Anomalous fine structures of the 000 band of tetracene in large He droplets and their dependence on droplet size

The spectrum of the 0(0)(0) band of the S-1 <-- S-0 transition of tetracene molecules embedded in superfluid helium droplets is studied via laser induced fluorescence spectroscopy. Measurements are reported for droplets of different average sizes from about 10(3) to 10(7) helium atoms. In agreement with previous observations in small droplets the 0(0)(0) band shows two components designated as alpha and beta peaks split by about 1 cm(-1). In addition the higher energy beta-peak shows some sharp superimposed peaks separated by about 0.05 cm(-1). A very similar fine structure of the 0(0)(0) band is found in the tetracene-argon complex and in pentacene which challenges a simple explanation in terms of a free molecule Hamiltonian. Presumably the rotational-translational coupling of very anisotropic molecules, such as tetracene in superfluid helium has a noticeable affect on the spectra. In droplets larger than about (N) over bar = 10(6), obtained in supercritical expansions, the beta peak has a broad feature shifted by 0.1 cm(-1) towards higher energies from the maximum found at smaller sizes which dominates the spectrum at (N) over bar = 10(7). Several mechanisms including the interaction with the phonons in the interior of the droplets and trapping by quantum vortices are discussed as possible explanations for the unexpected spectral behaviour in the large droplets.