Quantum Study of Helium Clusters
Doped with Electronically Excited
Li, Na, K and Rb Atoms
David Dell’Angelo
Abstract I shall present a systematic study of the lowest electronically excited
state of alkali (Li, Na, K, Rb) atoms in helium clusters with up to 15 helium atoms,
for which a diatomics-in-molecule model combined with an Importance-Sampling
Diffusion Monte Carlo (IS-DMC) technique to compute the vibrational ground
state energies has been used. Solvation structures are examined via various density
distributions. Results show a first solvation shell around the np-orbital completed
with 5≤ n ≤ 7 depending on the alkali atom and in agreement with other theoretical
works. Except for Li, a shift of one helium atom is observed between classical and
quantum first filled shells. Dealing with the number of helium atoms in the second
shell, for K and Rb a closure at eleven atoms can be easily concluded whereas number
ranging from 9 to 11 for Li and 9 or 10 for Na are found. The planar structures of
the Ak
He n obtained in the first shell are maintened up to the second shell and the
radius of the second shells are twice as large as the inner ones.
Keywords Excited states · Quantum Monte Carlo · Alkali · Helium clusters ·
Doping · Liquid helium · Quantum dynamics · Molecular physics
1 Introduction
Helium clusters have become these last three decades [1, 2] the subject of extensive
experimental and theoretical studies [3, 4]. The unique and particular properties of
these cold (T = 370 mK for
4 He N [5]) droplets permit one to study superfluidity [6]
and chemical reaction at low temperatures [7, 8]. As they offer a weakly perturbing
environment, helium nanodroplet isolation [1, 2, 9] (HENDI) has been established
as a useful tool for spectroscopic studies of molecules and weakly bound complexes
[1, 2, 10]. Dopants such as atoms and molecules can be added to these nanodroplets
and most submerge into the liquid helium. Alkali metal atoms represent an exception.
D. Dell’Angelo (B)
Department of Physics and Astronomy, University College of London,
Gower Street, London, WC1E 6BT, UK
e-mail: ucapdd0@ucl.ac.uk
© Springer Nature Switzerland AG 2020
L. Mammino et al. (eds.), Advances in Quantum Systems in Chemistry,
Physics, and Biology, Progress in Theoretical Chemistry and Physics 32,
https://doi.org/10.1007/978-3-030-34941-7_5
85
Doped with Electronically Excited
Li, Na, K and Rb Atoms
David Dell’Angelo
Abstract I shall present a systematic study of the lowest electronically excited
state of alkali (Li, Na, K, Rb) atoms in helium clusters with up to 15 helium atoms,
for which a diatomics-in-molecule model combined with an Importance-Sampling
Diffusion Monte Carlo (IS-DMC) technique to compute the vibrational ground
state energies has been used. Solvation structures are examined via various density
distributions. Results show a first solvation shell around the np-orbital completed
with 5≤ n ≤ 7 depending on the alkali atom and in agreement with other theoretical
works. Except for Li, a shift of one helium atom is observed between classical and
quantum first filled shells. Dealing with the number of helium atoms in the second
shell, for K and Rb a closure at eleven atoms can be easily concluded whereas number
ranging from 9 to 11 for Li and 9 or 10 for Na are found. The planar structures of
the Ak
He n obtained in the first shell are maintened up to the second shell and the
radius of the second shells are twice as large as the inner ones.
Keywords Excited states · Quantum Monte Carlo · Alkali · Helium clusters ·
Doping · Liquid helium · Quantum dynamics · Molecular physics
1 Introduction
Helium clusters have become these last three decades [1, 2] the subject of extensive
experimental and theoretical studies [3, 4]. The unique and particular properties of
these cold (T = 370 mK for
4 He N [5]) droplets permit one to study superfluidity [6]
and chemical reaction at low temperatures [7, 8]. As they offer a weakly perturbing
environment, helium nanodroplet isolation [1, 2, 9] (HENDI) has been established
as a useful tool for spectroscopic studies of molecules and weakly bound complexes
[1, 2, 10]. Dopants such as atoms and molecules can be added to these nanodroplets
and most submerge into the liquid helium. Alkali metal atoms represent an exception.
D. Dell’Angelo (B)
Department of Physics and Astronomy, University College of London,
Gower Street, London, WC1E 6BT, UK
e-mail: ucapdd0@ucl.ac.uk
© Springer Nature Switzerland AG 2020
L. Mammino et al. (eds.), Advances in Quantum Systems in Chemistry,
Physics, and Biology, Progress in Theoretical Chemistry and Physics 32,
https://doi.org/10.1007/978-3-030-34941-7_5
85