Fingerprint of Droplet Shape and Vortex in the Line Shape at the Electronic Band Origin of Phthalocyanine in Superfluid Helium Droplets.

X-ray and XUV diffraction experiments have visualized both the outer shape and quantum vortices inside individual superfluid helium droplets. Both features are effective on the helium induced signature observed as the spectral shape and position of the electronic transitions of molecules doped into helium droplets. In this article the helium induced signature at the electronic band origin of phthalocyanine is re-examined systematically comprising previous analytical results as well as newly reported experimental investigations. Helium-induced effects such as a nonmonotonous evolution of the solvent shift and the emergence of an optical anisotropy, both observed for rather large helium droplets, are the spectroscopic response on the analytical results reported from diffraction experiments. All helium induced spectroscopic features can be explained as an expression of London dispersion interaction under the varying structural conditions of helium droplets.