SPECTRAL HOLE-BURNING OF TETRACENE AND TETRACENE ARGON COMPLEXES IN A SUPERSONIC JET

The fluorescence excitation spectra of the tetracene molecule and the van der Waals clusters tetracene.Ar(n) with n = 1, 2, 3 cooled in a supersonic free jet expansion have been obtained with a spectral hole-burning technique. The supersonic beam contained a mixture of clusters up to n = 7. The influence of clustering on the intramolecular vibrations of tetracene up to a vibrational excess energy of 1250 cm-1 was found to be very small. For the three smallest (n = 1-3) clusters a low-frequency vibration with a wavenumber of about 40 cm-1 was found and assigned to the motion of the Ar atom perpendicular to the molecular plane. A vibration with a wavenumber of 8 cm-1 corresponding to the motion parallel to the plane of the tetracene molecule was distinctly observed only in the cluster containing a single Ar atom. It is shown that weak lines due to the minor species can be assigned unambiguously even if they are strongly overlapped by lines of another species.