Laser-induced fluorescence spectroscopy of van der Waals complexes of tetracene-ArN (N≤5) and pentacene-Ar within ultracold liquid He droplets

Van der Waals complexes of tetracene and pentacene with several Ar atoms have been assembled within cold (0.38 K) liquid He-4 droplets (approximate to 6000 atoms) and then studied spectroscopically via laser-induced fluorescence (LIF). The spectra reveal a series of well-resolved sharp peaks (linewidth delta nu < 0.5 cm(-1)) which are attributed to the band origins of different van der Waals complexes of the hydrocarbon molecule with up to five Ar atoms. From the analysis of the spectra it is possible to identify different isomers of the complexes corresponding to different local minima in the potential energy surface. The assignments of some of the spectral lines of the tetracene-Ar complex to different isomers is confirmed by spectral hole-burning measurements. The results are compared with earlier LIF experiments on free van der Waals complexes produced in seeded beams. The additional isomers found in the He-4 droplets were apparently suppressed in the earlier seeded beam experiments presumably because of the much higher internal temperatures. (C) 1998 Elsevier Science B.V. All rights reserved.