Resonant two-photon ionization of fluorene rare-gas van der Waals complexes

Resonant two-photon ionization combined with time-of-flight mass spectrometry was applied for the interrogation of the S⁰→S ₁ electronic-vibrational excitations of van der Waals complexes of fluorene (FL) with rare-gas atoms and N₂ in supersonic jets. Energy-resolved and mass-resolved spectra of FL · Ne, FL · Ar _n (n = 1-3), FL · Kr, FL · Xe, and FL · N ₂ were recorded over the energy range 0-800 cm^-1 above the electronic origin of S₁. The red microscopic spectral shifts of the electronic origins of FL · R (R = Ar, Kr, and Xe) complexes are dominated by dispersive interactions, being proportional to the polarizability of R. The vibrational level structure of FL · R_n (R = Ar, Kr, and Xe) complexes exhibits intramolecular vibrational excitations of FL, as well as intermolecular vibrations, which involve the relative motion of FL and R in the complex. The spectra of FL · Ne and FL · N₂ reveal a rich vibrational structure in the vicinity of the electronic origin, indicating a substantial change of the nuclear configuration upon electronic excitation. Upper and lower bounds on the dissociation energies of FL · R (R = Ne, Kr, and Xe) and FL · Ar₂ were inferred from the vibrational level structure in the mass-resolved spectra, where the disappearance of the signal of the parent van der Waals ion and the appearance of the ion signal of the fragments mark the onset of the vibrational predissociation process.