (TGA) analysis, whereas time resolved UV-Vis and XAFS... 

Figure 1-24. Angular resolved XPS analysis of a bilayer structure. Fe(II)/Fe(III) passive layer on iron developing at =-0.16 V in 1 M NaOH with passivation time tp (Haupt and Strehblow, 1987a).

In order to monitor the real-time dynamics of gas molecules interacting with surface, time-resolved study is required. It is generally known that the time domains for the gas adsorption/desorption on surface are within pico-second regime while the molecular vibration on surface is within femto-second regime. To accommodate this time-requirement as well as chemical analysis on surface, a type of pump and probe experiment is required, which makes use of synchronization between a laser pulse and a synchrotron radiation pulse of AP-XPS endstation. For example, the carrier dynamics and reaction mechanism of photocatalysts under AP conditions can be an ideal system to look at with this time-resolved experimental set-up. At present, the synchronization technique has been well developed as shown in a block diagram (Fig. 9.24). This time-resolved set-up can be further refined and adapted into advanced system when the free electron X-ray source is available. 

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