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Fast Reactions : Flash Photolysis :

Preamble
Many reactions in nature occur at an extremely fast time scale and are also complex. These reaction are not resolvable in the time scale that humans can act. For instance, the light-harvesting system of the photosynthetic reaction center, which is at the heart of the photosynthesis process, occurs in the femtosecond time scale (a femtosecond is one-millionth of one-billionth of a second). Similarly, the early chemical events in the process of vision occur in the picosecond (one-millionth of one-millionth of a second) time scale. The time scale of early events in the photosynthetic and vision process initiates a series of very complex processes, which ultimately leads to an event that occurs on the time scale that humans can decipher without any aids. New studies indicate that “the blink of an eye” could be as low as 15 milliseconds. Apart from these examples, a vast number of reactions, either initiated or uninitiated by light, occur at timescales that need special tools to investigate, to which this facility is geared.

The instrument
The timescale of a reaction is measured relative to a triggering event, which could be as simple as adding two reacting chemicals. Alternatively, a large class of reactions occur due to a “photo-trigger”, and are classfied as photochemical reactions. The initial events in a photochemical reaction are known to cover a large time window at least of about ten orders of magnitude (femtoseconds to microseconds) thus making it impractical for a single instrument to cover this entire dynamic range. The LASER FLASH PHOTOLYSIS facility is capable of probing light-induced events that occur on the time scale of nanoseconds (one-billionth of a second) to microseconds (one-millionth of a second). A generic version of the instrument consists of three major components, which are (i) photolysis laser, (ii) probe unit and (iii) detection unit. The three units are in turn connected by a master controller which also acts as a time-keeper.

Experiments
One illustrative example, among many experiments that are planned for UG/PG/PhD students, is the photo-redox reaction of quinone coupled with proton transfer. In this reaction quinone upon excitaion with light is converted to hydroquinone through an intermediate semiquinone. This is also a classical example of consecutive reactions, wherein the concentration of the intermediate semiquinone builds up in the first half and decays in the second half. This facility is aimed to teach and train students to understand the concepts in light-induced reactions.