The mechanism of isomerization has been the subject of some debate, with two pathways identified as viable: a "rotation" about the N-N bond, with disruption of the double bond, or via an "inversion", with a semi-linear and hybridized transition state. It has been suggested that the "trans"-to-"cis" conversion occurs via rotation into the "S2" state, whereas inversion gives rise to the "cis"-to-"trans" conversion. It is still under discussion which excited state plays a direct role in the series of the photoisomerization behavior. However, the latest research on femtosecond transition spectroscopy has suggested that the "S2" state undergoes internal conversion to the "S1" state, and then the "trans"-to-"cis" isomerization proceeds. Recently another isomerization pathway has been proposed by Diau, the "concerted inversion" pathway in which both CNN bond angles bend at the same time.