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
