The investigation of chemical processes that occur on the timescale of a femtosecond (10−15 s). Femtochemistry has become possible as a result of the development of lasers capable of being pulsed in femtoseconds. This has enabled observations to be made on very short-lived species, such as activated complexes, which only exist for about a picosecond (10−12 s). In a femtochemical experiment a femtosecond pulse causes dissociation of a molecule. A series of femtosecond pulses is then released, the frequency of the pulses being that of an absorption of one of the products of the dissociation. The absorption can be used as a measure of the abundance of the product of the dissociation. This type of study enables the course of the mechanism of a chemical reaction to be studied in detail.