Photoelectric threshold

Another phenomenon that was inexplicable in classical terms was the photoelectric effect discovered by Hertz in f 887. When ultraviolet light falls on an alkali metal surface, electrons are ejected from the surface only when the frequency of the radiation reaches the threshold... 

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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C07-0115. The photoelectric effect for magnesium metal has a threshold frequency of 8.95 X 10 s". Can Mg be used in photoelectric devices that sense visible light Do a calculation in support of your answer. 

G.W. Gobeli and F.G. Allen, Photoelectric Threshold and Work Function... 

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 settling of certain particular amplitude depends on the initial conditions. When the motion becomes stationary the amplitude s value practically does not depend on the wave s intensity when the latter changes over a significant range above a certain threshold value. This is reminiscent of Einstein s explanation of the photoelectric effect using Planck s quantization hypothesis. In this case the absorption is also independent of the incoming wave s intensity. Besides, the absorbed... 

The photoelectric effect is the emission of electrons from the surface of a metal when light shines on it. Electrons are emitted, however, only when the frequency of that light is greater than a certain threshold value characteristic of the particular metal. The alkali metals, with only one electron in their valence shells, have the lowest threshold values. 

E. Antoncik and J. Tauc, Quantum Efficiency of the Internal Photoelectric Effect in InSb G. W. Gobeli and /. G. Allen, Photoelectric Threshold and Work Function... 

Richardson and Young (Proc. Roy. Soc. A, evil. 377,1925) in an examination of the thermionic and photoelectric emission from surfaces of sodium and potassium have observed more than one threshold value for the work functions and suggest that in these cases also there are small patches of the surface associated with a low value of the work function. 

In the photoelectric method, the measured average work function is always less than the true since patches of high work function tend to be excluded from the emission process. Thus, the nonuniform distribution of adsorbate on a patch surface may cause a slight discrepancy in the evaluation of A. Experimentally, the photoelectric method has various limitations. Photocurrents of the order of 10 A. must be measured accurately in the region of vo, and for films of work function greater than 5 v., the threshold frequency lies in the far ultraviolet—a practical disadvantage. Furthermore, the method is inapplicable at pressures in excess of 10 mm. Hg because of ionization of the gas by collision. 

A photoelectric method was also used by Baker and Rideal (76) for studying the adsorption of Ha, CO, and CaH on evaporated metal films of Ta, Fe, Ni, and Co. Photocurrents near the threshold were too small to be measured accurately, so the threshold frequencies were obtained by in-... 

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... 

In Tables V and VI data for the work functions for different organic solid compounds are summarized. It is peculiar that for anthracene no reliable value could be found and the margin of

contact potential difference for anthracene is 0.8 v and from that value one should expect

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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. 

W being the work function of the photosensitive material. The threshold of the photoelectric effect corresponds to the condition hv = W, for which the velocity of emitted electrons is 0. 

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. 

Einstein s hypothesis, then, led to two definite predictions. In the first place, there should be a photoelectric threshold frequencies less than a certain limit, equal to 4>/h, should be incapable of ejecting photoelectrons from a metal. This prediction proved to be verified experimentally, and with more and more accurate determinations of work function it continues to hold true. It is interesting to see where this threshold comes in the spectrum. For this purpose, it is more convenient to find the wave length X = c/v corresponding to the frequency /h. If we express in electron volts, as is commonly done, (see Eq. (1.1), Chap. IX), we have the relation... 

All wave lengths shorter than the threshold of Eq. (3.2) can eject photoelectrons. Thus a metal with a small work function of two volts (which certain alkali metals have) has a threshold in the red and will react photoelectrically to visible light, while a metal with a work function of six volts would have a threshold about 2000 A, and would be sensitive only in the rather far ultraviolet. Most real metals lie between these limits. 

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