Relaxation dynamics of helium nanodroplets after photodissociation of a dopant homonuclear diatomic molecule. The case of Cl2@((4)He)(N).

To investigate the quantum dynamics of the relaxation process of excited helium nanodroplets, (4)HeN, arising from the photodissociation of Cl2 embedded molecules (B ← X electronic transition), here we have performed a time dependent density functional theory (TDDFT) study considering nanodroplets of different sizes (N = 50, 100, 200, 300 and 500), extending a previous study which was centered on the photodissociation step. The relaxation process takes place in the timescale of several hundred picoseconds and a simple dependence of this process on time has been found. The results have been satisfactorily analyzed in terms of a phenomenological model proposed here and also by applying the Rice-Ramsperger-Kassel (RRK) statistical chemical kinetic model for unimolecular reactions. From what we know, this is the first time that the dynamics of these de-excitation processes has been studied, opening up a window for understanding them. We expect that this work will encourage further research on this little known but interesting phenomenon.

Quantum dynamics of the pick up process of atoms by superfluid helium nanodroplets: the Ne + ((4)He)1000 system.

The capture dynamics of a Ne atom by a superfluid helium nanodroplet (((4)He)N=1000; T = 0.37 K), Ne + ((4)He)N→ Ne@((4)He)N', was investigated using a quantum approach (TDDFT (helium) + quantum wave packet (Ne)) at zero angular momentum and a rather wide range of Ne atom initial mean velocities (〈v0〉: 90-1300 m s(-1)). This is probably the first quantum dynamics study focusing on the pick up process and the evolution of the dopant inside the nanodroplet and the second more detailed investigation on this topic. For 〈v0〉 = 210 m s(-1) and above the atom is always captured, but for lower velocities the probability of capture is somewhat below the unity and decreases as 〈v0〉 diminishes. The main energy exchange begins with the collision of the atom with the nanodroplet surface, and the excess of energy placed in the doped nanodroplet is progressively released by the evaporation of a small amount of (4)He atoms. Once the atom has entered into the nanodroplet its mean position follows an oscillatory trajectory, due to multiple sequential collisions with the inner surface of the nanodroplet, and its mean velocity reaches values which are below Landau's critical velocity. This probably corresponds to the general behavior of nanodroplets with a bulk-like region when moderate collision energies (i.e., similar to the ones considered here) are involved. In the future we hope to investigate the influence of angular momentum on the mechanism of the pick up process, using the same quantum dynamics method.

Photodissociation Dynamics of Homonuclear Diatomic Molecules in Helium Nanodroplets. The Case of Cl2@(4He)N.

To investigate the photodissociation dynamics of diatomic homonuclear molecules in helium nanodroplets, a hybrid quantum mechanical theoretical method that combines time dependent density functional theory (helium) and quantum dynamics (molecule) has been developed. This method has been applied to investigate the Cl2 photodissociation arising from the B ← X electronic transition, considering Cl2(v = 0,X)@((4)He)N nanodroplets with N = 50, 100, 200, 300, and 500 (initial configuration for the dynamics). A time scale of a few picoseconds has been determined, and the time required for the dissociating atoms to reach the nanodroplet surface increases with N. Moreover, at the high velocities involved (orders of magnitude above the Landau's critical velocity), an efficient energy exchange between the chlorine atoms and the helium takes place, releasing up to 91% of the energy of the excited diatomics for the bigger nanodroplet considered; and the energy exchange mechanism is the same for all the nanodroplets. Finally, simple (linear) relations for the average Cl final velocity and the (small) number of evaporated He atoms with respect to the radius of the droplets have been reported, together with the existence of confinement resonances. We hope that these results will encourage the experimentalists to investigate this kind of systems.

Quantum interferences in the photodissociation of Cl2(B) in superfluid helium nanodroplets ((4)He)N.

Quantum interferences are probably one of the most fascinating phenomena in chemical physics and, particularly, in reaction dynamics, where they are often very elusive from an experimental perspective. Here, we have theoretically investigated, using a hybrid method recently proposed by us, the dynamics of the formation of confinement quantum interferences in the photodissociation of a Cl2 molecule (B ← X electronic excitation) embedded in a superfluid helium nanodroplet of different sizes (50-500 (4)He atoms), which is to the best of our knowledge the first time that this type of interference is described in reaction dynamics. Thus, we have widely extended a recent contribution of our group, where interferences were not the main target, identifying the way they are formed and lead to the production of strongly oscillating velocity distributions in the Cl dissociating atoms, and also paying attention to the energy transfer processes involved. This probably corresponds to a rather general behavior in the photodissociation of molecules in helium nanodroplets. We hope that the present study will encourage the experimentalists to investigate this captivating phenomenon, although the technical difficulties involved are very high.

Unraveling the absorption spectra of alkali metal atoms attached to helium nanodroplets.

The absorption spectra of the first electronic exited state of alkali metal atoms on helium nanodroplets formed of both 4He and 3He isotopes were studied experimentally as well as theoretically. In the experimental part new data on the 2p<--2s transition of lithium on 3He nanodroplets are presented. The absorption spectrum changes drastically when compared to 4He droplets, in contrast to sodium where only marginal differences were observed in former studies. To explain these large differences and to answer some still open questions concerning the interaction of alkali metal atoms with helium nanodroplets, a model calculation was performed. New helium density profiles as well as a refined model allowed us to achieve good agreement with the experimental findings. For the first time the red-shifted intensities in the lithium and sodium spectra are explained in terms of enhanced binding configurations in the excited state displaced spatially from the ground state configurations.

Explosion of electron bubbles attached to quantized vortices in liquid 4He.

Electron bubbles in superfluid (4)He have been recently observed in low-temperature cavitation measurements under experimental conditions where quantized vortices are also present in the liquid, and which might be attached to the bubbles. We have calculated, within density functional theory, the structure and energetics of electron bubbles pinned to linear vortices in liquid (4)He at low temperature, and the pressure at which such structures become mechanically unstable. Our results are in semiquantitative agreement with the experiments. We discuss dynamical effects not included in the theoretical model used in the present calculations, and which could explain some discrepancies between our results and the experimental data.

The structure and energetics of 3He and 4He nanodroplets doped with alkaline earth atoms.

We present systematic results, based on density functional calculations, for the structure and energetics of 3He and 4He nanodroplets doped with alkaline earth atoms. We predict that alkaline earth atoms from Mg to Ba go to the center of 3He drops, whereas Ca, Sr, and Ba reside in a deep dimple at the surface of 4He drops, and Mg is at their center. For Ca and Sr, the structure of the dimples is shown to be very sensitive to the He-alkaline earth pair potentials used in the calculations. The 5s5p <-- 5s2 transition of strontium atoms attached to helium nanodroplets of either isotope has been probed in absorption experiments. The spectra show that strontium is solvated inside 3He nanodroplets, supporting the calculations. In the light of our findings, we emphasize the relevance of the heavier alkaline earth atoms for analyzing mixed 3He-4He nanodroplets, and in particular, we suggest their use to experimentally probe the 3He-4He interface.