In the first half of the 20th century, a lot of research work was devoted to the determination and theoretical analysis of these parameters and the study of reaction mechanism by using these parameters. However, the confirmation of reaction mechanism mainly depends on the ability to detect and analyze reaction intermediates. In the late 20th century, the extensive study of free radical chain reaction kinetics brought two development trends to chemical kinetics: one was the extensive study of meta-reaction kinetics; Secondly, it is urgent to establish a method for detecting active intermediates. This requirement and the development of electronics and laser technology promote the development of rapid reaction kinetics. The time resolution of detecting transient active intermediates has been improved from millisecond level in 1950s to picosecond level.