Electronic spectroscopy of polycyclic aromatic hydrocarbons in helium nanodroplets

Electronic spectra of biphenylene, acenaphtylene, fluoranthene, benzo(k)fluoranthene and perylene inside helium nanodroplets have been recorded with the beam depletion method. Although the beam depletion method allows one to study primarily non-fluorescent molecules like biphenylene, highly fluorescent molecules like perylene and benzo(k)fluoranthene can also be studied when the conditions are optimized. All studied transitions are excitations to the first singlet excited state and are in the 350-450 nm region; within the range of the second harmonic of the Ti:Al2O3 laser. The spectra of the argon and oxygen van der Waals complexes of biphenylene, fluoranthene and benzo(k)fluoranthene inside the droplets have also been reported.;Biphenylene has the largest zero phonon line (ZPL) splitting, which is retained even for the van der Waals complexes. This odd behavior is in contrast with previous studies on tetracene inside helium nanodroplets and intensifies the debate about the origin of the ZPL splitting.;Acenaphtylene, fluoranthene and benzo(k)fluoranthene differ by a benzene ring, and yet the ZPL splitting and the shape of the phonon wing are very different. The spectra and the peak shapes have been analyzed using atom-atom pair potentials, however it is not possible to fully address the solvation and the differences in the spectra without more accurate calculations.;Inside the helium droplets, oligomers of perylene have been formed and probed by electronic spectroscopy. The growth of the exciton band as a function of the pick-up pressure has been recorded. A distributed point dipole-point dipole model has been suggested and used to account for the excitonic interactions of the displaced parallel dimer.;The Hartree-Fock plus dispersion (HFD) method for modeling the intermolecular interaction potentials has been revisited for atom-atom systems including rare gas, alkali metal and alkali earth metal dimers. The method has been modified, to include anisotropy, with Legendre polynomials and has been extended to atom-molecule systems including Ar-HF, He-HF, Ne-HCN, He-N2O, and He-HCCCN. The HFD method can describe the interaction of atom-molecule systems within 5% around the minimum of the potential if the molecule contains up to 3 atoms.