The phonon wings in the (S1 ← S0) spectra of tetracene, pentacene, porphin and phthalocyanine in liquid helium droplets

Abstract

The vibronic spectra of the large organic molecules tetracene (C₁₈H₁₂), pentacene (C₂₄H₁₄), base-free porphin (C₂₀H₁₄N₄) and base-free phthalocyanine (C₃₂H₁₈N₈) have been investigated in large (N ≈ 10³–10⁴) ⁴He droplets using laser induced fluorescence. The spectra show sharp zero phonon lines (Δν ⩽ 0.2 cm⁻¹), accompanied by weaker phonon wings on the blue side. The very small inhomogeneous broadening of the zero phonon lines supports the unique properties of liquid He as an extremely gentle ultra-cold matrix for high resolution spectroscopy of larger molecules. Pronounced structures in the phonon wings, which are characteristic for each molecule, are attributed to the excitation of vibrational modes of the helium atoms located in shells surrounding the molecule.