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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.

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The stability against small azimuthal petturbatians of canfined modes of propagation of intense short-pulse radiation govemed by relativistic and chxge-displacement nonlinearities in underdense plasmas is examined theoretically. In the plane of the dimensionless parameters PO zs ~Q % , Q / c and II PQ/P,,, defined by the critical power (Per) and the initial conditions represented by the focal radius (ro) of the incident radiation, the unperturbed plasma frequency (w,,o), and the peak incident power (Po), zones corresponding to stable (single-channel) and unstable (strong filamentation) regimes of propagation are established. The genera! finding is that large regions of stable propagation exist The results show that for values of w sufficiently close to the dimensionless radius of the zeroth dpnmcde I)E,Q. the self-channelling is stable for all values of 7 =-1, a condition of exceptional robustness. I1 is also found lhat for the region 1 c q 5 IO, the propagation is stable for a very wide range of values of W . In addition, the location of the bounday separating the stable and unstable zones is largely independent of the curyamre of the phase front of the incident wave and weakly influenced both by the magniludes of the azimuthal permrbations'and the detailed radial profile of the incident radiation. Since selffocusing generated solely by relativistic mechanisms ten& strongly to unstable behaviour in the 7 >> 1 regime, these results demonstrate the crucial role of the ponderomotively driven charge displacement in stabilizing the propagation. Physically, the ponderomotive radial displacement of the electrons and the wnuaSting inertial confinement of the ions simultaneously produce the two chief characteristics of the channels. l l e y are the refractive self-foeusing ofthe propagating energy arising from the displaced electrons and the spatial stability of the channels produced by the immobile electrostatic spine farmed by the ions

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%.

Studies of multiphoton-induced X-ray generation in Kr and Xe clusters give direct information concerning (1) the atom-specific energy transfer rate, (2) the dependence of the X-ray yield on the strength of the intra-cluster inelastic electron scattering cross section, and (3) the threshold intensity for X-ray generation. Measurements of these three classes of observables with subpicosecond ( approximately 300 fs) ultraviolet (248 nm) radiation at a maximum intensity of approximately 1019 W cm-2 all indicate that the non-linear coupling to the cluster has an anomalous strength with respect to that derivable from conventional single-particle interactions. Five cases have been examined (Kr(M), Kr(L), Xe(N), Xe(M) and Xe(L)), spectrally spanning the range from approximately 80 eV to approximately 5 keV. In order to reconcile theoretical estimates with these experimental findings, three generalizations of the original formulation of the interaction were necessary. The most important involves an enhancement in the coupling arising from the coherent motion of the (Z) field-ionized electrons induced by the external driving field. The coherently energized electrons act like a quasi-particle possessing a charge Ze and a mass Zm, thereby presenting a sharply augmented coupling resembling that associated with energetic ion-atom collisions. The second involves the process of multiple electron ejection from an inner-shell, a mechanism that was found imperative to interpret the high level of ionization observed in the Xe M-shell. The third modification concerns multiple transits of the driven electrons in the cluster. The inclusion of these considerations consistently brings the theoretical analysis into agreement with the three measured properties. These results also indicate that energy deposition rates exceeding approximately 1 W/atom are feasible in appropriately designed molecules incorporating heavy atoms. The limiting magnitude of the excitation energy Delta Emax characteristic of the coherent coupling is estimated to be in the range Zamc2