Electron attachment to doped (semi-)liquid clusters

Atomic and molecular cluster matrices differ strongly in temperature and phase. Neutral helium clusters are known to be liquid (even super-fluid) independently of the presence of dopants in these clusters, while pure liquid neon- and hydrogen cluster start to be solid after embedment of few single molecules. The commonness of the mentioned (semi)-liquid and extremely cold cluster (~0.4-10 K) is their strongly repulsive interaction with an excess electron which leads for electron injection to the formation of an electron bubble inside the cluster, where the excess electron is localized. Moreover, the cluster surface represents an energetic barrier for an incoming electron, i.e. the electron needs sufficient kinetic energy to overcome this barrier. By doping these clusters with molecules which may form long-lived anions upon electron attachment, specific cluster properties upon electron capture can be studied over a wide range of cluster sizes. A pronounced modification of the cross sections for electron attachment and subsequent dissociation can be expected. In this project specific molecules will be embedded in the mentioned cluster matrices and anion formation will be studied for the first time by means of a high resolution electron monochromator which will be combined with a time-of-flight mass spectrometer. With the present experiment the electron injection into doped clusters will be characterized. In case of helium clusters precise theoretical predictions concerning the minimum cluster size for bubble formation have been done which may be verified here experimentally. In case of hydrogen clusters electron injection may also lead to a bubble containing a hydride which is formed by dissociative electron attachment to a hydrogen molecule of the cluster. Reactions of such hydride bubbles with dopants in hydrogen clusters will be investigated. Moreover, by the present study we will also accomplish to extend the knowledge of the electron attachment process to molecules and clusters and the stabilization of formed negative ions to an extremely low temperature range. In addition to temperature also solvation is a crucial parameter for the rates of (chemical) reactions and the formation of product ions.

Rare gases are well known to be a chemically unreactive species. They are not able to capture a free electron and keep it sufficiently long for detection by mass spectrometry. The lifetime of the anionic state also depends on the size of the rare gas and for the small atoms helium and neon, the lifetime is particularly short. Also experimentally condensed helium or neon was shown to have an unfavorable interaction with excess electrons. Once an electron is injected into the macroscopic liquid of helium or neon, the electron is sitting in a cavity and does not bind to a specific atom of the liquid. The electron also needs some amount of energy to enter the liquid. In the present project we investigated this capture process for helium and neon clusters (small agglomerates containing few tens up to a million atoms), in order to investigate if these systems follow rather the behavior of the single atom or of the macroscopic liquid. For helium clusters the processes occurring in the formation of anions by electron capture turned out to be strongly cluster-size dependent and dependent on the presence of a dopant in the clusters. For pure clusters of sufficiently large size an exotic anionic species was found to form efficiently: the electronically excited helium anion. Such species is formed when an electron first electronically excites a helium atom in the cluster, loses thereby its energy and then becomes captured by this excited atom. This state would be stable in the helium cluster; however a second electron with the same charge causes the emission of the anion out of the droplet. This process allows the detection of the bare anion. We also determined, how much energy the electron loses upon the entrance of the helium cluster. This energetic barrier turned out to be cluster size dependent and for the first time we showed that a certain threshold size exists. In case of a dopant we observed that the attachment dynamics changes dramatically. Finally, the excess electron attaches to the dopant, and this charged species leaves the cluster. Related to this aspect, helium clusters and neon clusters show the same behavior. Anion formation upon electron attachment to doped neon clusters was studied for the first time and in contrast to the helium cluster, no distinct threshold for the formation of an electronic barrier was obtainable. However, like in the case of helium clusters, an electron with energy very close to zero eV is not able to localize at the molecular dopant. Therefore, the present results provide comprehensive information on the electronic structure of condensed rare gases.