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Roatan Bilingual
Institute
Student: Adrian Alexander Jackson Wood
Teacher: Ms. Palache
Course: 10th
Class: Chemistry I
Assignment: Branch of Chemistry research paper
Date: 4/9/2019
Photochemistry
Thesis statement
The term photochemistry oftenly hands of to chemical alteration induced
by interplay of light (electromagnetic radiation) with matter. Therefore,
light is always one of the chemical substances in a photochemical system.
Introduction
Photochemistry is the branch of chemistry concerned with the chemical
effects of light. Generally, this term is used to describe a chemical reaction
caused by absorptionof ultraviolet wavelengths, visible light, or infrared
radiation, but the wavelength regions adjacent to this range are also of
interest for certain applications. With the advent of lasers.
Photochemistry
In nature photochemistry is of immense importance as it is the basis of
photosynthesis, vision, and the formation of Vitamin D with sunlight.
Photochemical reactions proceed differently than temperature-driven
reactions. Photochemical paths access high energy intermediates that
cannot be generated thermally, thereby overcoming large activation barriers
in a short period of time, and allowing reactions otherwise inaccessible by
the photo degradation of plastics.
Grotthuss-Draper law and Stark-Einstein
Photoexcitation is the first step in a photochemical
process where the reactant is elevated to a state of
higher energy, an excited state. The first law,
states that light must be absorbed by a chemical
substance in order for a photochemical reaction to
take place. According to the second law, each photon of light absorbed by a
chemical system, no more than one molecule is activated for a
photochemical reaction, also known as “The Quantum Yield”.
The application of quantum mechanics to molecular systems requires
approximate methods cause the Schrodinger equation cannot be solved
exactly for many-body systems. Fortunately, nuclei are much heavier than
the electrons, and their motion is much slower than that of the electrons.
The nuclei can be considered as fixed centres of potential and a description
of the electronic motion can be obtained by solving the Schrodinger
equation. Quantum mechanical treatment of nuclear motion within the
Born-Oppenheimer approximation requires a solution of the nuclear
Schrodinger equation with the electronic energy as the potential.
Flourescence & Phosphorescence
When a molecule or atom in the ground state (S0) absorbs light, one
electron is excited to a higher orbital level. This electron maintains its spin
according to the spin selection rule; other transitions would violate the law
of conservation of angular momentum. The excitation to a higher singlet
state can be from HOMO to LUMO or to a higher orbital, so that singlet
excitation states at a different energies are possible. Kasha’s rule stipulates
that higher singlet states would
quickly relax by radiationless decay
internal conversion.
or
Principles
The absorption of a photon of light by a reactant molecule may also permit
a reaction to occur no just bringing the molecule to the necessary activation
energy, but also by changing the symmetry of the molecule’s electronic
configuration, enabling an otherwise inaccessible reaction path. Some
photochemical reactions are several orders of magnitude faster than thermal
reactions; reactions as fast as 10-9 seconds and associated processes as fast
as 10-15 seconds are often observed.
The photon can be absorbed directly by the reactant or by a photosensitizer,
which absorbs the photon and transfers the energy to the reactant. The
opposite process is called quenching when photoexcited state is deactivated
by a chemical reagent.
Conclusion
Light provides the activation energy, simplified, light is one mechanism for
providing the activation energy required for many reactions.
Bibliography
https://en.wikipedia.org/wiki/Photochemistry
http://photobiology.info/Ilichev.html
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