Light-matter interactions electronic absorption

Resonant light—matter interaction occurs when the photon energy (or frequency of the oscillating field) matches the energy difference (transition frequency) between rotational, vibrational, or electronic states of the system leading to absorption or... 

The quality of fabrication depends on the laser-matter interaction, which is related to the laser parameters, that is, laser power, laser frequency, pulsing frequency, and pulse duration. The absorptivity of the laser energy inside the matter depends on the frequency of laser. Hence, the frequency of laser is decided such that maximal portion of incident laser photon is absorbed by the material being machined. The complete description of laser-matter interaction requires the solution of Maxwell equation for the laser fleld coupled with the matter. Hence, the total problem is interconnected by simpler problems involving absorption of laser light, ionization, energy transfer from photon to electrons and ions, heat conduction, and hydrodynamic... 

Photochemistry is the branch of chemistry which relates to the interactions between matter and photons of visible or ultraviolet light and the subsequent physical and chemical processes which occur from the electronically excited state formed by photon absorption. 

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 interaction between light and matter can be viewed as the creation of a coherent quantum superposition of initial and final electron states that has an associated polarization [3], as shown in Figure 1. The coherence between states with different wave vector requires an intermediate virtual state and the presence of a coherent phonon. A transition between the initial and final states may occur when the coherence of the system is broken either due to the finite width of an optical wave packet or by scattering from the environment. The transition results in the absorption of a photon and the creation of a hot electron-hole pair. Otherwise, the photon is re-radiated with a different phase and, perhaps, polarisation. 

Abstract Photochemistry is concerned with the interaction between light and matter. The present chapter outlines the basic concepts of photochemistry in order to provide a foundation for the various aspects of environmental photochemistry explored later in the book. Electronically excited states are produced by the absorption of radiation in the visible and ultraviolet regions of the spectrum. The excited states that can be produced depend on the electronic structure of the absorbing species. Excited molecules can suffer a variety of fates together, these fates make up the various aspects of photochemistry. They include dissociation, ionization and isomerization emission of luminescent radiation as fluorescence or phosphorescence and transfer of energy by intramolecular processes to generate electronic states different from those first excited, or by intermo-lecular processes to produce electronically excited states of molecules chemically different from those in which the absorption first occurred. Each of these processes is described in the chapter, and the ideas of quantum yields and photonic efficiencies are introduced to provide a quantitative expression of their relative contributions. 

Baev et al. review a theoretical framework which can be useful for simulations, design and characterization of multi-photon absorption-based materials which are useful for optical applications. This methodology involves quantum chemistry techniques, for the computation of electronic properties and cross-sections, as well as classical Maxwell s theory in order to study the interaction of electromagnetic fields with matter and the related properties. The authors note that their dynamical method, which is based on the density matrix formalism, can be useful for both fundamental and applied problems of non-linear optics (e.g. self-focusing, white light generation etc). 

NLO phenomena result from the interaction between light and matter and, more precisely, between the polarisable electron density and the strong electric field associated with a very intense laser beam. They were experimentally observed firstly in 1961 just after the development of intense laser sources in particular by Kaiser and Garett for two-photon absorption and by Fra n ken for They can be divided in two... 

When dealing with low-energy infrared radiation, the interaction with matter is limited to the absorption of light by the outer shell electrons, i.e. those used in forming compounds. Hence, particular bonds will absorb particular wavelengths. This is the principle used for infrared spectroscopy. There are equivalent techniques for ultraviolet radiation and visible radiation, but they are mostly used to provide information about concentration of a given compound, rather than for identification purposes such as XRF or IR techniques. 

---