BM22018/10.26125/efat-qe04

Highly Excited Molecules – The Unusual Properties of Micrometer-Sized Dimers

Author(s): Johannes Deiglmayr

Publication: Bunsenmagazin, Issue 2 2018, Aspekte, Seiten: 83 - 91

Publisher: Deutsche Bunsen-Gesellschaft für physikalische Chemie e.V., Frankfurt

Language: English

DOI: 10.26125/efat-qe04

 

Introduction

When we think about a molecule, we mostly consider bonds between elements in their electronic ground state. Can atoms in highly-excited electronic states also form “stable” chemical bonds? How can we calculate the electronic structure of such molecules accurately and efficiently? And how can we study them experimentally? These are questions that we are trying to answer in our research. The basic building blocks of the molecules we study are atoms with a valence electron in a highlyexcited orbital, so called “Rydberg atoms”. The semiclassical orbit of the Rydberg electron has a radius of n2a0, where n is the principal quantum number of the excited state and a0 is the Bohr radius. Already for n = 30, the diameter of a Rydberg atom approaches 100 nm. The resulting strong delocalization of the electronic wavefunction increases also the radiative lifetime, which scales as n3 and exceeds 30 μs at n = 30.

In principle, molecules containing Rydberg atoms could be stable on the same timescale as the Rydberg atom if the only decay mechanism would be radiative decay of the electronic excitation. However, experience tells us that highly-excited molecules typically dissociate or autoionize within pico- to femtoseconds. In the following we will discuss two types of molecules where this is not the case. For the first type of molecules, the binding results purely from long-range van der Waals forces. We demonstrate experimentally that such interactions between two atoms in a Rydberg state, with an internal excitation energy of a few electron volts each, lead to vibrationally stable dimers with binding energies in the micro-eV range, as predicted by theory [1, 2]. For the second type of molecules, the binding results from the scattering of a Rydberg-electron off neutral particles within its orbit. This interaction provides an even more exotic binding mechanism for long-range Rydberg molecules, which is neither ionic, nor of covalent or van der Waals type [3]. This novel binding mechanism, low-energy electron-neutral scattering, plays an important role in the kinetics of cold plasmas and interstellar clouds. We will demonstrate how an accurate modelling of long-range Rydberg molecules can yield reliable experimental values for the relevant scattering parameters.

 

Cite this: Johannes Deiglmayr (2018): Highly Excited Molecules – The Unusual Properties of Micrometer-Sized Dimers. Bunsenmagazin 2018, 2: 83-91. Frankfurt am Main: Deutsche Bunsen-Gesellschaft für physikalische Chemie e.V. DOI: 10.26125/efat-qe04

 

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