Interactions of Photons with Matter

Photons, also called X-rays or -y-rays, are electromagnetic radiation. Considered as particles, they travel with the speed of light c and they have zero rest mass and charge. The relationship between the energy of a photon, its wavelength A, and frequency is 

There is no clear distinction between X-rays and y rays. The term X-rays is applied generally to photons with 1 MeV. Gammas are the photons with 1 MeV. In what follows, the terms photon, y, and X-ray will be used interchangeably. 

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Either as a wave or a particle there are a number of possible interactions of photons with matter and for the moment we consider the atom, although any matter will do. The list of possible interactions is as follows (although it is not... 

Photons with energies lower than the work function, i.e. when < IV, do not have the capacity to cause the release of photoclectrons. Equation (1.8) is based on the first law of thermodynamics (i.e. the law of conservation of energy). The photoelectric effect is shown diagram-matically in Figure 1.2, and Figure 1.3 is a representation of two cases of the possible interactions of photons with matter. 

Because the interaction of photons with matter is quite complicated and depends strongly on the electronic structure, the various derivatives of the dipole moment and polarizability tensor cannot be estimated easily. Even calculations with the most advanced quantum methods are far from providing reliable estimations. Therefore, a major drawback of optical techniques is that intensities cannot be interpreted with confidence. 

To understand the production of laser light, it is necessary to consider the interaction of light with matter. Quanta of light (photons) of wavelength X have energy E given by Equation 18.1, in which h is Planck s constant (6.63 x 10 J-sec) and c is the velocity of light (3 x 10 m-sec-h-... 

Fluorescence and phosphorescence are particular cases of luminescence (Table 1.1). The mode of excitation is absorption of a photon, which brings the absorbing species into an electronic excited state. The emission of photons accompanying deexcitation is then called photoluminescence (fluorescence, phosphorescence or delayed fluorescence), which is one of the possible physical effects resulting from interaction of light with matter, as shown in Figure 1.1. 

The reduction of obtainable light-pulse durations down to subpicosecond pulses (halfwidth about 10 sec) allows fast transient phenomena which were not accessible before to be studied in the interaction of light with matter. One example is the extension of spin echoe-techniques, well known in nuclear-magnetic-resonance spectroscopy, to the photon echoes in the optical region. 

The absolute values of the photoabsorption, photoionization, and photodissociation cross sections are key quantities in investigating not only the interaction of photons with molecules but also the interaction of any high-energy charged particle with matter. The methods to measure these, the real-photon and virtual-photon methods, are described and compared with each other. An overview is presented of photoabsorption cross sections and photoionization quantum yields for normal alkanes, C H2 + 2 n = 1 ), as a function of the incident photon energy in the vacuum ultraviolet range and of the number of carbon atoms in the alkane molecule. Some future problems are also given. 

The interaction of light with matter gives rise to many varied and fascinating phenomena. Molecular photochemistry at the most basic level deals with interactions of molecules and photons to generate different electronic configurations, which may show substantially different chemical reactivity than ground-state species (1). Photochemical reactions may involve many electronic states, each of different character, which may be coupled strongly... 

We now consider the effect of exposing a system to electromagnetic radiation. Our treatment will involve approximations beyond that of replacing (3.13) with (3.16). A proper treatment of the interaction of radiation with matter must treat both the atom and the radiation field quantum-mechanically this gives what is called quantum field theory (or quantum electrodynamics). However, the quantum theory of radiation is beyond the scope of this book. We will treat the atom quantum-mechanically, but will treat the radiation field as a classical wave, ignoring its photon aspect. Thus our treatment is semiclassical. 

The interaction of electrons with matter is different from that of heavier charged particles for two reasons. One of them is the electron mass which is more than two orders of magnitude lower than that of the second lightest charged particle, the muon this makes photon radiation very important in the stopping power of electrons even at lower energies. The other reason is that at low energies, the interaction with shell electrons dominates and that is collision between identical particles, which has to be taken into account in the calculations. 

Most of what has just been stated for the interaction of light with matter (the absorption or excitation) still holds for the radiative deactivations (the luminescence mechanisms). The Frank Condon principle is applicable for downward radiative transitions, and all the other selection rules (spin and synunetry) are as valid for the absorption as for the emission of a photon. 

To observe a cosmic gamma ray, one must first devise a means to stop it (or at least to slow it down in some measurable way) within a detecting medium. In the gamma-ray regime, the primary interaction mechanisms of photons with matter are the photoelectric effect, the Compton effect, and pair production. Extensive analyses are available of these fundamental interaction processes. Here, we only briefly review their basic physical characteristics. 

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