The investigation of chemical processes that occur on the timescale of a femtosecond (10^-15s). Femtochemistry has become possible as a result of the development of lasers capable of being pulsed in femtoseconds.

In a femtochemical experiment a femtosecond pulses is then released, the frequency of the pulses being that of 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.