“…All ISC in the à 1 A u ( S 1 ) state of acetylene is described by doorway-mediated interaction. ,,, The model is supported by Zeeman anticrossing spectroscopy, ,, LIF Zeeman quantum beat spectroscopy, − high-resolution LIF spectroscopy, simultaneous LIF and SEELEM spectroscopy, − and photoelectron spectroscopy. , The doorway model treats matrix elements between vibrational levels of the S 1 and T 1,2 electronic states as negligible, and stipulates that all mixing between S 1 and T 1,2 levels is induced indirectly by nonzero S 1 ∼ T 3 and T 3 ∼ T 1,2 spin–orbit matrix elements.…”