In the following, we focus on more recent studies of isolated molecules in the gas phase, for example . Although these are less broadly relevant to synthetic applications (compared to solution phase studies), they allow for a high level of mechanistic understanding due to a lower ambiguity in the interpretation of experimental observables and a better comparison with theory (see below). The studies largely agree on an excited state lifetime of CHD of ≈140 fs, with the notable exception of the studies by Schalk et al. and Karashima et al. , which find significantly shorter lifetimes. Studies employing time-resolved photoelectron spectroscopy, which provides sensitivity to the electronic character of the excited state , such as the work by , largely agree upon a decay of population from the photoexcited ππ * diabatic state within ≈ 30 fs towards the π * π * state (37; 39). The doubly excited character of the π * π * state was confirmed in a seminal transient X-ray absorption spectroscopy study at the carbon K-edge by Attar et al. .