Reactions driven by He anions in doped He droplets

Anions of He were discovered already in 1939 but only in 2014 they were observed in helium nanodroplets (HND). This exotic anion is metastable and has an electronic energy 19.7eV above the ground state of neutral He and the free electron and a suitable description is a He+ cationic core which loosely binds two additional electrons. Laboratory measurements are proposed to study the reactions triggered by negatively charged helium ions formed via inelastic electron interactions with doped HND. In pristine helium nanodroplets He* and He2* are efficiently formed via a recently discovered process. He* is highly mobile and due to the extra charge as well as its potential energy of 19.7eV several novel reactions with dopants inside or at large He nanodroplets can be expected. He* can be described as a He+ cation which binds two loosely bound electrons and in a double electron transfer these two electrons can be transferred to a neutral dopant which has a high electron affinity. Thereby dianions are formed and for fullerene clusters this mechanism has been observed with surprisingly high probability. For He nanodroplets doped with alkali metals and halogen containing molecules He* is able to trigger the formation of salt nanocrystals. In the presently proposed studies it is planned to unravel the nature of different states of He* which are expected to exist due to the peculiar shape of the anion efficiency curve of this anion. Furthermore, the reactivity of He* with dopants depends strongly on the electron energy at which the anion is formed. A high-current electron gun with an electron energy resolution E<0.3eV is required and will be developed to probe these states of He*. Besides the dianion- and salt formation additional reactions driven by He* will be studied as a function of the droplet size as well as the dopant clusters.

The main objective of the project was to investigate the formation and the properties of metastable negative ions of He, i.e., He*- and He2*- since both species were recently discovered as abundant negatively charged species in large undoped helium nanodroplets (HND). The proposed study would follow with experiments on negatively charged HND doped with water, alcohol and metals. Finally, it was planned to acquire data of positively charged HNDs doped with neon and also with a mixture of metal atoms and C60 to observe salt formations or combustion reactions. The anion production was proposed to be done via electron attachment and, as a consequence, the project also included the necessary construction of a new ion source with enough electron energy resolution to probe the fine structure of the He electronic states. The data concerning metastable negative ions was partially acquired in a different setup than the initially proposed because it was verified that this apparatus exhibited a much better performance in cations measurements. The loss of novelty of the topic by measurements in another machine together with some unforeseen obstacles in the construction of the ion source and the best performance of the machine for cations resulted in a change of orientation. While the obstacles were dealt with the current and operational experimental setup was used to acquire cations data. This strategy had a surprisingly beneficial effect on the overall course of the project and opened up an new investigation path to an almost unexplored field the unusually strong chemical behaviour of gold atoms in ultracold temperatures. These contributions promoted the advancement to the field since the complexes experimentally detected, notably MAuM+ (M being rare gases, H2, C60 and aromatic molecules), among others, were either never expected to be observed in such low temperatures, only theoretically predicted or never predicted at all. The relevance of these results for other areas of science is immediate, especially for chemists and biochemists, that can now investigate ways to synthesize and verify the stability of the products in room conditions. Once proven stable, chemical and biochemical analysis can be performed in search of useful properties of such compounds.