Tris (methylidene)‐cyclopropane (“[3]Radialene”). Part 2. Electronic states of the molecular cation and revised UV.‐absorption spectrum of the parent neutral

The photoelectron spectrum of tris (methylidene)‐cyclopropane 1 (“[3]radialene”) is reported and the electronic states of 1+ assigned. Jahn‐teller activity in the degenerate states of 1+is discussed. Differences in the Franck‐Condon profile of the first PE band and the Rydberg series in the Vacuum‐UV./UV. absorption spectrum indicate for the Rydberg series (n=3) a 1A″2‐species, which supports earlier tentative proposals. The high intensity absorption in the 5.5 eV‐6 eV energy range of the latter spectrum recorded earlier are definitely due to impurities. The vibrational fine structure of a weak band system around 5.5 eV in our spectrum suggests for this transition S0→S2(1A′1), which is dipole forbidden but borrows intensity from S1(1E′) trough vibronic coupling via an e′‐mode. From vapor pressure measurements ε(λmax=289 nm) = 9390± 1170 for gaseous 1 was found.