Molecules with Light

Optical absorption spectrum of chlorophyll a superimposed on the solar spectrum, showing how efficiently this key photosynthetic pigment absorbs a significant fraction of the available light. The color of green leaves is due to chlorophyll s relatively weak abosorption of green light. 

Bond force constant, from vibrational spectroscopy 

Pathways of energy flow through molecules, from electronic excitation 

Identification of bonding groups and their interactions, from magnetic resonance spectroscopy 

We conclude this chapter with a brief introduction to photosynthesis, the natural process through which light energy from the sun is harvested by plants. Energy stored in plants is converted to other forms by combustion, which is one contributor to the changes induced in the atmosphere through industrialization. 

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In photochemistry, we are interested in the system dynamics after the interaction of a molecule with light. The absorption specbum of a molecule is thus of primary interest which, as will be shown here, can be related to the nuclear motion after excitation by tbe capture of a photon. Experimentally, the spectrum is given by the Beer-Lambert law... 

It was pointed out in Chapter 3 that conventional vulcanised rubbers were composed of highly flexible long chain molecules with light cross-linking... 

Fluorescence is a process that occurs after excitation of a molecule with light. It involves transitions of the outermost electrons between different electronic states of the molecule, resulting in emission of a photon of lower energy than the previously absorbed photon. This is represented in the Jablonski diagram (see Fig. 6.1). As every molecule has different energy levels, the fluorescent properties vary from one fluorophore to the other. The main characteristics of a fluorescent dye are absorption and emission wavelengths, extinction... 

Even though sensors based on the measurement of fluorescence intensity are the most numerous, the high-intensity light sources employed accelerate the possible photodecomposition of the active molecules. Fluorescence-lifetime-based sensors avoid this problem by the excitation of molecules with light pulses. 

Electrochemical methods are selective they are able to control the potential of an electrode, which makes it possible to determine the electrochemical spectrum of electroactive species in solution, analogous to probing the energy states of a molecule with light via spectroscopy. 

In the first volume of Advances in Quantum Chemistry [1] I published an article called Spin-Free Quantum Chemistry. Since then I have broadened this concept considerably and have changed the name of the subject to "freeon dynamics". The word "freeon1 means "free-of-spin" and not the common refrigerant. Freeon dynamics is a viable alternative (for light atoms and for molecules with light atoms) to the more-conventional fermion dynamics. The raison d etre for freeon dynamics is that it is conceptually and computationally simpler than fermion dynamics and so is consistent with Ockham s razor ... 

Comprehensive treatments of the physics of NLO originating from interaction of atoms and molecules with light can be found elsewhere. Here, we will simply define the quantities that we will use further. Electric field (Ea), such as an applied dc field or a propagating electromagnetic wave, always... 

Ab initio MO methods based on HF or small multiconfigurational wave-functions have been the method of choice, up to the present, for studies of organic systems and other molecules with light nuclei. The properties of stable species on the PES are often reproduced very well by calculations with just HF wavefunctions. Studies of reactions usually require the more sophisticated and expensive techniques, such as Cl or MP perturbation theory, that take into account the effects of the correlation between the electrons that is omitted from the HF approximation. The additional energy lowering computed with these methods with respect to that obtained with an HF calculation is called the correlation energy. A detailed and up-to-date discussion of the accuracy of state-of-the-art MO methods when applied to a variety of problems may be found in the book by Hehre et al. 

In elementary treatments of the interaction of atoms and molecules with light, the radiation field is taken as a classical phenomenon. Its interaction with a molecule is often expressed by... 

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