Photoelectric threshold energy

In routine spectrophotometers, photomultiplier tubes are replaced by photodiodes (Fig. 11.11), which have excellent sensitivity, linearity and dynamic range. The photoelectric threshold, in the order of 1 eV, allows detection up to wavelengths of 1.1 pm. In diode array systems, each rectangular rectangular diode (15 pm x 2.5 mm) is associated with a capacitor. The electronic circuit sequentially samples the charge of each capacitor. While a photomultiplier tube measures the instant intensity in watts, a diode measures the emitted energy in joules over a time interval. 

The energy required to remove an electron from the surface of a phthalo-cyanine crystal to infinity (surface ionization energy) has been measured by Pope (286) by an electrostatic method, and by Kearns and Calvin (176) by the photoelectric threshold method. The surface ionization energy is 5 eV and is independent of the presence or absence of the central metal ion. 

However, photodiode readout is inferior in terms of the low energy detection threshold which can be achieved. This is due to the lack of internal gain within the photodiode, which means that electronic readout noise will dominate the detector performance at low energies. A typical threshold for a Icc CsI(Tl) detector will be in the range 30-50 keV. At energies below 50 keV a bare silicon photodiode becomes a useful X-ray detector due to photoelectric absorption within the bulk of the silicon. 

Suchet J (1965) Chemical physics of semiconductors. Van Nostrand, Princeton Phillips JC (1970) Ionicity of the chemical bond in crystals. Rev Modem Phys 42 31-356 Nethercot AH Jr (1974) Prediction of Fermi energies and photoelectric thresholds based on electronegativity concepts. Phys Rev Lett 33 1088-1091... 

Poole RT, Williams D, Riley J et al (1975) Electronegativity as a unifying concept in the determination of Fermi energies and photoelectric thresholds. Chem Phys Lett 36 401 03 Chen ECM, Wentworth WE, Ayala JA (1977) The relationship between the Miilliken electronegativities of the elements and the work functions of metals and nonmetals. J Chem Phys 67 2642-2647... 

Solution. The photoelectric threshold of sodium metal is 650 nm. Accordingly, the photoelectrons that are produced have no kinetic energy the amount of energy in the photon is just enough to remove the electron from the metal. Hence an extremely small retarding potential would stop the flow of photoelectrons under these conditions. 

Jortner - has obtained the following relations between the solvation energy, the binding energy of the electron — E, and the outer photoelectric threshold a. 

The relation between photoelectric threshold and heat of solution has been discussed already in Parts 1-C and 2-C. The photoelectric experiments of Teal indicate that the photoelectric threshold in concentrated solutions is not very different from that in dilute solutions. This is to be expected if for the metal-like concentrated solutions, the cohesive energy due to the metallic binding between dimer-cluster units is small. There is independent evidence for this from the observed values of small viscosity and large compressibility in concentrated solutions, as discussed in Part 3-A. 

K, Busch etal. [5]. Metastable disordered films produced at 4.2 K have only = 3.85 0.1 OeV in the antiferromagnetic state. Possible origins for this behavior have been discussed in the paper [4]. On other polycrystalline films (produced at room temperature) the photoelectric threshold energy and the work function had. the same low value of 2.8 0.3 eV due to photoemission from impurity states at the Fermi level Ep extending down to 1.4 eV below Ep, Eastman et al. [6]. 

Photoelectron spectroscopy involves the ejection of electrons from atoms or molecules following bombardment by monochromatic photons. The ejected electrons are called photoelectrons and were mentioned, in the context of the photoelectric effect, in Section 1.2. The effect was observed originally on surfaces of easily ionizable metals, such as the alkali metals. Bombardment of the surface with photons of tunable frequency does not produce any photoelectrons until the threshold frequency is reached (see Figure 1.2). At this frequency, v, the photon energy is just sufficient to overcome the work function

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We were first introduced to the photoelectric effect as the emission of electrons when a surface is irradiated with light. The threshold is defined by hv = W where v is the frequency of the light and W a characteristic binding energy for the electron. It was soon realized that... 

The Maxwell-Heaviside theory seen as a U(l) symmetry gauge field theory has no explanation for the photoelectric effect, which is the emission of electrons from metals on ultraviolet irradiation [39]. Above a threshold frequency, the emission is instantaneous and independent of radiation intensity. Below the threshold, there is no emission, however intense the radiation. In U(l), electrodynamics energy is proportional to intensity and there is, consequently, no possible explanation for the photoelectric effect, which is conventionally regarded as an archetypical quantum effect. In classical 0(3) electrodynamics, the effect is simply... 

Two papers by Albert Einstein ultimately led to acceptance of the idea of quantization of energy for radiation, and were central to the development of the quantum theory (ironically, in later years Einstein became the most implacable critic of this same theory). The first of these papers, in 1905, concerned the photoelectric effect. Light ejected electrons from a metallic surface if the light had a greater frequency than some threshold frequency v0 which depended on the particular metal. The kinetic energy K of the emitted electrons was proportional to the excess frequency, v — v0 (Figure 5.4). Only the number of emitted electrons, not the kinetic energy, increased as the intensity increased. 

Now the electrons in a real metal, however, are situated as if they were in a potential box in which the value of the distance from the highest level to the edge is again equal to the energy of a light quantum of the threshold wave length of the photoelectric effect, but where the bottom of the potential box... 

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