Solvation and spectral line shifts of chromium atoms in helium droplets based on a density functional theory approach.

Martin Ratschek,Wolfgang E Ernst,Johann V Pototschnig,Andreas W Hauser

doi:10.1021/jp5034036

Martin Ratschek, Wolfgang E Ernst + Show 2 more

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https://doi.org/10.1021/jp5034036

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Abstract

The interaction of an electronically excited, single chromium (Cr) atom with superfluid helium nanodroplets of various size (10 to 2000 helium (He) atoms) is studied with helium density functional theory. Solvation energies and pseudo-diatomic potential energy surfaces are determined for Cr in its ground state as well as in the y7P, a5S, and y5P excited states. The necessary Cr–He pair potentials are calculated by standard methods of molecular orbital-based electronic structure theory. In its electronic ground state the Cr atom is found to be fully submerged in the droplet. A solvation shell structure is derived from fluctuations in the radial helium density. Electronic excitations of an embedded Cr atom are simulated by confronting the relaxed helium density (ρHe), obtained for Cr in the ground state, with interaction pair potentials of excited states. The resulting energy shifts for the transitions z7P ← a7S, y7P ← a7S, z5P ← a5S, and y5P ← a5S are compared to recent fluorescence and photoionization experiments.

Highlights

Chromium, a very versatile transition metal due to its half filled 3d and 4s shells, is found in 9 different oxidation states, ranging from −2 to 6
We investigate the influence of the helium nanodroplets (HeN) environment on Cr in its ground state and selected electronically excited states of the septet and quintet spin manifold by means of helium density functional theory (DFT)
We focus on a theoretical description of the interaction between a single Cr atom and He2000

Summary

Introduction

A very versatile transition metal due to its half filled 3d and 4s shells, is found in 9 different oxidation states, ranging from −2 to 6. While Cr clusters are mostly formed in antiferromagnetic states,[1,2] the principal possibility to create high-spin nanoparticles[3] makes them a technologically interesting target. In this context, the experimental methods of helium nanodroplet isolation spectroscopy[4] offer a 2-fold advantage for studying cluster formation and manipulation. It has been shown that cluster formation on helium nanodroplets (HeN) is spin selective, with a clear preference for weakly bound high spin states.[3,5−8] Second, single species of the formed clusters are made spectroscopically accessible at very low temperature due to the cold He environment.[4,9] Recent experimental investigations of Cr on HeN in our group were based on mass spectroscopy,[10] followed by photoionization and fluorescence studies.[11−13] The complex interaction between Cr and the He environment, with Cr turning out to be a borderline species in terms of its actual position on the droplet, triggered this theoretical study

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