High resolution electronic spectroscopy of 4-fluoroaniline in a molecular beam: new experimental results and their interpretation in terms of molecular geometry

We measured two vibronic bands in the SI <-- So electronic transition of 4-fluoroaniline with high resolution technique and obtained accurate rotational constants by a direct assignment of the rotational quantum numbers to the levels involved in the transitions. An attempt was made to evaluate the change of the molecular geometry in the different vibrational levels of the SI state, with particular emphasis on the problem of the planarity of the molecular skeleton in both the ground and the first singlet excited state. Evidence is given for non-planar equilibrium geometry of both the molecules in the S-1 electronically excited state.