Absorption by molecules

Quantum mechanics provides a theoretical basis for understanding the relative energy levels of molecular orbitals and how they vary with structure. Quantum mechanics also generates a set of selection rules to predict what transitions occur in molecules. The transitions that occur in molecules are governed by quantum mechanical selection rules. Some transitions are allowed by the selection rules, while others are forbidden . The selection rules are beyond the scope of this text, but may be found in most physical chemistry texts or in the 

When molecules are electronically excited, an electron moves from the highest occupied molecular orbital to the lowest unoccupied orbital, which is usually an antibonding orbital. Electrons in tt bonds are excited to antibonding tt orbitals, and n electrons are excited to either a or tt orbitals. 

Functional group Chemical structure Electronic transitions 

Transition metal compounds are often colored, indicating that they absorb light in the visible portion of the spectrum. This is due to the presence of unfilled d orbitals. The exact wavelength of the absorption band maximum depends on the number of d electrons, the geometry of the compound, and the atoms coordinated to the transition metal. 

Beer s Law, which relates absorbance of a sample to the path length and concentration of absorbing species, was covered in Chapter 2. The proportionality constant, a, in Beer s Law is the absorptivity of the absorbing species. 

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The aim of this chapter is to recall the basic principles of light absorption by molecules. The reader is referred to more specialized books for further details. 

Band spectra are produced in emission or absorption by molecules coutaining two or more atoms, and line spectra by single atoms or monatomic ions. The structure of bands is related to the vibration of the nuclei of the atoms within the molecule and to the rotation of the molecule. 

In addition, it ought to be kept in mind, considering the classical concept of light absorption by molecules, that, owing to the inertia of molecules, the light field is not capable of turning the molecule and thus excites the vibrations of the dipole oscillator only to the extent to which... 

While dense IS regions are generally well-shielded against high-energy photons, the interaction between molecular ions and photons is relevant to the question of the survival of such ions in the diffuse interstellar medium, where UV irradiation might be expected to be a powerful destructive force. Such effects are, of course, important also for the fate of neutral molecules in the diffuse interstellar radiation field, but UV photo absorption by molecules of moderate size is often more likely to lead to photoionization (itself an important topic, but not covered herein) than to photodissociation. 

To a first approximation, absorption by free atoms is similar to absorption by molecules and there is a linear relationship between absorbance and the... 

In this chapter we are concerned primarily with the physical nature of light and the mechanism of light absorption by molecules. We will discuss... 

Figure 5-10. Resonance transfer of excitation from molecule A to molecule B. After light absorption by molecule A, a radiationless transition occurs to the lowest vibrational sublevel of its excited state. Next, resonance transfer of the excitation takes place from A to B, causing the second molecule to go to an excited state, while molecule A returns to its ground state. After a radiationless transition to the lowest vibrational sublevel in the excited state, fluorescence can then be emitted by molecule B as it returns to its ground state. Based on the energy level diagrams (which include the vibrational sublevels for each of these two different pigments), we can conclude that generally the excitation rapidly decreases in energy after each intennolec-ular transfer between dissimilar molecules.

Decomposition mechanisms are of two kinds, direct and indirect decomposition of polymers. Direct decomposition takes place after energy absorption by the polymer molecules themeselves. Indirect decomposition is a secondary decomposition of the polymers, involving energy absorption by molecules which are usually impurities, followed by energy-transfer processes and/or radical reactions. These direct and indirect processes are considered below. 

Characterize light absorption by molecules and its role in stratospheric chemistry of ozone and other reactants. 

In discussing light absorption by molecules, we can focus on two MOs. The highest occupied molecular orbital (HOMO) is the MO of highest energy that has electrons in it The lowest unoccupied molecular orbital (LUMO) is the MO of lowest energy that does not have electrons in it. In N2, for example, the HOMO is the a2p MO and the LUMO is the Ttjp MO (Figure 9.43). 

A major advance in the utility of laser spectroscopy came as a result of the development of multiphoton ionization MPI as a means of detection of multiphoton absorption by molecules [1]. The resonance encountered as the n-photon energy of a scanning laser becomes coincident with that of a molecular excited state is evidenced by a large increase in ionization rate. Since single ionization events can be detected with near unit efficiency, this results in a very sensitive means of detecting weak multiphoton absorption. MPI is a more widely applicable method than laser induced fluorescence since it can be used for non-emitting states. 

Figure 5.8 The reiative energy ieveis of a set of a, it, and n orbitais and the associated antibonding orbitais. 5.1.2 Absorption by Molecules...

Absorption by molecules removes the absorbed wavelengths from white light, and a sample will appear to our eyes as the combination of reflected wavelengths. 

Physical Basis of Light Absorption by Molecules The Franck-Condon Principle... 

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