Electromagnetic radiation electron transitions

X-ray Electromagnetic radiation of wave length c. 1 k. X-rays are generated in various ways, including the bombarding of solids with electrons, when they are emitted as a result of electron transitions in the inner orbits of the atoms bombarded. Each element has a characteristic X-ray spectrum. 

Section 13 21 Transitions between electronic energy levels involving electromagnetic radiation m the 200-800 nm range form the basis of UV VIS spec troscopy The absorption peaks tend to be broad but are often useful m indicating the presence of particular tt electron systems within a mole cule... 

Absorption of a photon is accompanied by the excitation of an electron from a lower-energy atomic orbital to an orbital of higher energy. Not all possible transitions between atomic orbitals are allowed. For sodium the only allowed transitions are those in which there is a change of +1 in the orbital quantum number ) thus transitions from s—orbitals are allowed, but transitions from s d orbitals are forbidden. The wavelengths of electromagnetic radiation that must be absorbed to cause several allowed transitions are shown in Figure 10.18. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

Electronic transitions mostly involve interaction between the molecule and the electric component of the electromagnetic radiation (Section 2.1). The selection rules are, therefore. 

Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to excited states. For the lanthanides the most common of such transitions involve excited states which are either components of the ground term or else belong to excited terms which arise from the same 4f" configuration as the ground term. In either case the transitions therefore involve only a redistribution of electrons within the 4f orbitals (i.e. f—>f transitions) and so are orbitally forbidden just like d—>d transitions. In the case of the latter the rule is partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion. However, it has already been pointed out that crystal field effects are very much smaller in the case of ions and they... 

If the sample is balhed in electromagnetic radiation of frequency v, then a transition between the (wo spin states, corresponding to the electron changing its component of spin, can take place providing the resonance condition ... 

Absorption of radiation in the radio-frequency, RF, region of the electromagnetic spectrum can be observed for those nuclei which are considered to spin about their own axes. The energy changes are associated with the orientation of the nuclear axis in space relative to an external applied magnetic field and are of the order of 0.1 J moH, 10-600 MHz (50 cm-30 m or 3 x 1(M to 2 x 10 2 cm1). This is considerably smaller than the energy changes associated with vibrational and electronic transitions (pp. 364, 378). 

Some salts, particularly of transition metals, are highly colored and absorb in the UV and visible regions. Salts and complexes that have the highest molar absorptivities tend to absorb electromagnetic radiation by a charge-transfer process. In the charge-transfer process, an electron is promoted from one part of a complex to another causing one part of the complex to be oxidized and the other to be reduced as in Eq. (5.11). 

Molecular spectroscopy. This spectroscopy deals with the interaction of electromagnetic radiation with molecules. This results in transition between rotational and vibrational energy levels besides electronic transitions. 

The allowed transition in ESR is diagrammed in Figure 2. The ESR experiment is commonly conducted at a fixed frequency near 9.5 x 109 Hz by scanning through a magnetic field range until absorption of electromagnetic radiation is detected at H0. The value of H0 can then be used to calculate the electron g-factor. 

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