Femtochemistry is the branch of chemistry in which chemical reactions are detected as they occur and the reaction times are measured. Femtochemistry gets its name from the fact that these times are measured in femtoseconds. One femtosecond is one-millionth of one-billionth of a second. This number is written out as a decimal point followed by 14 zeros and a 1. Scientists believe that no chemical reaction can take place more rapidly than this.