Electronic-vibrational excitations of aromatic molecules in large argon clusters

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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.

Original languageEnglish
Pages (from-to)3345-3358
Number of pages14
JournalJournal of Physical Chemistry
Issue number17
StatePublished - 1982


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