Photo electric effect

R. A. Millikan (California Institute of Technology, Pasadena) work on the elementary charge of electricity and on the photo-electric effect. 

The fundamental principle of PES is the photo-electric effect. A molecule M in the gas phase is irradiated with monochromatic UV light which is usually generated by a helium discharge source (Hel 21.22 eV, 58.43 nm Hell 40.81 eV, 30.38 nm). Electrons can be ejected when their binding energy is lower than the photon energy leaving behind a radical cation M+ in a certain electronic and vibrational state. 

Photo-electric effect no yes liquids, powders, granular solids. Electromagnetic signal in visible light or infra-red region of electromagnetic spectrum is sensed by photo-electric cell. Beam is attenuated by liquids and cut off by solids. Liquids can transmit or reflect the beam. 

FIGURE 5.8 The photo- electric effect. A plot of the number of electrons ejected from a metal surface versus light frequency shows a threshold value. Increasing the intensity of the light while keeping the frequency the same increases the number of electrons ejected but does not change the threshold value. 

Considering the first equality, Is measurable. It Is the Isothermal reversible work required to extract ions from phase a for electrons In a metal It represents the electronic work function. For metals, a, can also be obtained from thermo-emission or the photo-electric effect. Sometimes a is called the real (Gibbs) energy of hydration of ion i. The logic behind this last definition stems from the second equality In [3.9.61. The standard molar Gibbs energy of solvation of an Ion [1.5.3.11 equals when Is referred to the gas... 

An experimental investigation of the lag in the photo-electric effect led to the conclusion that the sum of the lag of the Kerr effect in carbon bisulphide and the lag of the photo-electric effect from a potassium surface is not greater than 3(10 ) sec We read a paper on this research at the April meeting of the American Physical Society (abstract in press) and shall publish details later. 

Photo-electric effect> Maximum kinetic energy... 

The decrease in electrical resistance of selenium during exposure to light was first observed more than hundred years ago. Later a similar behaviour was also found with a number of other materials, but at that time the potential possibilities of the photo-electrical effect were not utilized practically. Today, however, the situation has changed and, for example, energy conversion with photo-voltaic solar cells has become an important technology. 

The time-dependent case is the easiest to interpret because it is absolutely basic to the quantum theory that there are quantum effects where very large changes in electron distribution are caused by time-dependent perturbations of arbitrarily small magnitude indeed, one of these processes — the photo-electric effect — was among the key experiments in demolishing the classical model at the electronic level. 

It is not possible within the scope of this collection of methods to deal with the theoretical processes in the interaction of radioactive emitters with matter (e.g. ionization, photo-electric effect, Compton effect, pair creation), the construction and operation of detectors, the structure and circuitry of the measuring equipment, or the electrical evaluation of the signals received. The reader should consult the specialized radiochemical literature. 

Behari [10] gives a useful general review of many solid state properties of bone, both human and non-human, many of which are not dealt with here. These properties include the Hall effect, photo-electric effects, electron paramagnetic resonance effects and so on. 

The photo-electric effect, on which XRF is based, and in which the original photon disappears by transferring all its energy to an electron. 

In the photo-electric effect, the energy of the incoming photon is totally transferred to an electron of the atom and the electron leaves the atom (Figure 2). 

The intensity of fluorescence is a function of the probability of a photo-electric effect, and since this is proportional to Eq, it decreases rapidly as Eo increases. Fig. 2 illustrates this decrease, if the photo-electric effect probability for Eo = 35 keV is normalised to 100 %. There is therefore an advantage in using a source with an excitation energy Eo as close as possible to the value of W, since in conditions of identical photon flux, the closer Eo to W, the higher the fluorescent signal. This fact is weighted, however, by two physical phenomenon the Compton scattered photons which become all the more numerous in the ray as Eo approaches Wj, and the tissue attenuation effect on incident photons which is all the greater as Eo approaches Wj. ... 

If Figures 2 and 5 are compared, it can be seen that between 35 and 60 keV, the probability of photo-electric effect drops from 100 % to 20 %, whereas beam transmission at a thickness of 50 mm rises, respectively, from 100 % to 146 %. Thus overall, considering the intensity of the fluorescent signal, it is better to work with values for E that are close to W. ... 

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