Dynamics of photoexcited Ba+ cations in 4He nanodroplets

Abstract

We present a joint experimental and theoretical study on the desolvation of Ba(+) cations in (4)He nanodroplets excited via the 6p ← 6s transition. The experiments reveal an efficient desolvation process yielding mainly bare Ba(+) cations and Ba(+)Hen exciplexes with n = 1 and 2. The speed distributions of the ions are well described by Maxwell-Boltzmann distributions with temperatures ranging from 60 to 178 K depending on the excitation frequency and Ba(+) Hen exciplex size. These results have been analyzed by calculations based on a time-dependent density functional description for the helium droplet combined with classical dynamics for the Ba(+). In agreement with experiment, the calculations reveal the dynamical formation of exciplexes following excitation of the Ba(+) cation. In contrast to experimental observation, the calculations do not reveal desolvation of excited Ba(+) cations or exciplexes, even when relaxation pathways to lower lying states are included.

Capture of Xe and Ar atoms by quantized vortices in4He nanodroplets

“…where T [r] is the kinetic energy of a fictitious system of noninteracting particles (with mass m 4 ) constituting a BEC in the present case. As in recent applications of the TDDFT approach, 23,[26][27][28][29][30] we use the correlation energy density functional E c proposed in ref. 31.…”

Helium-induced electronic transitions in photo-excited Ba+–Hen exciplexes

“…However, these exciplexes are formed at rather long distances since the Franck-Condon region for their excitation corresponds to the bubble state of the ground electronic level. As shown by Fiedler et al, 65 Leal et al, 66 and more recently by Batulin et al, 67 the bubble radius is about 4.5 Å, and DFT calculations 44,65,66 show that it is surrounded by a high density shell structure. This would locate the Franck-Condon region for Ba + in liquid helium at about the same He-Ba + distance as for the He• • • Ba + dimer, 4 to 8 Å with a maximum at ∼5.2 Å, as materialized by the wave function in Figs.…”

“…Intramonomer correlation and dispersion interaction between Ba + and He has been accounted for by including single and double excitations starting from an active space of 11 electrons: the 1s 2 pair of He plus 9 electrons coming from Ba + (5s 2 , 5p 6 plus the outermost shells electron). Perturbative computations 44 proved this variational scheme to be well grounded. Interaction energies and wave functions have been computed at 33 internuclear distances ranging from 1.5 to 10.0 Å, plus the asymptote at 100.0 Å, sampling more finely the regions where the potential energy surfaces undergo sudden changes.…”

“…In order to study the potential energies and couplings in exciplexes with n > 1 helium atoms, we use an extended version of the diatomics in molecules (DIM) Hamiltonians used previously 44,48 for the valence electron of Ba + . The total electronic Hamiltonian for Ba + interacting with n helium atoms is written as…”

“…More recently Leal et al 44 have used helium timedependent density functional theory (He-TDDFT) to characterize the dynamics following photoexcitation of Ba + 6p 2 P in 4 He droplets. The experimental excitation spectrum was well reproduced using Mella and Cargnoni's He-Ba + interaction energy curves.…”