86
D. Dell’Angelo
In their electronic ground state are known to reside on the surface of the helium
droplet [11]. Upon excitation, they can remain on the droplet, detach from it or go
inside the droplet, depending on the helium interaction potential in the corresponding
electronically excited state. Whitin the variety of possible dopants, alkali atoms
appear to be one of the best candidates to study electronic excitations due to their
relatively simple electronic configuration with a single electron in the valence orbitals
and the optically accessible electronic transitions. Because of their simple, well
known, absorption spectrum and because they introduce only weak perturbations,
alkali atoms constitute a minimally perturbing probe capable of investigating the
surface of a helium cluster. These alkalis features permit to address the question
of solvation versus surface location for an impurity atom in liquid helium or how
the finite size of a helium cluster affects the superfluidity [12]. Spectra of the D
lines (nP (1/2,3/2) ← nS,n being the principal quantum number in the ground state)
of alkali-metal atoms on the surface of superfluid helium nanodroplets obtained via
Laser-induced-fluorescence (LIF) and beam-depletion (BD) spectroscopy have been
measured for Li [13–16], Na [13–15, 17–19], K [13, 15, 18, 20, 21], Rb [15, 21–
24], and Cs [21, 25–29]. Excitation spectra of alkalis on helium nanodroplets in the
neighborhood of the free alkali D lines have been interpreted in terms of an excitation
of the alkali s electrons into a p σ or a p π orbital, referring to the orbital angular
momentum being perpendicular or parallel, respectively, to the helium nanodroplet
surface. In fact, a peculiarity in the excitation of light alkali atoms on the surface of the
helium droplet is the formation of alkali-helium exciplexes which form when one or
more He atoms are attracted into the nodal plane of the excited p orbital [30]. Alkalihelium exciplexes are not formed when the p orbital is aligned perpendicularly to
the helium surface ( excitation); aligning the p orbital parallel, as an increase in
both attraction and repulsion leads to a bound-bound transition, mainly the Ak
He
diatomic exciplex forms.
In bulk liquid helium, fluorescence of laser-excited light alkalis has not been found
and the formation of exciplexes like Ak
He 5 has been proposed [20] as the cause of
the quenching of fluorescence emission, because of the crossing between the excited
and the ground state potential energy surface and the possibility of a decay via nonradiative transitions. In contrast to bulk helium, fluorescence spectra of light alkalis
attached to helium droplets have been recorded upon p ← s excitation [13]. When
Ak = Rb (5P (1/2,3/2) ←5S transition), as argued by Reho et al. [20] and experimentally verified through LIF excitation spectroscopy by Brühl et al. [22], a high barrier
along the 5
2 P 1/2 Rb-He potential prohibiting the tunneling of a helium atom from
the droplet toward the Rb atom prevents formation of the exciplex. For Rb on helium only the excitation of a He n−1 -HeRb 5
2 P 3/2 complex would yeld fluorescence
emission from Rb
He while the barrier would prevent desorption of the diatomic
in its 5
2 P 1/2 state within its fluorescence lifetime. The desorption of Rb
off the
droplet [31] competes with the submersion of the Rb
+ ion into the droplet interior,
because of the repulsive interaction of the droplet with the Rb atom in an excited
state and the attractive interaction of the droplet with the Rb
+ [32]. The formation of
Rb
He n exciplexes has been observed in pressurized liquid He [33] and explained as
a consequence of the quenching of the Rb 5
2 P 1/2 state. Also in solid He, Rb atoms
D. Dell’Angelo
In their electronic ground state are known to reside on the surface of the helium
droplet [11]. Upon excitation, they can remain on the droplet, detach from it or go
inside the droplet, depending on the helium interaction potential in the corresponding
electronically excited state. Whitin the variety of possible dopants, alkali atoms
appear to be one of the best candidates to study electronic excitations due to their
relatively simple electronic configuration with a single electron in the valence orbitals
and the optically accessible electronic transitions. Because of their simple, well
known, absorption spectrum and because they introduce only weak perturbations,
alkali atoms constitute a minimally perturbing probe capable of investigating the
surface of a helium cluster. These alkalis features permit to address the question
of solvation versus surface location for an impurity atom in liquid helium or how
the finite size of a helium cluster affects the superfluidity [12]. Spectra of the D
lines (nP (1/2,3/2) ← nS,n being the principal quantum number in the ground state)
of alkali-metal atoms on the surface of superfluid helium nanodroplets obtained via
Laser-induced-fluorescence (LIF) and beam-depletion (BD) spectroscopy have been
measured for Li [13–16], Na [13–15, 17–19], K [13, 15, 18, 20, 21], Rb [15, 21–
24], and Cs [21, 25–29]. Excitation spectra of alkalis on helium nanodroplets in the
neighborhood of the free alkali D lines have been interpreted in terms of an excitation
of the alkali s electrons into a p σ or a p π orbital, referring to the orbital angular
momentum being perpendicular or parallel, respectively, to the helium nanodroplet
surface. In fact, a peculiarity in the excitation of light alkali atoms on the surface of the
helium droplet is the formation of alkali-helium exciplexes which form when one or
more He atoms are attracted into the nodal plane of the excited p orbital [30]. Alkalihelium exciplexes are not formed when the p orbital is aligned perpendicularly to
the helium surface ( excitation); aligning the p orbital parallel, as an increase in
both attraction and repulsion leads to a bound-bound transition, mainly the Ak
He
diatomic exciplex forms.
In bulk liquid helium, fluorescence of laser-excited light alkalis has not been found
and the formation of exciplexes like Ak
He 5 has been proposed [20] as the cause of
the quenching of fluorescence emission, because of the crossing between the excited
and the ground state potential energy surface and the possibility of a decay via nonradiative transitions. In contrast to bulk helium, fluorescence spectra of light alkalis
attached to helium droplets have been recorded upon p ← s excitation [13]. When
Ak = Rb (5P (1/2,3/2) ←5S transition), as argued by Reho et al. [20] and experimentally verified through LIF excitation spectroscopy by Brühl et al. [22], a high barrier
along the 5
2 P 1/2 Rb-He potential prohibiting the tunneling of a helium atom from
the droplet toward the Rb atom prevents formation of the exciplex. For Rb on helium only the excitation of a He n−1 -HeRb 5
2 P 3/2 complex would yeld fluorescence
emission from Rb
He while the barrier would prevent desorption of the diatomic
in its 5
2 P 1/2 state within its fluorescence lifetime. The desorption of Rb
off the
droplet [31] competes with the submersion of the Rb
+ ion into the droplet interior,
because of the repulsive interaction of the droplet with the Rb atom in an excited
state and the attractive interaction of the droplet with the Rb
+ [32]. The formation of
Rb
He n exciplexes has been observed in pressurized liquid He [33] and explained as
a consequence of the quenching of the Rb 5
2 P 1/2 state. Also in solid He, Rb atoms