Emission photo-electron

As has been demonstrated by various authors an expression for the photo electron current intensity l E,k,ms uj,q, ) observed in VB-photo emission can be derived by... 

The sensitivity of a photo-emissive cell (phototube) may be considerably increased by means of the so-called photomultiplier tube. The latter consists of an electrode covered with a photo-emissive material and a series of positively charged plates, each charged at a successively higher potential. The plates are covered with a material which emits several (2-5) electrons for each electron collected on its surface. When the electrons hit the first plate, secondary electrons are emitted in greater number than initially struck the plate, with the net result of a large amplification (up to 106) in the current output of the cell. The output of a photomultiplier tube is limited to several milliamperes, and for this reason only low incident radiant energy intensities can be employed. It can measure intensities about 200 times weaker than those measurable with an ordinary photoelectric cell and amplifier. 

Coherent lattice motions can create periodic modulation of the electronic band structure. Time-resolved photo-emission (TRPE) studies [20-22] demonstrated the capability to detect coherent phonons as an oscillatory shift of... 

Photoemissive tubes are necessary for work in the ultraviolet range and they show greater sensitivity and precision than photoelectric cells. A simple photo-emissive tube consists of two electrodes in a vacuum. A silver cathode coated with an alkali metal is maintained at a potential difference of about 100 V from the anode, which is a plain silver wire and serves to collect the electrons (Figure 2.26(a)). 

The characterization of graphene often involves several techniques in conjunction in order to build up a complete picture of the material. The techniques typically include electron microscopy, Raman spectroscopy, X-ray photo-emission spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and thermal-gravimetric analysis (TGA). 

This chapter is devoted to photoemission spectroscopy and the related inverse photo-emission spectroscopy, which are well developed experimental tools to study occupied and empty electronic levels, respectively. Special emphasis is given to the 5f electrons and their localized or delocalized character. 

One deals with the ab initio description of electronic excited states. These include the attachment or removal of electrons, the account of direct or inverse photo-emission spectra, and the electron-hole excitations of the d -> d or charge transfer type. Advanced methods are presently under development to account for them the GW method, the SIC method, the LDA-I-U method, etc. However, they imply an increased computation cost, which is not routinely accessible for complex systems, such as most oxide surfaces. These methods are also expected to open the field of strongly correlated materials, among which transition metal oxides, which have important technological applications high-Tc superconductivity, giant magneto-resistance, etc. 

The electronic structure of the Cu 100 -c(2x2)-Pd, Au and Mn systems have been probed by ARUPS. Wang et al. [16] have compared valence band photo-emission spectra using synchrotron radiation from the Cu 100 -c(2x2)-Au surface alloy with a CusAullOO bulk alloy terminated by a mixed c(2x2) CuAu monolayer. A surface-induced Au d-band narrowing of 0.45 eV was foimd for the Cu 100 -c(2x2)-Au surface alloy, despite the smaller lattice... 

A photomultiplier (PM) tube is more sensitive than a phototube for the visible and ultraviolet regions. It consists of a photoemissive cathode, which the photon strikes, and a series of electrodes (dynodes), each at a more positive potential (50 to 90 V) than the one before it. When an electron strikes the photo-emissive surface, a primary electron is emitted (this is the photoelectric effect— Albert Einstein received the 1921 Nobel Prize in Physics for its discovery in 1905, not for the special theory of relativity which he also introduced in 1905—see www.lucidcafe.com/lucidcafe/librarv/96mar/einstein.htmD. The primary electron released from the photoemissive surface is accelerated toward the first dynode. The impact of the electron on the dynode surface causes the release of many secondary electrons, which in turn are accelerated to the next electrode where each secondary electron releases more electrons, and so on, up to about 10 stages of amplication. The electrons are finally collected by the anode. The final output of the photomultiplier tube may, in turn, be electronically amplified. 

Photoemission and neutron diffraction measurements are limited only to UBe13. The combined XPS-BIS experiments of Wuilloud et al. (1984) displayed a picture similar to other very narrow-band compounds with 5f states extended to 2 eV below Ep, while the 5f intensity is spread above 5 eV about EF. Pronounced satellites corresponding to poorly screened final states accompany the core 4f lines of U, but the XPS spectrum of the Be Is level remains unaffected by hybridization with U electron states. A temperature-dependent narrow feature was distinguished at EF by means of high-resolution studies (Arko et al. 1984). The existence of a low intensity 5f-tail extended far below EF, which can be resolved by resonant photo-emission, is taken as an indication for the hybridization of 5f states with Be-derived conduction-band states (Parks et al. 1984). 

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