Interface Excitons in Krmnen Clusters : The Role of Electron Affinity in the Formation of Electronic Structure

The formation of the electronic structure of small Kr_m clusters (m<150) embedded inside Ne_N clusters (1200<N<7500) has been investigated with the help of fluorescence excitation spectroscopy using synchrotron radiation. Electronically excited states, assigned to excitons at the Ne/Kr interface, 1i and 1'i were observed. The absorption bands, which are related to the lowest spin-orbit split atomic Kr 3P1 and 1P1 states, initially appear and shift towards lower energy when the krypton cluster size m increases. The characteristic bulk 1t and 1't excitons appear in the spectra, when the cluster radius exceeds some critical value, R_cl>Delta_1i . Kr clusters comprising up to 70 atoms do not exhibit bulk absorption bands. We suggest that this is due to the penetration of the interface excitons into the Kr_m cluster volume, because of the negative electron affinity of surrounding Ne atoms. From the energy shift of the interface absorption bands with cluster size an unexpectedly large penetration depth of delta_1i =7.0+/-0.1 A is estimated, which can be explained by the interplay between the electron affinities of the guest and the host cluster

Experimental Observation of Resonance Effects in Intensely Irradiated Atomic Clusters

We have resolved the expansion of intensely irradiated atomic clusters on a femtosecond time scale. These data show evidence for resonant heating, similar to resonance absorption, in spherical cluster plasmas

Transitory Microbial Habitat in the Hyperarid Atacama Desert

Traces of life are nearly ubiquitous on Earth. However, a central unresolved question is whether these traces always indicate an active microbial community or whether, in extreme environments, such as hyperarid deserts, they instead reflect just dormant or dead cells. Although microbial biomass and diversity decrease with increasing aridity in the Atacama Desert, we provide multiple lines of evidence for the presence of an at times metabolically active, microbial community in one of the driest places on Earth. We base this observation on four major lines of evidence: a physico-chemical characterization of the soil habitability after an exceptional rain event, identified biomolecules indicative of potentially active cells [e.g., presence of ATP, phospholipid fatty acids (PLFAs), metabolites, and enzymatic activity], measurements of in situ replication rates of genomes of uncultivated bacteria reconstructed from selected samples, and microbial community patterns specific to soil parameters and depths. We infer that the microbial populations have undergone selection and adaptation in response to their specific soil microenvironment and in particular to the degree of aridity. Collectively, our results highlight that even the hyperarid Atacama Desert can provide a habitable environment for microorganisms that allows them to become metabolically active following an episodic increase in moisture and that once it decreases, so does the activity of the microbiota. These results have implications for the prospect of life on other planets such as Mars, which has transitioned from an earlier wetter environment to today's extreme hyperaridity. [Abstract copyright: Copyright © 2018 the Author(s). Published by PNAS.

Temperature limits to deep subseafloor life in the Nankai Trough subduction zone

No embargo required.Microorganisms in marine subsurface sediments substantially contribute to global biomass. Sediments warmer than 40°C account for roughly half the marine sediment volume, but the processes mediated by microbial populations in these hard-to-access environments are poorly understood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hot sediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetative cells drop two orders of magnitude and endospores become more than 6000 times more abundant than vegetative cells. Methane is biologically produced and oxidized until sediments reach 80° to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrations demonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zones alternate with zones up to 192 meters thick where microbes were undetectable.</jats:p

Evolution of Electronic Energy Levels in Krypton Clusters from the Atom to the Solid

We deduce the evolution of electronic excitations in Kr clusters (N=2−3000) from fluorescence excitation spectroscopy with synchrotron radiation. In small clusters (N≤30), a broad absorption band appears, slightly red shifted compared with the atomic 5s($\dfrac {3}{2})_1$ resonance line. Intermediate-size clusters (50$<$N$≤$200) show absorption profiles which are mainly correlated with the surface excitons of the solid. The bulk excitons are pronounced only in large clusters (N≥200). Absorption processes at the surface and inside the cluster correlate well to the surface-to-volume ratio of the cluster

On the nature of bulk and surface excitations in Argon clusters

Absorption spectra of Ar clusters containing between 10 and 10$^6$ atoms are dominated by strong transitions into bulk and surface states. The intensity variation of bulk and surface excitations is analyzed within a model, which divides the cluster into a surface layer and into an interior part. The thickness of the surface layer is determined by the intensity ratio of bulk and surface excitations. For then=2, 2′ excitons a reasonable value ranging between the radius of then=2 exciton and the nearest neighbour distance is obtained. In case of then=1 excitons the thickness of the surface layer is much smaller than the nearest-neighbour distance indicating that then=1 surface excitons might be interpreted as excitations of atoms on the cluster surface

Fluorescence excitation spectroscopy of xenon clusters in the VUV

Fluorescence excitation spectra of xenon clusters with an average cluster size N up to 600 atoms/cluster have been recorded in the VUV spectral range from 100 to 155 nm. Below the ionization limit strong absorption bands corresponding for small clusters to molecular and for large clusters to excitonic bands of the solid are observed. Above ≈ 9.5 eV a broad continuum appears for clusters larger than approximately 20 atoms/cluster which is related to the formation of the conduction band

Evolution of excitonic energy levels in ArN\mathrm{Ar_N} clusters: Confinement of bulk, surface, and deep valence shell excitons

The evolution of excitonic energy levels (Wannier and Frenkel type) is investigated for ArN clusters in the range N=200–106 using fluorescence excitation spectroscopy. In the case of Wannier excitons, a pronounced blue shift of the absorption bands relative to the position in the infinite solid is observed. As a consequence of the lower dimensionality, the shift of the transition energy of surface excitons is considerably smaller than that of the bulk states of clusters. The evolution with size is discussed within several theoretical models for exciton confinement. In addition, model calculations are performed for bulk excitons which give good quantitative agreement with the experimental results. In the case of n=1 Frenkel or intermediate type excitons, there are blue and red shifts observed. The spectral shift of (3p→4s) and deep valence (3s→4p) excitations differs considerably. From the shift of the transition energies the exciton mass of the (3p→4s) exciton is derived

Observation of cluster-specific excitations in XeN\mathrm{Xe_N} clusters

The absorption spectra of Xe$_N$ clusters (N=50–500) exhibit extra absorption bands not seen in condensed Xe gas. They show up if the cluster radius is comparable to the radius of the electronic excitation. From the sharpness of the bands and from the energy-level positions it is concluded that these excitations have a character which combines features seen in molecular Rydberg states and excitons of the solid. This interpretation is supported by simple model calculations. For small clusters (N≤150) the number of extra absorption bands reflects the shell structure of the clusters