Detailed molecular structural information of the living state is of enormous significance to the medical and biological communities. Since hydrated biologically active structures are small delicate complex three-dimensional (3D) entities, it is essential to have molecular scale spatial resolution, high contrast, distortionless, direct 3D modalities of visualization of naturally functioning specimens in order to faithfully reveal their full molecular architectures. An x-ray holographic microscope equipped with an x-ray laser as the illuminator would be uniquely capable of providing these images. A quantitative interlocking concordance of physical evidence, that includes (a) the observation of strong enhancement of selected spectral components of several Xeq+ hollow-atom transition arrays (q = 31, 32, 34, 35, 36, 37) radiated axially from confined plasma channels, (b) the measurement of line narrowing that is spectrally correlated with the amplified transitions, (c) evidence for spectral hole-burning in the spontaneous emission, a manifestation of saturated amplification, that corresponds spectrally with the amplified lines, and (d) the detection of an intense narrow (δθx ∼ 0.2 mrad) directed beam of radiation, (1) experimentally demonstrates in the λ ∼ = 2.71–2.93 Å range (ℏωx ∼ = 4230–4570 eV) the operation of a new concept capable of producing the ideal conditions for amplification of multikilovolt x-rays and (2) proves the feasibility of a compact x-ray illuminator that can cost-effectively achieve the mission of biological x-ray microholography. The measurements also (α) establish the property of tunability in the quantum energy over a substantial fraction of the spectral region exhibiting amplification (Δℏωx ∼ 345 eV) and (β) demonstrate the coherence of the x-ray output through the observation of a canonical spatial mode pattern. An analysis of the physical scaling revealed by these results indicates that the capability of the x-ray source potentially includes single-molecule microimaging, the key for the in situ structural analysis of membrane proteins, a cardinal class of drug targets. An estimate of the peak brightness achieved in these initial experiments gives a value of ∼1031–1032 photons s−1 mm−2 mrad−2/(0.1% bandwidth), a magnitude that is ∼107–108-fold higher than presently available synchrotron technology.

Single-pulse measurements of spectral hole burning of Xe(L) 3d → 2p hollow atom transition arrays observed from a self-trapped plasma channel provide new information on the dynamics of saturated amplification in the λ ∼ 2.8-2.9 Å region. The spectral hole burning on transitions in the Xe 34+ and Xe 35+ arrays reaches full suppression of the spontaneous emission and presents a corresponding width hω x ∼ = 60 eV, a value adequate for efficient amplification of multikilovolt x-ray pulses down to a limiting length τ x ∼ 30 as. The depth of the suppression at 2.86 Å indicates that the gain-to-loss ratio is 10. An independent determination of the x-ray pulse energy from damage produced on the surface of a Ti foil in the far field of the source gives a pulse energy of 20-30 µJ, a range that correlates well with the observation of the spectral hole burning and indicates an overall extraction efficiency of ∼10%.

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Arsenic pollution in waters has become a worldwide issue, constituting a severe hazard to whole ecosystems and public health worldwide. Accordingly, it is highly desirable to design high-performance adsorbents for arsenic decontamination. Herein, a feasible strategy is developed for in situ growth of β-FeOOH nanorods (NRs) on a three-dimensional (3D) carbon foam (CF) skeleton via a simple calcination process and subsequent hydrothermal treatment. The as-fabricated 3D β-FeOOH NRs/CF monolith can be innovatively utilized for arsenic remediation from contaminated aqueous systems, accompanied by remarkably high uptake capacity of 103.4 mg/g for arsenite and 172.9 mg/g for arsenate. The superior arsenic uptake performance can be ascribed to abundant active sites and hydroxyl functional groups available as well as efficient mass transfer associated with interconnected hierarchical porous networks. In addition, the as-obtained material exhibits exceptional sorption selectivity toward arsenic over other coexisting anions at high levels, which can be ascribed to strong affinity between active sites and arsenic. More importantly, the free-standing 3D porous monolith not only makes it easy for separation and collection after treatment but also efficiently prevents the undesirable potential release of nanoparticles into aquatic environments while maintaining the outstanding properties of nanometer-scale building blocks. Furthermore, the monolith absorbent is able to be effectively regenerated and reused for five cycles with negligible decrease in arsenic removal. In view of extremely high adsorption capacities, preferable sorption selectivity, satisfactory recyclability, as well as facile separation nature, the obtained 3D β-FeOOH NRs/CF monolith holds a great potential for arsenic decontamination in practical applications.

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The Xe(L) system at λ ∼ 2.9 Å uniformly exhibits all of the canonical attributes of a strongly saturated amplifier on the full ensemble of single-vacancy Xeq+ transition arrays (q = 31, 32, 34, 35, 36) that exhibit gain. The key observables are (1) sharp spectral narrowing, (2) the detection of a narrow directed beam (δθx≅200 µrad), (3) an increase in the amplitude of the emission and the development of an intense output (⩾106 enhancement) and (4) the observation of deep spectral hole-burning on the inhomogeneously broadened spontaneous emission profile. Experimentally determined by two methods, (a) line narrowing and (b) signal enhancement, the observations for several single-vacancy 3d→2p transitions indicate a range of values for the effective small signal (linear) gain constant given by go≅25−100 cm−1. Quantitative analysis shows that this result stands in clear conflict with the corresponding upper bound go≅40−80 cm−1 that is based on available spectroscopic data and estimated with conventional theory. Overall, the observed values deviate substantially from expectations scaled to the spectral density of the measured Xe(L) spontaneous emission profile; they are systematically too high. The most extreme example is the heavily saturated Xe32+ transition at λ = 2.71 Å, a case that fails to reconcile the lower bound of the measured signal strength with the corresponding theoretically predicted maximal value; the former falls above the latter by a factor exceeding 400 giving an enormous gap. Moreover, although saturation is a prominent characteristic of the amplification at λ≅2.71 Å, as demonstrated by spectral hole-burning, the theoretical upper bound of go given for this transition is far too small for saturation to be reached. The Xe31+ transition at λ≅2.93 Å exhibits comparably pronounced anomalous behaviour. This double paradox is resolved with the Ansatz that the amplification is governed principally by the saturated gain gs, not the conventionally described small signal value go. This interpretation is further supported by the observation of deep spectral hole-burning, the signature of strong saturation, that occurs uniformly across the spectrum of the spontaneous emission profile. The effective amplification exhibits an anomalously weak dependence on the spectral density; saturation is the rule, not the exception. A lucid manifestation of the saturation is the recording of spectrally resolved x-ray yields on the Xe31+ array that are sufficiently high to produce gross structural damage to the material in the film plane of the spectrograph. The behaviour of the amplifier can be best described as an explosive supersaturated amplification. The source of this exceptionally strong amplification can be traced to the dynamically enhanced radiative response of the excited Xe hollow atom states located in the clusters that are mode coupled to the plasma waveguide forming the amplifying channel.

double-vacancy states undergo strong amplification in relativistic self-trapped plasma channels on 3d → 2p transitions in the λ = 2.78–2.81 Å region. The 2P3/2 → 2S1/2 component at λ ≅ 2.786 Å exhibits saturated amplification demonstrated by both (1) the observation of spectral hole-burning in the spontaneous emission profile and (2) the correlated enhancement of 3p → 2s cascade transitions (2S1/2 → 2Pj; j = 1/2, 3/2) at λ = 2.558 Å and λ = 2.600 Å. The condition of saturation places a lower limit of ∼1017 W cm−2 on the intensity of the x-ray beam produced by the amplification in the channel. The anomalous strength of the amplification signalled by the saturation mirrors the equivalently anomalous behaviour observed for all 3d → 2p transitions corresponding to single-vacancy Xeq+ arrays (q = 31, 32, 34, 35, 36) that exhibit gain. The conspicuous absence of amplification involving states with double-vacancy configurations suggests the operation of a selective interaction that enhances the production of states. Overall, the generation of double-vacancy states of this genre demonstrates that an excitation rate approaching ∼1 W/atom for ionic species is achievable in self-trapped plasma channels.

Uniform europium-based infinite coordination polymer nanospheres have been successfully fabricated as an effective fluorescence probe for phosphate sensing.