Electronic Structure of the Helium Molecular Anion<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math>

Properties of ground state He(=He(1s 2 1 S)), He + (=He(1s 2 S)), He + 2 (=He 2 (1σ 2 g 1σ u 2 + u )) and excited (metastable) He * (=He(1s2s 3 S)), He 2 * (=He 2 (1σ 2 g 1σ u 2σ g 3 + u )), He * − (=He(1s2s2p 4 P)) and He 2 * − (=He 2 (1σ 2 g 1σ u 2σ g 1π u 4 g )) are calculated using the coupled-cluster method and basis sets multiply augmented with diffuse functions. The aim of this work is to capture the essential physics needed to describe the qualitatively different behaviour of the above mentioned helium species dissolved in liquid helium. By studying their interaction with atomic ground state helium it is found that ground state He, He + , He 2 + and excited (metastable) He * − are well bound within a helium droplet. In comparison excited (metastable) He * , He 2 * and He 2 * − are found to be squeezed out due to the high energetic cost associated with the large volume they require inside a helium droplet. In particular, the molecular species He 2 * and He 2 * − consist of a positive core in the form of a He 2 + which is surrounded by a diffuse electronic cloud accounting for one or two electrons, respectively. The implications of these results for recent experimental studies on helium nanodroplets are discussed, particularly for the negatively charged species He * − and He 2 * − . We find that the latter species experience completely different dynamcis in a helium droplet although they are very similar in various other respects (e.g. diffuse electron clouds, size) in good agreement with experimental observations.

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“…He ( [32] basis sets are within 0.1 meV of each other. The results obtained with the q-aug-cc-pVQZ basis set are depicted in Figure 2.…”

Section: Atomic Speciesmentioning

confidence: 99%

On the properties of charged and neutral, atomic and molecular helium species in helium nanodroplets: interpreting recent experiments

Huber

Mauracher

2013

Molecular Physics

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“…These anions are well-known in the gas phase and have been studied from both experimental 12−14 and theoretical side. 15−20 The basis for the above-mentioned speculation about the relevance of these anions has been observations 21,22 of negative ion resonances upon electron impact on helium droplets at about 22 eV and multiples thereof. This could be brought in line with the excitation of ground state helium into the first excited triplet state, i.e., He(1s2s 3 S), 23 by taking into account the energy loss of the impact electron due to the intrusion into the helium droplet.…”

Section: Introductionmentioning

confidence: 99%

On the Formation of (Anionic) Excited Helium Dimers in Helium Droplets

Huber

Mauracher

2014

J. Phys. Chem. A

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Metastable atomic and molecular helium anions exhibiting high-spin quartet configurations can be produced in helium droplets via electron impact. Their lifetimes allow detection in mass spectrometric experiments. Formation of atomic helium anions comprises collision-induced excitation of ground state helium and concomitant electron capture. Yet the formation of molecular helium anions in helium droplets has been an unresolved issue. In this work, we explore the interaction of excited helium atoms exhibiting high-spin triplet configurations with ground state helium using the equation-of-motion coupled-cluster method. Transition barriers in the energetically lowest He*–He and He*––He interaction potentials prevent molecule formation at the extremely low temperatures present in helium droplets. In contrast, some excited states allow a barrier-free formation of molecular helium (anions). Moreover, we show that the necessary excitation energies pinpoint (higher) resonances in recently recorded mass spectra and emend the assignment of those resonances that have previously been assigned to electron-impact ionization of ground state helium necessitating subsequent double-electron capture. Embedding molecules or molecular clusters in helium droplets is a predestined experimental technique for the study of phenomena at very low temperatures. Profound knowledge about active processes in the helium environment is required for a proper assessment of experimental data.

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“…Theoretical works on the "Ils (los lo"' 2o' lm"') metastable state of He2 were reported by Michels [9,10] who used the configuration interaction method, and by us [11,12] where the coupled cluster method and numerical orbital techniques were employed. These studies indicate that the principle of the formation of this state rests in a two-electron attachment to the He2+ core, which results in the formation of a quartet state, where both valence electrons occupy very diffused orbitals with bonding character.…”

Section: Introductionmentioning

confidence: 99%

“…One can assess this diA'erence upon examining Fig. 1 where we present the comparison of the repulsive energy curve for the ground state of the neutral He& cluster, the energy curves of the ground state and the erst excited state of He2+, the curve for the long-lived He2 Hg state studied earlier [5,9,10] (at 0.5 a.u. ).…”

Section: Introductionmentioning

confidence: 99%

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MetastableHe2−ions formed by two-electron attachment to the excitedHe
Adamowicz¹,
Pluta
²

1991
Phys. Rev. A
16
0
5
0
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Four metastable states (1 H", 2 H", @",and I"),resulting from two-electron attachments to the excited He2+ core ( Xg+), are characterized using the numerical Hartree-Fock method. It is determined that such metastable states are formed when both valence electrons are placed into equally difFused orbitals, which have bonding charter, and whose angular momentum quantum numbers do not di8'er by more than 1.

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Electronic Structure of the Helium Molecular AnionHe2−

Cited by 30 publications

References 17 publications

On the properties of charged and neutral, atomic and molecular helium species in helium nanodroplets: interpreting recent experiments

On the properties of charged and neutral, atomic and molecular helium species in helium nanodroplets: interpreting recent experiments

On the Formation of (Anionic) Excited Helium Dimers in Helium Droplets

MetastableHe2−ions formed by two-electron attachment to the excitedHe
Adamowicz¹,
Pluta
²

1991
Phys. Rev. A
16
0
5
0
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Electronic Structure of the Helium Molecular AnionHe2−

Cited by 30 publications

References 17 publications

On the properties of charged and neutral, atomic and molecular helium species in helium nanodroplets: interpreting recent experiments

On the properties of charged and neutral, atomic and molecular helium species in helium nanodroplets: interpreting recent experiments

On the Formation of (Anionic) Excited Helium Dimers in Helium Droplets

MetastableHe2−ions formed by two-electron attachment to the excitedHeAdamowicz1, Pluta2 1991Phys. Rev. A16050View full textAdd to dashboardCite

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MetastableHe2−ions formed by two-electron attachment to the excitedHe
Adamowicz¹,
Pluta
²

1991
Phys. Rev. A
16
0
5
0
View full text Add to dashboard Cite