Energy transitions, electromagnetic

Figure 3.2 Illustrated energy transitions for several useful regions of the electromagnetic spectrum. (Adapted with permission from Figure 2.2 of Cowan, J. A. Inorganic Biochemistry, An Introduction, 2nd ed., Wiley-VCH, New York, 1997. Copyright 1997, Wiley-VCH.)...

Figures 2.13(a) and 2.13(b) illustrate the basis of a semiconductor diode laser. The laser action is produced by electronic transitions between the conduction and the valence bands at the p-n junction of a diode. When an electric current is sent in the forward direction through a p-n semiconductor diode, the electrons and holes can recombine within the p-n junction and may emit the recombination energy as electromagnetic radiation. Above a certain threshold current, the radiation field in the junction becomes sufficiently intense to make the stimulated emission rate exceed the spontaneous processes.

The energy of electromagnetic radiation is inversely proportional to its wavelength. Since excitation of an electron for the tt — rr transition of ethylene occurs at a shorter wavelength (Amax = 170 nm) than that of cis, trans- 1,3-cyclooctadiene (Amax= 230 nm), the HOMO-LUMO energy difference in ethylene is greater. 

Conventional spherical shell model calculations have been undertaken to describe 90 88zr and 90 88y in these large scale calculations valence orbitals included If5/2 2P3/2 2Pl/2 and 199/2 The d5/2 orbital was included for 98Y and for high-spin calculations in 98Zr. Restrictions were placed on orbital occupancy so that the basis set amounted to less than 2b,000 Slater determinants. Calculations were done with a local, state independent, two-body interaction with single Yukawa form factor. Predicted excitation energies and electromagnetic transition rates are compared with recent experimental results. 

When an atom makes a transition from a high-energy quantum state to a lower energy state, electromagnetic radiation with a definite frequency and a definite period is emitted. When properly detected, this frequency, or period, becomes the ticking of an atomic clock, just as the crystal vibration frequency and the swinging frequency are the inaudible ticks of a quartz clock and a pendulum clock. The frequency emanating from the atom, however, is much less influenced by environmental factors such as temperature, pressure, humidity, and acceleration than are the frequencies from quartz crystals or pendula. Thus, atomic clocks hold inherently the potential for reproducibility, stability, and accuracy. 

The horizontal lines represent energy levels of an atom. The vertical arrows represent energy transitions. These energy transitions can be either radiations (i.e. absorption or emission of electromagnetic radiation) or thermal (energy transfer through collisions... 

Extreme conditions in general enable a phase transition. These may not only be just high pressure alone, but also extreme temperatures, bombardment with electrons or other particles, or the apphcation of energy-rich electromagnetic radiation. The crucial step is to remove a carbon atom from its equUibrium position to enable the redeposition in the shape of another modification. In such a process, the product formed is not necessarily the one thermodynamically most stable as kinetic effects may influence the outcome. 

Molecules and molecular assemblies undergo energy transitions at defined resonance energies upon irradiation with appropriate electromagnetic waves. The theoretical and experimental details of the various spectroscopies are discussed in many modem textbooks and reviews. We shall limit the discussion here to a few remarks on those aspects of the final spectra that are important to the analysis of conformational changes of natural products and their intermolecular interactions in assemblies. 

TYPES OF ENERGY TRANSITIONS IN EACH REGION OF THE ELECTROMAGNETIC SPECTRUM... 

In addition to matching the energy of a photon with the energy difference between two levels, a second requirement must be met for the absorption of radiation by matter the energy transition in the molecule must be accompanied by a change in the electrical center of the molecule so that electric work can be carried out on the molecule by the electromagnetic radiation. Requirements for the absorption of radiation by matter are summarized in quantum-mechanical selection rules, which determine which transitions may take place. These rules, based on considerations of the symmetry of the system in the upper and lower states, point out that some transitions are more probable than others. 

The calculations give a reasonable description of the observed energy spectra, electromagnetic and P-decay transition rates, electromagnetic moments, and many other properties of nuclei with closed shells 2 nucleons. 

Table 12.3 Types of Energy Transitions Resulting from Absorption of Energy from Three Regions of the Electromagnetic Spectrum ...

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