Viewing the transition state:

The F + H₂ reaction has played an important role in the history of a field called reaction dynamics - the goal is to gain a complete understanding of how chemical reactions take place.

In 1992, the transition state of this benchmark elementary bimolecular chemical reaction was directly studied using a spectroscopic method called photoelectron spectroscopy.  The work (D. E. Manolopoulos, et al., Science, 262, 1852 (1993).) was published in Science, and a Perspective article (Science. 262, 1828(1993)) in the same issue provides a useful introduction to this experiment.

Another method for viewing the transition state of a chemical reaction is made possible using extremely fast laser pulses.  In order to catch the transition state in the process of falling apart, one laser "pump" pulse starts a unimolecular reaction and a second pulse "probes" the reaction as a function of time.  For this to work, both pulses must be extremely short compared to the transition state motions - a ultrafast shutter on a spectroscopic camera. 

The reaction ICN ---->  [I..CN]^¥  -->  I + CN was studied in 1987 using ultrafast spectroscopy in the 1999 Nobel Laureate, A. Zewail's lab