Electronic-vibrational excitations of aromatic molecules in large argon clusters

In this paper we report the results of an experimental study of excited-state energetics, line broadening, and vibrational relaxation (VR) dynamics of electronically-vibrationally excited states of anthracene, tetracene, and pentacene embedded in clusters of Ar. Large clusters of Ar, each containing a single aromatic molecule, were synthesized in supersonic jets of seeded Ar, expanded through a 150-μm nozzle in the pressure range p = 3000-14000 torr. We applied the interrogation techniques of laser spectroscopy in supersonic expansions, monitoring the fluorescence excitation spectra, the energy-resolved fluorescence, and the simultaneous time-resolved and energy-resolved emission. Information was obtained on the energetics and vibrational structure of the S₁ state of anthracene, tetracene, and pentacene in Ar clusters. We have also observed a structureless spectrum of the anthracene dimer embedded in Ar clusters. The red spectral shifts and the line widths of the electronic origin of the S₀ → S₁ transition of the anthracene, tetracene, and pentacene molecules increase with increasing stagnation pressure in the range p = 1000-2500 torr and are independent of the stagnation pressure in the range p = 6000-14000 torr. An intrinsic line broadening in the energy-resolved S₁(0) → S₀(0) emission of tetracene in clusters synthesized at p ≥ 8000 torr was observed, which provides evidence for homogeneous line broadening, originating from electron-phonon coupling, in these clusters. Vibrational relaxation of intramolecular vibrations of tetracene and anthracene in their S₁ state was explored by the observation of hot luminescence and by the measurements of its energy-resolved decay time. The vibrational relaxation process in clusters prepared at p = 6000-14000 torr is surprisingly slow, occurring on the nanosecond time scale. These vibrational relaxation lifetimes decrease with increasing stagnation pressure.