Microsolvation of aromatic molecules by argon: Structure of fluorene–(Ar)2 from high-resolution ultraviolet spectroscopy

We present the high-resolution UV spectrum of the 000(S1←S0) transition in fluorene–(Ar)2 recorded by mass-selective resonance-enhanced two-photon ionization with a pulsed narrow-band laser system (Δν=140 MHz). The measured b-type rotational structure of the strongest band at 33 692.51 cm−1 in fluorene–(Ar)2 allows the exclusion of a (2‖0) structure for this band with the two Ar atoms placed on one side of the substrate. The observation of a b-type rotational transition is proof of a specific opposite sided (1‖1) structure with the two Ar atoms placed above (and below) the central five-membered ring and one remaining sign ambiguity concerning the short in-plane axis coordinate.