Synthesis and characterization of ultracold molecular anions

Free helium droplets provide a novel approach to synthesize and characterize molecules and clusters. Nano-sized helium droplets, containing 105 - 108 atoms, constitute the ultimate matrix: They provide a stable temperature of 0.37 K at which they are superfluid. Thus, species that collide with these droplets are readily incorporated; they quickly diffuse to the center of the droplet where they will agglomerate with other dopants. Whereas the environment guarantees an ultralow temperature of the embedded complex via efficient heat transfer, it only minimally perturbs the morphology of the complex. We will apply the technique of embedding complexes into helium nanodroplets to unravel the energetics and dynamics of molecular and cluster anions. In particular, attachment of free, low-energy electrons to DNA bases initiate chemical reactions that are highly bond- and site-selective. The energy of the incoming electron controls which bond will be broken. So far the reactions have been explored on isolated molecules; they display a variety of dissociation channels in the range of 0 to 10 eV, below ionization threshold. Preliminary experiments indicate that the electron-triggered reactions change dramatically when the anion is embedded in a helium droplet. Additional structural information on the product ions will be obtained by applying photoelectron spectroscopy to these ultracold anions. Water cluster anions will be our second model system. Studies of water cluster anions help unravel structural and energetic details of the bulk hydrated electron, and the dynamics following vibrational and electronic excitation. Absorption and photoelectron spectra measured so far on free anions are complex because at least three different isomers are formed, with the electron bound in qualitatively different sites. Temperature and kinetics have a crucial impact on the relative population of different isomers that are formed in the experiment. Monitoring the temperature dependence of the different spectral features has helped in their assignment, but the temperature range that has been covered so far is rather narrow. Assigning spectral features to different states (i.e., dipolar, surface, and interior states) will be made easier when photoelectron spectra are measured on ultracold anions. Moreover, the helium matrix opens the possibility to anneal the neutral precursors of the anions, further helping with the assignment of spectral features.

Large He nanodroplets containing up to several million atoms were doped with fullerenes and most of the times with an additional molecular or atomic dopant. Ions formed via electron collisions were analysed with high precision and signal to noise ratio. Decoration of single fullerene ions or cluster ions of fullerenes with a few atoms and several low-mass molecules was investigated. Shell closures of layers as well as preferred positions on corrugated surfaces can easily be observed as intensity maxima or intensity drops at a certain number of the adsorbed species. The well-known properties of wetting and non-wetting substances on macroscopic surfaces were observed for fullerene surfaces, being ideal model surfaces of less than one nm2 in size. Polar molecules such as water form tiny droplets on the fullerenes whereas apolar molecules such as hydrogen form monolayer films. Effects of the curvature and corrugation of the surface were observed and conclusively interpreted with help of theoretical calculations. In several ways the present experiments represent benchmarks for existing and future calculation of the physisorption of atoms and molecules to graphite, grapheme, nanotubes and fullerenes. The results of this project have relevance for astrophysics and astrochemistry as fullerenes were detected in the interstellar medium in 2010. But also for hydrogen storage physisorption of molecular hydrogen or hydrogen rich molecules to carbon rich surfaces is a hot topic. Throughout the whole project a close collaboration of the experimental group in Innsbruck with theoreticians, both from the same institute but also from Europe and the USA turned out to be key for the detailed understanding of the results. Prof. Echt, the co-applicant of this proposal, spent in total more than one year in Innsbruck and contributed substantially to each of the 12 scientific papers that have been published in peer reviewed journals so far.