Dynamics of Isotopically Pure He Droplets Doped with Atomic Impurities

Abstract

Helium is the second lightest element and the second most abundant element in the universe. It is named for the Greek god of the sun, Helios, and it was first discovered by French astronomer Jules Janssen as an unknown yellow spectral line in sunlight during a solar eclipse in 1868.

Helium has a very simple atomic structure: two electrons around a nucleus formed of two protons and two neutrons (the case of He-4) or two protons and a single neutron (for He-3). Looking at the corresponding phase diagrams of both isotopes, it can be seen that they both have the unique property of maintaining the liquid phase down to T=0 K, thanks to their zero point energy, which is large enough to avoid solidification (due to their low mass and the weak He-He interaction). The zero point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. The existence of this energy is a prediction of Quantum Mechanics with no classical equivalent, and plays the role of a kinetic energy present even when there is no “motion” in the classical sense. The first liquefaction of helium was achieved in 1908, by Dutch physicist Heike Kamerlingh Onnes. Furthermore, under certain temperature conditions (below 2.17 K for He-4 and 2.7 mK for He-3) helium becomes superfluid, that is, its viscosity is almost zero and it can flow without any friction. This remarkable feature was first discovered by Pyotr Kapitsa, John F. Allen, and Don Misener in 1937.

Helium nanodroplets have been extensively studied in cluster physics and physical chemistry for more than 20 years, as they have the ability to capture atoms and molecules they collide with. This property, along with the weak interaction of superfluid He-4 with atoms and molecules, makes them ideal nanometric scale matrices for spectroscopic studies of molecules and other structures. Once captured, the impurities (also known as dopants) may solvate inside the droplet or, in some cases, remain in a dimple on the surface.

The five papers presented in this thesis are organized in three different sections: Chapter 2 addresses the dynamic study of Ba+ impurities at He-4 nanodroplets by two publications. The first paper focuses on the dynamic study of Ba+ upon ionization of Ba in a dimple on the surface of a droplet and the second paper studies the evolution of Ba+ in the bulk portion of the droplet when it is excited to 2P or 2D states.

Chapter 3 presents the dynamic study of alkali impurities (Rb and Cs) at the same droplets. This chapter is also represented by two papers, where the first one addresses the ejection of these impurities from the surface of He-4 nanodroplets upon excitation to the 6s state (for rubidium) or 7s state (in the case of cesium), and the second paper studies the dynamic evolution of Rb+ and Cs+ after the corresponding neutral atom is ionized on the surface of a droplet.

Lastly, Chapter 4 presents an article that focuses on the capture of Cs atoms by He-4 nanodroplets. We have studied this process by making cesium atoms collide with He droplets using different projectile velocities and impact parameters.

Esta tesis está compuesta de una colección de cinco de los artículos publicados desde 2013 a 2016, abarcados todos ellos en el campo de la física de bajas temperaturas y fluidos cuánticos. Cada uno de ellos constituye un paso adelante en la descripción teórica de los sistemas de helio en el marco de la teoría del funcional de la densidad.

Los dos primeros artículos están relacionados con estudios dinámicos de impurezas de Ba en nanogotas compuestas por 1000 átomos de helio. Se conoce experimentalmente, a partir del análisis del espectro de absorción de las impurezas, que los átomos de todos los elementos alcalinos y de todos los alcalinotérreos más pesados que el magnesio ---como es el caso del bario--- al ser capturados por las gotas de helio tienen un estado de equilibrio en la superficie de estas.

El siguiente bloque está compuesto por dos artículos dedicados a las dinámicas de alcalinos en nanogotas de He-4, en concreto rubidio y cesio, que comparten con el bario que su configuración de equilibrio en el estado fundamental está en la superficie de la gota.

El bloque que cierra esta tesis está compuesto por un único artículo que trata las colisiones de impurezas heliofóbicas ---en este caso átomos de cesio--- contra nanogotas de helio. Hemos estudiado estas colisiones cuando se utilizan dife-rentes velocidades para la impureza, así como distintos parámetros de impacto ---incluyendo las colisiones frontales con parámetro de impacto igual a cero---. Al modificar dichos parámetros hemos observado diferentes fenómenos que van desde la captura de la impureza por parte de la gota, permaneciendo en la superficie de la misma, hasta la ``transparencia'' de la gota para velocidades más altas ---la impureza penetra en la gota traspasándola de un extremo a otro---. En el caso de las colisiones con parámetro de impacto distinto de cero, que depositan una cierta cantidad de momento angular en las gotas, mediante el estudio de las corrientes de densidad hemos podido observarque la gota no gira como un cuerpo rígido, apreciándose un esquema de corrientes que corresponde al de un superfluido.