Electromagnetic field interaction with atom

So far considerable progress has been made. We have a fairly reasonable understanding of how the electromagnetic field interacts with an atom, and have in hand an expression that gives the transition probability for the absorption and emission of a photon by an atom. This expression has been demonstrated to be remarkably accurate in its description of the interaction of light with atomic structure. Additional features may be included to account for the permutation symmetry of various photons that interact with an atom. Explicit consideration may also be given for the probability that the atoms may also emit a photon once in the excited state. These considerations can be found in many textbooks on quantum electrodynamics. 

An electromagnetic field interacting with an atom or molecule can be represented by perturbation with an oscillatory time dependence of the form... 

An individual axisymmetric photon wavepacket that propagates in vacuo and meets a mirror surface, should be reflected in the same way as a plane wave, on account of the matching of the electromagnetic field components at the surface. Inside a material with a refraction index greater than that in vacuo, the transmission of the wavepacket is affected by interaction with atoms and molecules, in a way that is outside the scope of the discussion here. 

One can see that the full Hamiltonian consists of three terms, two which describe separately the parts for the atom and the field, and one which represents a coupling between the field (vector potential A) and terms from the atom (operator V,-). Obviously, it is this mixed term which is responsible for the photon-atom interaction. Provided perturbation theory can be applied, this term then acts as a transition operator between undisturbed initial and final states of the atom. Following this approach, one has to verify whether the disturbance caused by the electromagnetic field in the atom is small enough such that perturbation theory is applicable. Hence, one has to compare the terms which contain the vector potential A with an energy ch that is characteristic for the atomic Hamiltonian ... 

A volume of charged, current-containing matter, such as an atomic nucleus, interacts with the electromagnetic field. The electric field interacts with only the nuclear charge distribution, while the magnetic field interacts with the nuclear current distribution. The interaction energy, H%, between the electric field and the nuclear charge may be written as... 

Absorption of high energy electron beams occurs through interaction with the orbital electrons and the electromagnetic field at the atom. The processes are summarized in Table 6.1 and Figure 6.8. In order to distinguish between electrons from accelerators and those from /3-decay we refer to the latter as -particles. 

Theoretical Chemical Physics encompasses a broad spectrum of Science, where scientists of different extractions and aims jointly place special emphasis on theoretical methods in chemistry and physics. The topics were gathered into eight areas, each addressing a different aspect of the field 1 - electronic structure of atoms and molecules (ESAM) 2 - atomic and molecular spectra and interactions with electromagnetic fields (AMSI) 3 - atomic and molecular interactions, collisions and reactions (AMIC) 4 - atomic and molecular complexes and clusters, crystals and polymers (AMCP) 5 - physi / chemi-sorption, solvent effects, homogeneous and heterogeneous catalyses (PCSE) 6 - chemical thermodynamics, statistical mechanics and kinetics, reaction mechanisms (CTRM) 7 -molecular materials (MM), and 8 - molecular biophysics (MB). There was also room for contributions on electrochemistry, photochemistry, and radiochemistry (EPRC), but very few were presented. 

These fluctuations will affect the motion of charged particles. A major part of the Lamb shift in a hydrogen atom can be understood as the contribution to the energy from the interaction of the electron with these zero point oscillations of the electromagnetic field. The qualitative explanation runs as follows the mean square of the electric and magnetic field intensities in the vacuum state is equal to... 

A fourth possibility is electrodynamic bonding. This arises because atoms and molecules are not static, but are dynamically polarizable into dipoles. Each dipole oscillates, sending out an electromagnetic field which interacts with other nearby dipoles causing them to oscillate. As the dipoles exchange electro-magnetic energy (photons), they attract one another (London, 1937). 

Suppose now that the system is subjected to an oscillating electromagnetic field with a representative Fourier component of the electric field F0( >] cos cot. The predominant term in the interaction energy V is usually the electric dipole term Ei , e.g. for an electron in an atom... 

The Time Dependent Processes Section uses time-dependent perturbation theory, combined with the classical electric and magnetic fields that arise due to the interaction of photons with the nuclei and electrons of a molecule, to derive expressions for the rates of transitions among atomic or molecular electronic, vibrational, and rotational states induced by photon absorption or emission. Sources of line broadening and time correlation function treatments of absorption lineshapes are briefly introduced. Finally, transitions induced by collisions rather than by electromagnetic fields are briefly treated to provide an introduction to the subject of theoretical chemical dynamics. 

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