In this paper we report an experimental study of the time-resolved response of the molecule 1,4-diazabicyclo[2.2.2]octane (DABCO) to 266.3 nm electronic excitation of the S-1 state with a femtosecond laser. Rotational decoherence and vibrational oscillation within the S-1 state are observed. We performed state-of-the-art ab initio calculations on the ground and low electronic states of the neutral molecule and the cation, which assist in the assignment of the observed photoelectron signals. Using our theoretical and spectroscopic data, the experimental findings are interpreted in terms of an unusual quantum interference between two different vibrational modes, with only the v = 1 level of each mode being populated.