Quantum Study of Helium Clusters Doped with Electronically Excited …
87
do not emit visible fluorescence whereas Cs atoms do. Moroshkin et al. [34] presented spectroscopic studies of Cs
He n exciplexes produced by laser excitation of Cs
atoms isolated in a solid
4 He matrix. In these experiments the laser-excited 6P 3/2 state
is completely quenched and does not emit any fluorescence on the 6P 3/2 → 6S 1/2
transition. The Cs(6P 3/2 )He n=2 exciplex, formed directly from the 6P 3/2 state, is
quenched by a radiationless transformation into the larger Cs(6P 1/2 )He n=6,7 complex through the attachment of additional He atoms. Exciplex formation from the
6P 1/2 state proceeds via a strongly suppressed tunnelling transition.
Theoretical calculations for pure and atom- or molecule-doped helium clusters
have been widely performed, using the quantum Monte Carlo (QMC) method [35,
36], the path integral Monte Carlo (PIMC) method [37–39], or density functional approaches [12, 32, 40–43]. They confirmed that alkali atoms in the ground electronic
state are attached to the surface of helium clusters because of the weaker Ak-He
interaction (0.7–1.5 cm
−1 ) with respect to the He-He bond. A deep description of
the solvation process can be obtained by solving exactly the Schrödinger equation of
nuclear motion using an explicit many-body algorithm. In systems where accurate
interaction potentials between helium and the impurity are available, the quantum
Monte Carlo QMC approach is probably the best suited, as the DFT approach does
not take into account properly the discrete nature and the anisotropic deformation [44,
45] of the helium aggregates, and this might lead to overestimate the overall interaction energy with the impurity. Furthermore, we have to consider the strong dependence of the DFT on the choice of exchange-correlation functional which implies
that the description of van der Waals bonding within DFT is unreliable, given the
local nature of the approximations to the exchange-correlation functional. Quantum
Monte Carlo (QMC) techniques are useful in this regard as they can provide a highly accurate description of electron correlation effects. Although QMC methods are
computationally expensive, they can be applied to systems which are large enough
to model condensed matter. This was shown for instance by Drummond and Needs
on solid neon [46]. In their work, they showed that standard DFT methods do not
describe van der Waals bonding accurately, but they might be expected to work quite
well at high densities, where the short-range repulsion dominates. Alkali atoms are
displaced by about 5–6 Å from the droplet surface and the helium surface near alkali
atoms is slightly distorted. As reported by Ancilotto et al. [41], when an alkali atom is
attached to a large He-cluster, the Ak atom deforms the surface of the cluster giving
it a form of a dimple. The depth of the dimple increases with the mass of the alkali
metal atom, and it is larger for
3 He than for
4 He. Ak
He n exciplexes with the alkali
atom in the first excited state, consist of a ring of helium atoms located at the waist
of the alkali atom p orbital. Theoretical studies predict different numbers of helium
atoms in the ring. For Na, Dupont-Roc [47] found from five to six helium in the
ring, Kanorsky et al. [48] five, while De-Toffol et al. [49] estimated at least eight
helium atoms. Quantum Monte Carlo studies of K
He n [50] and of Rb
He n [51, 52]
led to rings formed by six or seven, and seven helium atoms for K and Rb, respectively. Anyway, to the best of my knowledge, no detailed theoretical studies after
this first ring-shaped structure have been conducted so far. As the number of helium
atoms in these ring structures varies depending on the alkali species, I investigated
87
do not emit visible fluorescence whereas Cs atoms do. Moroshkin et al. [34] presented spectroscopic studies of Cs
He n exciplexes produced by laser excitation of Cs
atoms isolated in a solid
4 He matrix. In these experiments the laser-excited 6P 3/2 state
is completely quenched and does not emit any fluorescence on the 6P 3/2 → 6S 1/2
transition. The Cs(6P 3/2 )He n=2 exciplex, formed directly from the 6P 3/2 state, is
quenched by a radiationless transformation into the larger Cs(6P 1/2 )He n=6,7 complex through the attachment of additional He atoms. Exciplex formation from the
6P 1/2 state proceeds via a strongly suppressed tunnelling transition.
Theoretical calculations for pure and atom- or molecule-doped helium clusters
have been widely performed, using the quantum Monte Carlo (QMC) method [35,
36], the path integral Monte Carlo (PIMC) method [37–39], or density functional approaches [12, 32, 40–43]. They confirmed that alkali atoms in the ground electronic
state are attached to the surface of helium clusters because of the weaker Ak-He
interaction (0.7–1.5 cm
−1 ) with respect to the He-He bond. A deep description of
the solvation process can be obtained by solving exactly the Schrödinger equation of
nuclear motion using an explicit many-body algorithm. In systems where accurate
interaction potentials between helium and the impurity are available, the quantum
Monte Carlo QMC approach is probably the best suited, as the DFT approach does
not take into account properly the discrete nature and the anisotropic deformation [44,
45] of the helium aggregates, and this might lead to overestimate the overall interaction energy with the impurity. Furthermore, we have to consider the strong dependence of the DFT on the choice of exchange-correlation functional which implies
that the description of van der Waals bonding within DFT is unreliable, given the
local nature of the approximations to the exchange-correlation functional. Quantum
Monte Carlo (QMC) techniques are useful in this regard as they can provide a highly accurate description of electron correlation effects. Although QMC methods are
computationally expensive, they can be applied to systems which are large enough
to model condensed matter. This was shown for instance by Drummond and Needs
on solid neon [46]. In their work, they showed that standard DFT methods do not
describe van der Waals bonding accurately, but they might be expected to work quite
well at high densities, where the short-range repulsion dominates. Alkali atoms are
displaced by about 5–6 Å from the droplet surface and the helium surface near alkali
atoms is slightly distorted. As reported by Ancilotto et al. [41], when an alkali atom is
attached to a large He-cluster, the Ak atom deforms the surface of the cluster giving
it a form of a dimple. The depth of the dimple increases with the mass of the alkali
metal atom, and it is larger for
3 He than for
4 He. Ak
He n exciplexes with the alkali
atom in the first excited state, consist of a ring of helium atoms located at the waist
of the alkali atom p orbital. Theoretical studies predict different numbers of helium
atoms in the ring. For Na, Dupont-Roc [47] found from five to six helium in the
ring, Kanorsky et al. [48] five, while De-Toffol et al. [49] estimated at least eight
helium atoms. Quantum Monte Carlo studies of K
He n [50] and of Rb
He n [51, 52]
led to rings formed by six or seven, and seven helium atoms for K and Rb, respectively. Anyway, to the best of my knowledge, no detailed theoretical studies after
this first ring-shaped structure have been conducted so far. As the number of helium
atoms in these ring structures varies depending on the alkali species, I investigated