Adsorption of Helium on Small Cationic PAHs: Influence of Hydrocarbon Structure on the Microsolvation Pattern.

Arne Schiller,Fabio Zappa,Paul Scheier,Serge A Krasnokutski,Florent Calvo,Paul Martini,Miriam Meyer

doi:10.1021/acs.jpca.1c05150

Abstract

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data. The calculated structures of the helium–PAH complexes indicate that the arrangement of adsorbed helium atoms is highly sensitive toward the structure of the solvated PAH cation. Closures of the first solvation shell around the studied PAH cations are suggested to lie between 29 and 37 adsorbed helium atoms depending on the specific PAH cation. Helium atoms are found to preferentially adsorb on these PAHs following the commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

Highlights

The adsorption of atoms and molecules on carbonaceous materials has been widely studied for a variety of reasons, ranging from probing fundamental chemistry and physics[1−4] to practical applications like hydrogen storage.[5−7] Helium is a highly interesting adsorbant species from a fundamental standpoint due to its extremely weak binding and its quantum nature, which becomes relevant at the low temperatures required to bind helium atoms.[8]
In addition to the classical and quantum structures and their associated energies, we explored the extent of statistical delocalization in the nuclear wave function probed by the path-integral molecular dynamics (PIMD) simulations, by performing regular quenches from the instantaneous centroid positions
The best agreement between experiment and theory is found when nuclear delocalization is accounted for by using PIMD simulations, which are capable of revealing features that are not recognized by classical methods

Summary

Introduction

The adsorption of atoms and molecules on carbonaceous materials has been widely studied for a variety of reasons, ranging from probing fundamental chemistry and physics[1−4] to practical applications like hydrogen storage.[5−7] Helium is a highly interesting adsorbant species from a fundamental standpoint due to its extremely weak binding and its quantum nature, which becomes relevant at the low temperatures required to bind helium atoms.[8]. The helium hydride ion is considered to be the first molecule being formed in relevant quantities in the early universe.[9] In laboratory experiments, He-tagged ions represent ideal targets for action spectroscopy, as recently demonstrated in several laboratories.[10−21] The weak binding energy leads to small line shifts with respect to the isolated ions and is at the same time a confirmation for vibrationally cold ions. Helium adsorption has been studied extensively on graphite,[22−26] graphene,[27,28] and graphene derivatives[29−32] as well as carbon nanotubes.[33,34] Going down in size, molecules such as fullerenes and polycyclic aromatic hydrocarbons (PAHs) offer the possibility to study helium adsorption on finite planar and curved graphene-like flakes. Whereas helium adsorption on neutral fullerenes has only been studied theoretically far,[35−43] helium adsorption on neutral PAHs has been studied by using various spectroscopic and theoretical methods.[44−49]

Methods

Results

Conclusion