Laser induced fluorescence of Mg-phthalocyanine in He droplets: Evidence for fluxionality of large H(2) clusters at 0.38 K

The formation of Ar and H(2) clusters, having up to 900 particles in helium droplets, has been studied via laser induced fluorescence of attached Mg-phthalocyanine (Mg-Pc) molecules. In the experiments, one Mg-Pc molecule in average was added to each He droplet either before or after the cluster species, and the shift of the spectrum of the Mg-Pc molecules was studied as a function of the cluster size. For Ar clusters, about a factor of 2 smaller matrix shift was observed for the late pickup of the Mg-Pc molecules as compared with the prior pickup, indicating that in the former case, the Mg-Pc molecules reside on the surface of the preformed Ar clusters. On the other hand, the spectra of the Mg-Pc molecules attached to H(2) clusters are independent of the pickup order, which is consistent with Mg-Pc molecules residing near the center of the H(2) clusters in both cases. Therefore H(2) clusters remain fluxional in helium droplets at T=0.38 K. No significant differences in the spectra were observed between the para-H(2) and ortho-H(2) clusters.