Structure and dynamics of liquid helium systems and their interaction with atomic dopants and free electrons

Abstract

[spa] En esta tesis se presenta una colección de cuatro artículos publicados y un manuscrito aún no publicado, todos ellos en el campo de la física de bajas temperaturas y fluidos cuánticos. Cada uno de ellos reporta un paso adelante en la descripción teórica de los sistemas de helio por medio de la teoría del funcional de la densidad. Los primeros dos artículos están clasificados como “estructura" ya que tratan cuestiones relacionadas con la descripción del estado fundamental de complejos de helio dopados con impurezas atómicas. En ellos hemos calculado la estructura y determinado su efecto sobre el espectro de absorción dipolar del Na en agregados de (3)He-(4)He y del Mg en el líquido homogéneo e isotópicamente mezclado. Para el caso de Na en gotas se ha encontrado que, a pesar de necesitar una gran cantidad de (3)He para que la capa exterior de la gota sature, el espectro de la impureza es muy insensible a la composición isotópica y rápidamente satura al valor que toma en las gotas de (3)He puras

[eng] This thesis presents a collection of four papers published in peer-reviewed scientific journals plus a manuscript yet to be submitted, all of them in the field of low temperature physics and quantum fluids. Each of these works reports a step forward in the ever-developing theoretical description of helium systems by means of density functional theory. The first two papers deal with questions related to the groundstate description of helium complexes around atomic impurities. We have computed such structure and determined its effect on the dipole absorption spectrum of Na in (3)He—(4)He clusters and of Mg in the homogeneous, isotopically mixed liquid. We have also explored the limits of density functional calculations for a small number of helium atoms interacting with a linear carbonyl sulfide (OCS) molecule. To this end we have implemented a Kohn-Sham scheme for 3He and computed the structure of OCS@3HeN clusters for N up to 40. The next three papers deal with the real-time description of dynamical processes in helium systems of experimental interest. We present an efficient and quantitatively accurate procedure to compute dynamical processes, namely the dynamics of an excited electron bubble and of an excited silver impurity, following a time-dependent density functional theory (TDDFT) for helium coupled to the appropiate dynamics of impurities. In the case of the electron bubble, we have related the experimental disappearance of 1P bubbles at high pressures with the existence of a nonradiative de-excitation path involving the bubble splitting about 20 picoseconds after the excitation. In the case of the desorption of a silver atom from a He drop, our dynamical calculations predict a range of velocities for the ejected impurity consistent with the experimental velocity distribution, which can be taken as indirect evidence of the superfluidity of helium nanodroplets