Spectroscopy of gold atoms and gold oligomers in helium nanodroplets.

Roman Messner,Alexander Schiffmann,Martin Schnedlitz,Wolfgang E Ernst,Johann V Pototschnig,Maximilian Lasserus,Florian Lackner

doi:10.1063/1.5026480

Roman Messner, Alexander Schiffmann + Show 5 more

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https://doi.org/10.1063/1.5026480

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Abstract

The 6p 2P1/2 ← 6s 2S1/2 and 6p 2P3/2 ← 6s 2S1/2 transitions (D lines) of gold atoms embedded in superfluid helium nanodroplets have been investigated using resonant two-photon ionization spectroscopy. Both transitions are strongly blue-shifted and broadened due to the repulsive interaction between the Au valence electron and the surrounding helium. The in-droplet D lines are superimposed by the spectral signature of Au atoms relaxed into the metastable 2D states. These features are narrower than the in-droplet D lines and exhibit sharp rising edges that coincide with bare atom transitions. It is concluded that they originate from metastable 2D state AuHen exciplexes that have been ejected from the helium droplets during a relaxation process. Interestingly, the mechanism that leads to the formation of these complexes is suppressed for very large helium droplets consisting of about 2 × 106 He atoms, corresponding to a droplet diameter on the order of 50 nm. The assignment of the observed spectral features is supported by ab initio calculations employing a multiconfigurational self-consistent field method and a multi-reference configuration interaction calculation. For large helium droplets doped with Au oligomers, excitation spectra for mass channels corresponding to Aun with n = 2, 3, 4, 5, 7, and 9 are presented. The mass spectrum reveals even-odd oscillations in the number of Au atoms that constitute the oligomer, which is characteristic for coinage metal clusters. Resonances are observed close by the in-droplet D1 and D2 transitions, and the corresponding peak forms are very similar for different oligomer sizes.

Highlights

D-lines in the following, in analogy to the alkali systems
The 6p 2P1/2 ← 6s 2S1/2 and 6p 2P3/2 ← 6s 2S1/2 transitions (D lines) of gold atoms embedded in superfluid helium nanodroplets have been investigated using resonant two-photon ionization spectroscopy
These features are narrower than the in-droplet D lines and exhibit sharp rising edges that coincide with bare atom transitions

Summary

Introduction

D-lines in the following, in analogy to the alkali systems. In contrast to the alkalis, Cu and Ag are located inside the helium droplet and, according to recent calculations, ground state gold atoms are expected to be located in the interior of the droplets.. A location of the atom inside the droplet typically gives rise to the observation of strongly blue-shifted valence electron transitions in the excitation spectra as a consequence of the strong repulsive interaction of the electron with the surrounding helium. AgHeN 18,19 and CrHeN 23–25 are important benchmark systems concerning the relaxation dynamics following photoexcitation of dopant atoms inside helium nanodroplets. It was found that during such an excitation-relaxation process, the atoms can (i) desorb from the droplet as bare atoms or.

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