Penning spectroscopy and structure of acetylene oligomers in He nanodroplets.

Embedded atoms or molecules in a photoexcited He nanodroplet are well-known to be ionized through inter-atomic relaxation in a Penning process. In this work, we investigate the Penning ionization of acetylene oligomers occurring from the photoexcitation bands of He nanodroplets. In close analogy to conventional Penning electron spectroscopy by thermal atomic collisions, the n = 2 photoexcitation band plays the role of the metastable atomic 1s2s 3,1S He*. This facilitates electron spectroscopy of acetylene aggregates in the sub-Kelvin He environment, providing the following insight into their structure: the molecules in the dopant cluster are loosely bound van der Waals complexes rather than forming covalent compounds. In addition, this work reveals a Penning process stemming from the n = 4 band where charge-transfer from autoionized He in the droplets is known to be the dominant relaxation channel. This allows for excited states of the remnant dopant oligomer Penning-ions to be studied. Hence, we demonstrate Penning ionization electron spectroscopy of doped droplets as an effective technique for investigating dopant oligomers which are easily formed by attachment to the host cluster.

Electron transfer mediated decay of alkali dimers attached to He nanodroplets.

Alkali metal dimers attached to the surface of helium nanodroplets are found to be efficiently doubly ionized by electron transfer mediated decay (ETMD) when photoionizing the helium droplets. This process is evidenced by detecting in coincidence two energetic ions created by Coulomb explosion and one low-kinetic energy electron. The kinetic energy spectra of ions and electrons are reproduced by simple model calculations based on diatomic potential energy curves, and are in agreement with ab initio calculations for the He-Na2 and He-KRb systems. This work demonstrates that ETMD is an important decay channel in heterogeneous nanosystems exposed to ionizing radiation.

Charge Exchange Dominates Long-Range Interatomic Coulombic Decay of Excited Metal-Doped Helium Nanodroplets.

Atoms and molecules attached to rare-gas clusters are ionized by an interatomic autoionization process traditionally termed "Penning ionization" when the host cluster is resonantly excited. Here we analyze this process in the light of the interatomic Coulombic decay (ICD) mechanism, which usually contains a contribution from charge exchange at a short interatomic distance and one from virtual photon transfer at a large interatomic distance. For helium (He) nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range and short-range contributions to the interatomic autoionization can be clearly distinguished by detecting electrons and ions in coincidence. Surprisingly, ab initio calculations show that even for alkali metal atoms floating in dimples at a large distance from the nanodroplet surface, autoionization is largely dominated by charge-exchange ICD. Furthermore, the measured electron spectra manifest the ultrafast internal relaxation of the droplet mainly into the 1s2s1S state and partially into the metastable 1s2s3S state.

Real-Time Dynamics of the Formation of Hydrated Electrons upon Irradiation of Water Clusters with Extreme Ultraviolet Light.

Free electrons in a polar liquid can form a bound state via interaction with the molecular environment. This so-called hydrated electron state in water is of fundamental importance, e.g., in cellular biology or radiation chemistry. Hydrated electrons are highly reactive radicals that can either directly interact with DNA or enzymes, or form highly excited hydrogen (H^{*}) after being captured by protons. Here, we investigate the formation of the hydrated electron in real-time employing extreme ultraviolet femtosecond pulses from a free electron laser, in this way observing the initial steps of the hydration process. Using time-resolved photoelectron spectroscopy we find formation timescales in the low picosecond range and resolve the prominent dynamics of forming excited hydrogen states.

Inelastic scattering of photoelectrons from He nanodroplets.

We present a detailed study of inelastic energy-loss collisions of photoelectrons emitted from He nanodroplets by tunable extreme ultraviolet (XUV) radiation. Using coincidence imaging detection of electrons and ions, we probe the lowest He droplet excited states up to the electron impact ionization threshold. We find significant signal contributions from photoelectrons emitted from free He atoms accompanying the He nanodroplet beam. Furthermore, signal contributions from photoionization and electron impact excitation/ionization occurring in pairs of nearest-neighbor atoms in the He droplets are detected. This work highlights the importance of inelastic electron scattering in the interaction of nanoparticles with XUV radiation.

Penning Ionization of Acene Molecules by Helium Nanodroplets.

Acene molecules (anthracene, tetracene, pentacene) and fullerene (C60) are embedded in He nanodroplets (HeN) and probed by EUV synchrotron radiation. When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction HeN* + M → HeN + M+ + e-. However, the Penning electron spectra are all broad and structureless, largely differing from those measured by binary Penning collisions, as well as from those measured for dopants bound to the He droplet surface. Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra only when assuming unexpectedly large He droplets, indicating that electron spectra of molecules embedded in helium nanodroplets are severely affected by collective electron-helium interactions.