Abstract
Superfluid helium nanodroplets comprised of thousands to millions of helium atoms can serve as a reactor for the synthesis of heterogeneous molecular clusters at cryogenic conditions. The cluster synthesis occurs via consecutive pick-up of the cluster building blocks by the helium droplet and their subsequent coalescence within the droplet. The effective collision cross section of the building blocks is determined by the helium droplet size and thus exceeds by orders of magnitude that of a reactive collision in the gas phase. Moreover, the cryogenic helium environment (at 0.38 K) as a host promotes the formation of metastable cluster configurations. The question arises as to the extent of the actual involvement of the helium environment in the cluster formation. The present study deals with clusters of single phthalocyanine (Pc) molecules with single water molecules. A large fluorophore such as Pc offers several sites where the water molecule can attach. The resulting isomeric variants of the Pc-H2O complex can be selectively identified by electronic spectroscopy. We compare the experimental electronic spectra of the Pc-H2O complex generated in superfluid helium nanodroplets with the results of quantum-chemical calculations on the same cluster but under gas-phase conditions. The number of isomeric variants observed in the helium droplet experiment comes out the same as that obtained from our gas-phase calculations.
Highlights
Exploratory spectroscopic investigations have established that van der Waals clusters in helium nanodroplets form metastable configurations that are absent in the gas phase.[1−5] It is the superfluid, high-thermal conductivity environment that gives rise to the helium-induced configurations, and the involvement of He atoms as building blocks of the clusters
This is attested by the moments of inertia of dopants such as glyoxal, OCS, or sulfur hexafluoride and numerous other dopant species that exhibit a characteristic increase due to a corotating, rigidly bound helium solvation layer.[6−12] Apart from the effect on the moments of inertia as revealed by rotationally resolved infrared spectra, the influence of the helium host environment on the clusters transpires in electronic spectra as helium-induced fine structure.[13−15] This manifests itself in the line shape at the electronic origin known as the zero phonon line (ZPL) and in a combined excitation of the dopant molecule and the helium environment known as the phonon wing (PW)
There are a total of five resolved peaks within this range, some corresponding to electronic origins, others to PW or vibronic excitations
Summary
Exploratory spectroscopic investigations have established that van der Waals clusters in helium nanodroplets form metastable configurations that are absent in the gas phase.[1−5] It is the superfluid, high-thermal conductivity environment that gives rise to the helium-induced configurations, and the involvement of He atoms as building blocks of the clusters This is attested by the moments of inertia of dopants such as glyoxal, OCS, or sulfur hexafluoride and numerous other dopant species that exhibit a characteristic increase due to a corotating, rigidly bound helium solvation layer.[6−12] Apart from the effect on the moments of inertia as revealed by rotationally resolved infrared spectra, the influence of the helium host environment on the clusters transpires in electronic spectra as helium-induced fine structure.[13−15] This manifests itself in the line shape at the electronic origin known as the zero phonon line (ZPL) and in a combined excitation of the dopant molecule and the helium environment known as the phonon wing (PW). Both the helium droplet experiment and the gas-phase quantum chemical calculations have identified three stable isomeric variants of Pc−H2O
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