The Photoelectron Spectrometer

The aim of the XPS-experiment is the determination of binding energies by measuring the velocity of the emitted photoelectrons. Fig. 3 shows the in- 

Schematic presentation of a typical photoelectron spectrometer. The sample is introduced from the top. For detailed discussion see text 

Electrons from the heated tungsten filament are accelerated to the annular anode. Depending on the anticathode material a characteristic fluorescence radiation is emitted, passes through a thin Aluminum window and induces photoelectrons on the surface of the analytical sample. These photoelectrons are deflected in the spherical electrostatic analyzer, double focussed to eliminate stray electrons and finally counted by the electron multiplier. The whole system works under a vacuum of 10-s to 10 7 torr or even 10 10 torr, if surface properties have to be studied. This vacuum is generated by a Titanium 

The line broadening Aa introduced by an electrostatic analyzer is a function of the kinetic energy EK of the photoelectrons 

There are basically two different ways of sweeping a spectrum. Either the potential between the two spheres of the analyzer is increased continuously, thus bringing electrons of higher energy into the focus. Or the analyzer energy Ea, i.e. the kinetic energy of the electrons that are focussed at the specific ana- 

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The photoelectron spectrometer is an instrument which scans the range 0 < <... 

A number of optical photoion-photoelectron coincidence experiments have been reported107 giving qualitative information on molecular breakdown. However, in these experiments little or no attempt has been made to correct for either the transmission efficiency of the photoelectron spectrometer or the ion kinetic-energy discrimination. Fragment ions usually... 

X-ray photoelectron spectroscopy (XPS) was used for elemental analysis of plasma-deposited polymer films. The photoelectron spectrometer (Physical Electronics, Model 548) was used with an X-ray source of Mg Ka (1253.6 eV). Fourier transform infrared (FTIR) spectra of plasma polymers deposited on the steel substrate were recorded on a Perkin-Elmer Model 1750 spectrophotometer using the attenuated total reflection (ATR) technique. The silane plasma-deposited steel sample was cut to match precisely the surface of the reflection element, which was a high refractive index KRS-5 crystal. 

Bodmer, A. The photoelectron spectrometer. In Conference abstracts X-ray photoelectron spectroscopy , p. 1. Zurich, October 1971. 

Molecules may be subjected to various stresses before they are ionized. Examples include pyrolysis, rf or mw heating, electron attachment, laser irradiation, collisions with radicals or other highly reactive species, etc. The unstable products and transients, which may also be highly excited, often constitute the sample introduced into the photoelectron spectrometer. The investigation of these transients, which may be referred to as active electron spectroscopy has provided much information about the existence, formation and properties of many previously unknown neutral, ionic and radical species. 

The photoelectron spectrometer is an instrument which scans the range 0 < kin < hv of die kinetic energies kin of the ejected photoelectrons and thus—according to equation 4—the range 0 < I < hv of ionization energies, recording for each value of 7 the count rate cps (cps = counts per second), i.e. the number of electrons ejected per second from a stream of molecules M in the gas phase. The plot of cps vs 7 is known as the photoelectron (pE) spectrum of M. Figure 1 shows the pE spectrum of a hypothetical molecule M. 

The PES measurements are performed with reference to the Eermi level of the photoelectron spectrometer, in solid specimens, as dealt with here, by the way the spectroscopy works. Thus, in cases when the Eermi level shifts due to some chemical modifications of the sample, i.e., in the intercalation of graphite or other layered compound [16] or in the doping of conjugated polymers [17], it is necessary to account for the change in the Eermi energy level before interpreting... 

An example of a spectrum with a chemical shift is that of the tin 3d peaks in Eig. 2.8. A thin layer of oxide on the metallic tin surface enables photoelectrons from both the underlying metal and the oxide to appear together. Resolution of the doublet 3 ds/2, 3 dii2 into the components from the metal (Sn ) and from the oxide Sn " is shown in Eig. 2.8 B. The shift in this instance is 1.6-1.7 eV. Curve resolution is an operation that can be performed routinely by data processing systems associated with photoelectron spectrometers. 

In principle, therefore, the surface concentration of an element can be calculated from the intensity of a particular photoelectron emission, according to Eq. (2.6). In practice, the method of relative sensitivity factors is in common use. If spectra were recorded from reference samples of pure elements A and B on the same spectrometer and the corresponding line intensities are and respectively, Eq. (2.6) can be written as... 

Some X-ray photoelectron spectrometers are equipped with monochromators that can be used to remove unwanted radiation, such as the continuous radiation and even some of the weaker characteristic X-rays such as K<,3, K 4, Kas, and Ko,6, from the emission spectrum of the anode. A monochromator can also be used to resolve the K i,2 line into its two components K i and Ka2- Using a monochromator has at least two beneficial effects. It enables the narrow, intense K<, line to be used to excite spectra at very high resolution. A monochromator also prevents unnecessary radiation (continuous, K<,2, Ka3, K<,4, Kas, and Ka6) that might contribute to thermal or photochemical degradation from impinging on the sample. 

In the majority of spectrometers, A1 and Mg are commonly used as x-ray target materials. With two anodes, A1 and Mg, it is possible to resolve overlapping photoelectron and Auger electron peaks. This is because in an XPS spectrum the position of the Auger peaks changes if Al radiation is replaced by Mg K radiation, but the positions of the photoelectron peaks are unaltered. 

One of the most direct methods is photoelectron spectroscopy (PES), an adaptation of the photoelectric effect (Section 1.2). A photoelectron spectrometer (see illustration below) contains a source of high-frequency, short-wavelength radiation. Ultraviolet radiation is used most often for molecules, but x-rays are used to explore orbitals buried deeply inside solids. Photons in both frequency ranges have so much energy that they can eject electrons from the molecular orbitals they occupy. 

The Fe-B nanocomposite was synthesized by the so-called pillaring technique using layered bentonite clay as the starting material. The detailed procedures were described in our previous study [4]. X-ray diffraction (XRD) analysis revealed that the Fe-B nanocomposite mainly consists of Fc203 (hematite) and Si02 (quartz). The bulk Fe concentration of the Fe-B nanocomposite measured by a JOEL X-ray Reflective Fluorescence spectrometer (Model JSX 3201Z) is 31.8%. The Fe surface atomic concentration of Fe-B nanocomposite determined by an X-ray photoelectron spectrometer (Model PHI5600) is 12.25 (at%). The BET specific surface area is 280 m /g. The particle size determined by a transmission electron microscope (JOEL 2010) is from 20 to 200 nm. 

TEM observation and elemental analysis of the catalysts were performed by means of a transmission electron microscope (JEOL, JEM-201 OF) with energy dispersion spectrometer (EDS). The surface property of catalysts was analyzed by an X-ray photoelectron spectrometer (JEOL, JPS-90SX) using an A1 Ka radiation (1486.6 eV, 120 W). Carbon Is peak at binding energy of 284.6 eV due to adventitious carbon was used as an internal reference. Temperature programmed oxidation (TPO) with 5 vol.% 02/He was also performed on the catalyst after reaction, and the consumption of O2 was detected by thermal conductivity detector. The temperature was ramped at 10 K min to 1273 K. 

We acknowledge financial support of this work by the Gas Research Institute (Contract No. 5982-260-0756). NSF support of the x-ray photoelectron spectrometer by an equipment grant, CHE-8201179, Is also acknowledged. 

The Ti02 (001) surface was cleaned and reduced by cycles of ion bombardment as previously described [3]. The distribution of titanium oxidation states was determined from cxirve fitting the Ti(2p3/2) envelope in x-ray photoelectron spectra [3]. After surface preparation, reaction experiments were conducted in either the TPD or steady state mode. TPD experiments have been described [1]. XPS spectra were also obtained following a saturation exposure of the sample using the same procedure as that for the TPD experiments. After pump down, the crystal was placed under the Mg X-ray source and the Ti(2p), 0(ls), and C(ls) regions were scanned. For steady-state experiments a dosing needle was aligned perpendicular to the axis of the mass spectrometer. It was used to direct a steady beam of methylacetylene (Linde, 95%) at the crystal surface when the sample was placed at the aperture of the mass spectrometer. Steady state reaction experiments were... 

The XPS spectra of the freshly sulfided Co-Mo/NaY catalysts were measured on an XPS-7000 photoelectron spectrometer (Rigaku, A1 anode 1486.6 eV). The sample mounted on a holder was transferred from a glove bag into a pretreatment chamber attached to the spectrometer as possible as carefully not to be contacted with air. The binding energies (BE) were referenced to the Si2p band at 103.0 eV for the NaY zeolite, which had teen determined by the Cls reference level at 285.0 eV due to adventitious carbon. 

Analytical Techniques. Sessile drop contact angles were measured with a NRL C.A. Goniometer (Rame -Hart, Inc.) using triply distilled water. The contact angles reported are averages of 2-8 identically treated samples with at least three measurements taken on each sample. ESCA spectra were obtained on a Kratos ES-300 X-ray Photoelectron Spectrometer under the control of a DS-300 Data System. Peak area measurements and band resolutions were performed with a DuPont 310 Curve Resolver. 

The photoelectron spectra of 3 and 25 (but not 1) showed evidence of thermolytic dissociation of the disilenes into silylenes in the spectrometer. This type of dissociation will be treated in Section VIII.B. 

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