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Register a new userMicrosolvation of cationic dimers in $^4$He droplets: geometries of A$_2^+$(He)$_N$ (A=Li,Na,K) from optimized energies
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Ab initio computed interaction forces are employed in order to describe the microsolvation of the A$_2^+(^2Sigma)$ (A=Li,Na,K) molecular ion in $^4$He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.
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A structural study of the smaller Li$^+$He$_n$ clusters with $nle30$ has been carried out using different theoretical methods. The structures and the energetics of the clusters have been obtained using both classical energy minimization methods and quantum Diffusion Monte Carlo. The total interaction acting within the clusters has been obtained as a sum of pairwise potentials: Li$^+$-He and He-He. This approximation had been shown in our earlier study cite{8} to give substantially correct results for energies and geometries once compared to full ab-initio calculations. The general features of the spatial structures, and their energetics, are discussed in details for the clusters up to $n=30$ and the first solvation shell is shown to be essentially completed by the first ten helium atoms.
We predict $s-$wave elastic cross-sections $sigma$ for low-energy atom-molecule collisions with kinetic energies up to 40 mK, for the $^4$He collision with weakly bound diatomic molecules formed by $^4$He with $^7$Li, $^6$Li and $^{23}$Na. Our scattering calculations are performed by using diatomic and triatomic molecular binding energies obtained from several available realistic models as input in a renormalized zero-range model, as well as a finite-range one-term separable potential in order to quantify the relevance of range corrections to our predictions. Of particular relevance for possible experimental realization, we show the occurrence of a zero in $sigma$ for the collision of cold $^4$He on $^4$He$-^{23}$Na molecule below 20 mK. Also our results for the elastic collision $^4$He on $^4$He$-^{6,7}$Li molecules suggest that $sigma$ varies considerably for the realistic models studied. As the chosen molecules are weakly bound and the scattering energies are very low, our results are interpreted on the light of the Efimov physics, which explains the model independent and robustness of our predictions, despite some sensitivity on the potential range.
Alkali metal dimers attached to the surface of helium nanodroplets are found to be efficiently doubly ionized by electron transfer-mediated decay (ETMD) when photoionizing the helium droplets. This process is evidenced by detecting in coincidence two energetic ions created by Coulomb explosion and one low-kinetic energy electron. The kinetic energy spectra of ions and electrons are reproduced by simple model calculations based on diatomic potential energy curves, and are in agreement with ab initio calculations for the He-Na_2 and He-KRb systems. This work demonstrates that ETMD is an important decay channel in heterogeneous nanosystems exposed to ionizing radiation.
We present a diffusion Monte Carlo study of a vortex line excitation attached to the center of a $^4$He droplet at zero temperature. The vortex energy is estimated for droplets of increasing number of atoms, from N=70 up to 300 showing a monotonous increase with $N$. The evolution of the core radius and its associated energy, the core energy, is also studied as a function of $N$. The core radius is $sim 1$ AA in the center and increases when approaching the droplet surface; the core energy per unit volume stabilizes at a value 2.8 K$sigma^{-3}$ ($sigma=2.556$ AA) for $N ge 200$.
The thermal resistivity and its scaling function in quasi-2D $^4$He systems are studied by Monte Carlo and spin-dynamics simulation. We use the classical 3D XY model on $Ltimes Ltimes H$ lattices with $Lgg H$, applying open boundary condition along the $H$ direction and periodic boundary conditions along the $L$ directions. A hybrid Monte Carlo algorithm is adopted to efficiently deal with the critical slowing down and to produce initial states for time integration. Fourth-order Suzuki-Trotter decomposition method of exponential operators is used to solve numerically the coupled equations of motion for each spin. The thermal conductivity is calculated by a dynamic current-current correlation function. Our results show the validity of the finite-size scaling theory, and the calculated scaling function agrees well with the available experimental results for slabs using the temperature scale and the thermal resistivity scale as free fitting parameters.
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