Metals photoelectric effect with

R. Pohl and P. Pringsheim 32 located a selective maximum photoelectric effect with four alkali metals for wave-lengths A in fj.fi... 

Use ideas presented in this chapter to indicate (a) three metals that you would expect to exhibit the photoelectric effect with visible light and three that you would not G ) the noble gas element that should have the highest density in the liquid state (c) the approximate first ionization energy j of fermium (Z = 100) (d) the approximate density of solid radium (Z = 88). 

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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In the photoelectric effect, energy absorbed from photons provides information about the binding energies of electrons to metal surfaces. When light interacts with free atoms, the interaction reveals information about electrons bound to individual atoms. 

Based on the photoelectric effect, electrons in evacuated tubes (photoelectrons) are released from a metal surface if it is irradiated with photons of sufficient quantum energy. These are simple photocells. Photomultipliers are more sophisticated and used in modem spectrophotometers where, via high voltage, the photoelectrons are accelerated to another electrode (dynode) where one electron releases several electrons more, and by repetition up to more than ten times a signal amplification on the order of 10 can be obtained. This means that one photon finally achieves the release of 10 electrons from the anode, which easily can be measured as an electric current. The sensitivity of such a photomultiplier resembles the sensitivity of the human eye adapted to darkness. The devices described are mainly used in laboratory-bound spectrophotometers. 

The photoelectric method is based on the photoelectric effect. The kinetic energy of the electrons emitted during illumination of a metal with light having a frequency v obeys the Einstein equation... 

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. 

Figure 12.5 The photoelectric effect. Light with photons of energy hc/k approaches from the left, strikes the atoms in the metal, and ejects a photoelectron with a kinetic energy equal to the photon energy minus the work function of the metal. This demonstrates the particulate nature of light.

The photoelectric effect Light incident on a clean metal surface ejects electrons with energies that depend only on the frequency but not on the intensity of the light. The intensity affects the number of electrons emitted. 

The photoelectric effect has received its simplest explanation in terms of the electron theory in which the phenomenon is attributed to the emission of electrons under the influence of ultra-violet light. J. J. Thomson 8 proved that the carriers of negative electricity from an illuminated metal plate are identical with the cathode rays of a Crookes tube, and consist of negatively electrified corpuscles or negative electrons carrying an electrical charge equal to that concerned in electrolytic... 

A distinction must be made between Ihe coniacl potentials in air and the so-called "intrinsic contact potentials in a vacuum with all adsorbed gases removed According to Millikan, the intrinsic potential difference between two metals A and B is expressed by VAfl = (i(v4 - vg /e. in which It is Planck s constant. t, i and ug are the critical frequencies of phutoeleclric emission for the two metals (see Photoelectric Effect), and e is the electronic charge. In any ease, if (he electronic work funclions of the metals are )>,t and ps- the contact potential difference is V. sa = [p./, - pn)/e. The work funclions. and hence V, . are in general dependent upon the medium surrounding the metals. Accurate measurements of these potentials are. unlorlunnlely, very difficult. 

The characteristics of the photoelectric effect are easy to explain in terms of photons. If the incident radiation has frequency v, it consists of a stream of photons of energy hv. These particles collide with the electrons in the metal ... 

Suhrmann (62) explains the strong increase of the normal photoelectric effect of metals caused by the adsorption of water molecules and also by the molecules of ammonia, by accepting similar coordinate links to function in the chemisorption of these molecules. Dipoles are formed which point with their positive poles away from the surface, thereby decreasing the work function and, consequently, increasing the normal photoelectric effect ... 

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