The development of femtosecond laser technology enabled the investigation of photochemical reaction dynamics using time-resolved spectroscopy. In particular, the femtosecond time resolution of these methods motivated studies on the time scale of internuclear motion in molecules. The majority of time-resolved studies of electrocyclic photochemistry have focused on CHD, with few exceptions. An early study with picosecond time resolution by Mathies and coworkers using transient UV resonance Raman spectroscopy placed the electrocyclic ringopening of CHD on the ultrafast timescale . Later studies using femtosecond time-resolved spectroscopy in both gas and solution phases demonstrated that the reaction takes place on the femtosecond timescale. Additionally, derivatives containing the CHD chromophore, such as α-terpinene (αTp) , α-phellandrene (αPh) , and 7-dehydrocholesterol (provitamin D3) , have been investigated by time-resolved spectroscopy. These and other studies on CHD and derivatives published until the early 2010s are well-summarized in reviews by Weber and coworkers and Sension and coworkers .