Microsolvation of porphine molecules in superfluid helium nanodroplets as revealed by optical line shape at the electronic origin

Abstract

We investigate the line shape at the electronic origin of single porphine molecules doped into superfluid helium droplets as a function of the droplet size. Helium droplets comprised of less than 105 atoms are generated from an expansion of gaseous helium, while droplets with more than 105 atoms originate from liquid helium. In contrast to our recent study on phthalocyanine, porphine is found to exhibit a solvent shift to the blue with respect to the gas-phase resonance frequency as well as a multiplet splitting. A comparison of the helium-induced features of phthalocyanine and porphine with those obtained in similar studies on tetracene and pentacene reveals that these occur chiefly as two kinds of excitations distinguished by their linewidths and their dependence on the droplet size. Moreover, at quasi-bulk conditions achieved with droplets in excess of 106 helium atoms, none of these four dopant species yields an optical spectrum that can be assigned to a plausible rotational band structure.

Heterogeneous Clusters of Phthalocyanine and Water Prepared and Probed in Superfluid Helium Nanodroplets

“…The attachment of polar water molecule(s) to the bare Pc molecule appears to have little effect−other than a shift in the transition energies. As we have shown in more detail for bare Pc and porphine, 26,27 the asymmetry in these line shapes is helium-induced and reflects the inhomogeneity caused by the droplet size distribution. The perseverance of the line shape despite the attachment of a water molecule supports the idea of an effect that is caused by the droplet volume rather than just the helium solvation layer surrounding the dopant cluster.…”

“…For larger droplets, the rotational fine structure is obscured by pronounced line broadening . Recent investigations of the ZPL at the electronic origin of phthalocyanine and porphine led to the conclusion that inhomogeneous line broadening due to the droplet size distribution is responsible for the asymmetric line shape found. , Simulations of the line shape by means of the excluded volume model developed for molecules embedded in a polarizable environment suffice to reproduce the experimental line shape without including particular spectral features of the rotational band system. , Apparently, molecular rotation is readily resolved in helium droplets when coupled to vibrational excitation. , However, upon coupling to electronic excitation, helium-induced perturbations mask spectral contributions expected for a free rotation of the dopant species.…”

Electronic Spectroscopy in Superfluid Helium Droplets

“…Similar as discussed for phthalocyanine, the line shape of the ZPL at the electronic origin of porphin is asymmetric, varies with the droplet size distribution, and, therefore, suffers inhomogeneous broadening [45]. However, the steep edge is at the blue side and a tail expands to the red which is inverted compared to the asymmetry found for phthalocyanine.…”

“…Another remarkable feature in the droplet size dependence of the line shape was a sharp peak about 0.04 cm −1 in width right on top of the inhomogeneous broadened ZPL [45]. It was observed for average droplet sizes below 10,000 helium atoms.…”

“…Accordingly, the inverted asymmetry is indicative for a helium induced solvent shift to the blue. And in fact, applying the excluded volume model with a blue shift the line shape and its dependence on the droplet size distribution can be simulated [45]. As closed shell organic molecule porphin is heliophilic in both electronic states.…”