Discovery of dumbbell-shaped Cs*Hen exciplexes in solid 4He

Dynamics of Photoexcited Cs Atoms Attached to Helium Nanodroplets

We present an experimental study of the dynamics following the photoexcitation and subsequent photoionization of single Cs atoms on the surface of helium nanodroplets. The dynamics of excited Cs atom desorption and readsorption as well as CsHe exciplex formation are measured by using femtosecond pump-probe velocity map imaging spectroscopy and ion time-of-flight spectrometry. The time scales for the desorption of excited Cs atoms off helium nanodroplets as well as the time scales for CsHe exciplex formation are experimentally determined for the 6p states of Cs. For the 6p ²Π_1/2 state, our results confirm that the excited Cs atoms only desorb from the nanodroplet when the excitation wavenumber is blue-shifted from the 6p ²Π_1/2 ← 6s ²Σ_1/2 resonance. Our results suggest that the dynamics following excitation to the 6p ²Π_3/2 state can be described by an evaporation-like desorption mechanism, whereas the dynamics arising from excitation to the 6p ²Σ_1/2 state is indicative for a more impulsive desorption process. Furthermore, our results suggest a helium-induced spin-orbit relaxation from the 6p ²Σ_1/2 state to the 6p ²Π_1/2 state. Our findings largely agree with the results of time-dependent ⁴He density functional theory (DFT) simulations published earlier [Eur. Phys. J. D 2019, 73, 94].

4s to 5s and 4p photoexcitation dynamics of K atoms from the surface of helium nanodroplets: a theoretical study

We study the photodissociation of the potassium atom from a superfluid helium nanodroplet upon 5s ²S or 4p ²P excitation using the time-dependent helium density functional method (He-TDDFT). The results show possible K spin–orbit relaxation.

Dynamics of photoexcited Ba(+) cations in (4)He nanodroplets

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.

Coinage metal exciplexes with helium atoms: a theoretical study of M*(2L)He(n) (M = Cu, Ag, Au; L = P,D)

The structure and energetics of exciplexes M*((2)L)He(n) (M = Cu, Ag and Au; L = P and D) in their vibrational ground state are studied by employing diffusion Monte Carlo (DMC). Interaction potentials between the excited coinage metals and He atoms are built using the Diatomics-in-Molecule (DIM) approach and ab initio potential curves for the M((2)L)-He dimers. Extending our previous work [Cargnoni et al., J. Phys. Chem. A, 2011, 115, 7141], we computed the dimer potential for Au in the (2)P and (2)D states, as well for Cu and Ag in the (2)D state, employing basis set superposition error-corrected Configuration Interaction calculations. We found that the (2)Π potential correlating with the (2)P state of Au is substantially less binding than for Ag and Cu, a trend well supported by the M(+) ionic radiuses. Conversely, the interaction potentials between a (n - 1)d(9)ns(2 2)D metal and He present a very weak dependency on M itself or the projection of the angular momentum along the dimer axis. This is due to the screening exerted by the ns(2) electrons on the hole in the (n - 1)d shell. Including the spin-orbit coupling perturbatively in the DIM energy matrix has a major effect on the lowest potential energy surface of the (2)P manifold, the one for Cu allowing the formation of a "belt" of five He atoms while the one for Au being completely repulsive. Conversely, spin-orbit coupling has only a weak effect on the (2)D manifold due to the nearly degenerate nature of the diatomic potentials. Structural and energetic results from DMC have been used to support experimental indications for the formation of metastable exciplexes or the opening of non-radiative depopulation channels in bulk and cold gaseous He.

Spectroscopy of Cs2, RbCs, and Rb2 in solid 4He

We present comparative experimental and theoretical studies of the absorption and fluorescence spectra of the alkali-metal dimer molecules Cs(2) and RbCs immersed in a solid helium matrix, thereby extending our recent observations of Rb(2) in solid (4)He. The laser-excited molecular states are mostly quenched by the interaction with the He matrix. The quenching efficiently populates the second lowest excited state of the molecule, i.e., (1)  (3)Π((u)) that is metastable in the homonuclear dimers. Molecular excitation and emission bands are modeled by calculating Franck-Condon factors that give a reasonable agreement with the experimental findings.

Cs*Hen exciplexes in solid He4

We present a theoretical and experimental study of the laser-induced formation process and of the emission spectra of Cs*He(n) exciplexes in the hcp and bcc phases of solid helium. Two different exciplex molecules are detected: a linear triatomic Cs*He2, which can exist in two electronic states: APi(1/2) and BPi(3/2), and a larger complex, where six or seven He atoms form a ring around a single cesium atom in the 6P(1/2) state. A theoretical model is presented, which allows the interpretation of the experimentally observed spectra.