Light interaction with electrons

We discuss colour in Chapter 9, so we restrict ourselves here to saying the colour of a substance depends on the way its electrons interact with light crucially, absorption of a photon causes an electron to promote between the two frontier orbitals. The separation in energy between these two orbitals is E, the magnitude of which relates to the wavelength of the light absorbed X according to the Planck-Einstein equation, E = hc/X, where h is the Planck constant and c is the... 

The colour of a chromophore depends on the way its valence-shell electrons interact with light, i.e. its colour depends on the way it absorbs photons. Photons are absorbed during the promotion of an electron between wave-mechanically allowed (i.e. quantized) energy levels. The magnitude of the energy required to achieve this, E, is given by the Planck equation, as follows ... 

Delocalized electrons in a metal are free to move, keeping metallic bonds intact. The movement of mobile electrons around positive metallic cations explains why metals are good conductors. The delocalized electrons move heat from one place to another much more quickly than the electrons in a material that does not contain mobile electrons. Mobile electrons easily move as a part of an electric current when electrical potential is applied to a metal. These same delocalized electrons interact with light, absorbing and releasing photons, thereby creating the property of luster in metals. 

New materials can hardly compete with existing ones if they merely copy their properties without being distinctly cheaper. Instead it is necessary to use novel, hitherto unavailable combinations of properties in the case of the PAni developed by us, for example, its metal-like conductivity combined with transparency in the red and violet parts of the spectrum, its elasticity and ease of processing even for complicated shapes, its chemical and electronic interactions with light, and its extremely high surface tension. The material can also be reversibly reduced and oxidised, changing its colour in the process (dispersed PAni is normally green the reduced form is yellow, the oxidised form blue). Both an acid state and a base state exist. 

Interaction with light changes the quantum state a molecule is in, and in photochemistry this is an electronic excitation. As a result, the system will no longer be in an eigenstate of the Hamiltonian and this nonstationaiy state evolves, governed by the time-dependent Schrddinger equation... 

Perhaps the most obvious metallic property is reflectivity or luster. With few exceptions (gold, copper, bismuth, manganese) all metals have a silvery white color which results from reflecting all frequencies of light. We have said previously that the electron configuration of a substance determines the way in which it interacts with light. Apparently the characteristic reflectivity of metals indicates that all metals have a special type of electron configuration in common. 

The appearance of a plasmon resonance is strictly related to a distinct size of the corresponding metal, based on the presence of a confined electron gas that interacts with light and so results in typical colours. Is there also a minimum size where plasmon resonance is no longer possible In any case this must happen if a particle reaches a typical molecular status. There are no longer freely mobile... 

Besides these electronic phenomena, chemical reactions can be induced when stannylenes interact with light. M, F. Lappert et al. have described that certain stable stannylenes form radicals when irradiated 179,180). 

As the lattice interacts with light only through electrons, both DECP and ISRS should rely on the electron-phonon coupling in the material. Distinction between the two models lies solely in the nature of the electronic transition. In this context, Merlin and coworkers proposed DECP to be a resonant case of ISRS with the excited state having an infinitely long lifetime [26,28]. This original resonant ISRS model failed to explain different initial phases for different coherent phonon modes in the same crystal [21,25]. Recently, the model was modified to include finite electronic lifetime [29] to have more flexibility to reproduce the experimental observations. 

Fig. 1 The effect of size on metals. Whereas bulk metal and metal nanoparticles have a continuous band of energy levels, the limited number of atoms in metal clusters results in discrete energy levels, allowing interaction with light by electronic transitions between energy levels. Metal clusters bridge the gap between single atoms and nanoparticles. Even though in the figure the energy levels are denoted as singlets, we must remark that the spin state of the silver clusters is not yet firmly established...

When the size of metals is comparable or smaller than the electron mean free path, for example in metal nanoparticles, then the motion of electrons becomes limited by the size of the nanoparticle and interactions are expected to be mostly with the surface. This gives rise to surface plasmon resonance effects, in which the optical properties are determined by the collective oscillation of conduction electrons resulting from the interaction with light. Plasmonic metal nanoparticles and nanostructures are known to absorb light strongly, but they typically are not or only weakly luminescent [22-24]. 

It should be stated here that the term dye is used in this context in its broadest meaning, and is meant to include all organic compounds with conjugated double bonds since the mechanism of their interaction with light always involve the excitation or deactivation of -electrons. 

Photoexcitation, in orbital terms, can be described as the promotion of a single electron from HOMO to LUMO by the interaction with light. When either transox cw-2-butene is subjected to UV light, the same mixture of cis and trans isomers results. Explain. 

According to the reaction scheme (3.257) of the irreversible quenching by electron transfer, the electron donor has three electronic states involved in the interaction with light and electron acceptor, while the latter has only two states participating in the charge transfer ... 

For simplicity, the QD and SM indices in the e-ph constants have been omitted in Eq. (118) however, the frequencies and e-ph constants are obviously different in the both subsystems. In the proposed description, we assume that equilibrium Green s functions of the semiconductor and the quantum dot are known. However, to find QD equilibrium Green s function in a time-dependent field is not an easy task because it is not even clear whether Dyson equations for SM and QD Keldysh functions exist for different types of fermions interacting with each other. This problem is rather complicated even for molecular wires [54], Thus, we expect this problem to be even more complicated for solar cell systems where the interaction with light makes the problem essentially time dependent. In this section, we prove that Dyson equations for nonequilibrium Green s functions do exist. In our description, we adopt a graduate approach to the problem introducing different approximations step by step. As the first and the easiest step, we consider only uncorrelated electrons. 

Silver nanoparticles are of great importance due to their ability to efficiently interact with light because of plasmon resonances.15 These are collective oscillations of the conducting electrons in the metal. Indeed, Ag nanoparticles are envisaged to be vital components of optical and photonic devices in the fiiture. Over... 

Photodissociation involves electronic transitions initiated by the absorption of light. The key molecular property that mediates the interaction with light is the transition dipole moment. The electronic transition dipole moment between the jth and kth electronic state is defined by the integral over the electronic degrees of freedom for the operator where the dipole moment is sandwiched between the two electronic wave functions given by... 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

This understanding was finally achieved in the quantum theory of 1925, which provided for the first time an adequate explanation of how matter is constructed of atoms and molecules, how atoms are constructed of nuclei and electrons, and how atoms interact with light. Each of the major developments of nineteenth-century physical science played critical roles in leading up to quantum theory. These developments included electromagnetic theory, molecular theory of matter, and statistical thermodynamics. ... 

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