Absorption and Emission of Light

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- 

This chapter deals with basic considerations about absorption and emission of electromagnetic waves interacting with matter. Especially emphasized are those aspects that are important for the spectroscopy of gaseous media. The discussion starts with thermal radiation fields and the concept of cavity modes in order to elucidate differences and connections between spontaneous and induced emission and absorption. This leads to the definition of the Einstein coefficients and their mutual relations. The next section explains some definitions used in photometry such as radiation power, intensity, spectral power density and polarization of electromagnetic waves. 

It is possible to understand many phenomena in optics and spectroscopy in terms of classical models based on concepts of classical electrodynamics. For example, the absorption and emission of electromagnetic waves in matter can be described using the model of damped oscillators for the atomic electrons. In most cases, it is not too difficult to give a quantum-mechanical formulation of the classical results. The semiclassical approach will be outlined briefly in Sect. 2.8. 

Many experiments in laser spectroscopy depend on the coherence properties of the radiation and on the coherent excitation of atomic or molecular levels. Some basic ideas about temporal and spatial coherence of optical fields and the density-matrix formalism for the description of coherence in atoms are therefore discussed at the end of this chapter. 

Throughout this text the term light is frequently used for electromagnetic radiation in all spectral regions. Likewise, the term molecule in general statements includes atoms as weU. We shall, however, restrict the discussion and most of the examples to gaseous media, which means essentially free atoms or molecules. 

For more detailed or more advanced presentations of the subjects summarized in this chapter, the reader is referred to the extensive literature on spectroscopy [37-47]. Those interested in light scattering from solids are directed to the sequence of Topics volumes edited by Cardona and coworkers [48]. 

As stated previously, spectrochemical methods of analysis involve the absorption and emission of light. Absorption is considered in Sections 7.4.1 to 7.4.3. Emission is considered in Section 7.4.4. 

Thermal equilibrium means that a body (molecule, particle, surface) receives from its environment as much energy as it emits. The higher the temperature of the body, the higher is the radiant flux density of emitted (thermal) radiation. The percentage of absorbed radiation is called absorptivity (or absorbance) degree e (and emissivity, respectively), complementary with the reflectivity (1 - e). For each temperature the emitted radiation is linear with the absorption degree to obtain the thermal 

This called Kirchhoff s law. Hence, a body which does not absorb radiation of a given wavelength also cannot emit on this wavelength. 

the earth obtains energy from solar radiation and loses it (to maintain a thermal equilibrium) by reflection, emission and terrestrial radiation the temperature of the earth s surface and atmosphere thus corresponds to the thermal budget. 

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The principle of the electronic processes in molecules can be schematically illustrated with the classical Jablonski diagram, which was first proposed by Prof. A. Jablonski in 1935 to describe absorption and emission of light. Figure 3.8 illustrates the electronic processes of the host-guest molecules. 

These facts were explained by Einstein5 in 1905 in a way that now appears very simple, but in fact relies on concepts that were at the time revolutionary. Einstein went beyond Planck and postulated that not only was the process of absorption and emission of light quantized, but that light itself was quantized, consisting in effect of particles of energy... 

Absorption and emission of light Interaction of light with matter Particles and waves... 

Substituting (2.7) into Eq. (2.2), we obtain the angular dependence G(0, ip) of the probability of absorption and emission of light with polarization vector E ... 

In concluding this chapter, we can say that the description presented thus permits us to connect the probability of absorption and emission of light of any polarization with the orientation of the angular momentum of the molecule. This may serve as a basis for more detailed analysis of the processes of creating an anisotropic distribution of angular momenta both on the upper, as well as on the lower, level of an optical transition (Chapter 3), including the effect of an external field (Chapters 4, 5)... 

As previously mentioned, the interaction between protruding, doubly occupied dz orbitals critically affects the photophysical properties upon aggregate formation. Absorption and emission of light, excited-state lifetimes, and redox properties are dramatically affected. A particularly interesting feature is represented by the possibility of tuning the distance between the monomeric units and, consequently, the degree of electronic coupling... 

Metal nanostructures can act as small antennas that aid in the reception and broadcasting (absorption and emission) of light from nearby fluorophores. Whether fluorescence enhancement or quenching is observed in a given system is determined by the relative extent of excitation enhancement (increased light absorption), emission enhancement (increased radiative decay), and quenching (increased non-... 

Miishchenko MI, Travis LD, Lads AA. Scattering, absorption, and emission of light by small particles. E ed, Cambridge Cambridge University Press, 2002 480pp. 

Fig 3. Absorption of photon and the various possibie fates of an excited state (A). Straight arrows represent absorption and emission of light, wavy arrows represent non-radiative transitions. (B) and (C) show the electron spin states in the singlet ground state and in the singlet and triplet excited states. (D) shows simplified absorption and emission spectra numbered to correspond to steps in (A). See text for discussion. 

It should be emphasized here that the electron transfer in the activated state is a very fast process which occurs within a time interval of about 10" s. The relaxation times for the solvent and the reacting nuclei are much longer, typically 10"" to 10" - s for vibrational motion and 10" to 10"" s for rotational motion. Accordingly, it is a reasonable approximation that the positions of the nuclei are unchanged in the course of the electron transfer. This condition is called the Franck-Condon principle. It is well known from studies of absorption and emission of light by molecules. 

Analytical atomic spectroscopy is based on the absorption and emission of light by atoms (4,6-8). These processes originate with the promotion of atoms in their ground electronic states to electronically excited states and the return of electronically excited atoms to their ground electronic states. Because the frequencies as well as the intensities of the light absorbed and emitted are determined by the... 

Eq. (1.125) shows that the transition rate of absorption, fVab, is proportional to the number of photons, nkv, while Eq. (1.126) suggests that, for the transition rate of emission, IVem, there exist two terms, one which is proportional to the number of photons and one independent of the number of photons emitted. Thus, absorption and emission of light are not symmetric phenomena, as suggested already by Einstein in his kinetic formulation for absorption and emission of light. The term proportional to the number of photons in the emission process is called stimulated emission. The term independent of nh. is called spontaneous emission. The stimulated emission is the basis of the laser oscillation, as will be described in Section 1.4.5. 

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