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

Van der Waals complexes of tetracene and pentacene with several Ar atoms ha ve been assembled within cold (0.38 K) liquid He-4 droplets (approximate to 6000 atoms) and then studied spectroscopically via laser-induced fluoresce nce (LIF). The spectra reveal a series of well-resolved sharp peaks (linewi dth delta nu < 0.5 cm(-1)) which are attributed to the band origins of diff erent van der Waals complexes of the hydrocarbon molecule with up to five A r atoms. From the analysis of the spectra it is possible to identify differ ent 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 hol e-burning measurements. The results are compared with earlier LIF experimen ts on free van der Waals complexes produced in seeded beams. The additional isomers found in the He-4 droplets were apparently suppressed in the earli er seeded beam experiments presumably because of the much higher internal t emperatures. (C) 1998 Elsevier Science B.V. All rights reserved.