James W. Longworth

Photosensitization of aqueous model systems by hypericin

Absorption and fluorescence measurements of purified hypericin (HY) were made in various media. Photosensitization of two aqueous systems was investigated: resealed red blood cell membranes (ghosts) and hen lysozyme (Lys). Solubilization of HY by ghost membranes was shown by means of diffuse reflectance spectroscopy. Visible light irradiation of the ghosts incorporating HY led to lipid peroxidation with evidence of singlet oxygen involvement. A binding model applicable for insoluble ligands is indicative of strong HY binding to HSA. The HY-HSA complex photosensitized inactivation of Lys. The pseudo-first-order reaction kinetics with protection by azide ion are consistent with a Type II mechanism mediated by singlet oxygen. The results are discussed in the context of the HY photodynamic and antiretroviral activities.

Dynamical orbital collapse drives super x-ray emission

Experimental studies of the characteristics of Xe(M) emission ( - 19 A) produced by multiphoton excitation of Xe clusters indicate that the nonlinear interaction automatically acts as a template leading to the preparation of the maximally radiating configurations for that spectral range. The new mechanism deduced is a general multiphoton multi-electron process which dynamically combines rapid multiphoton ionization, 4f-orbital collapse, and correlated electron motion.

An efficient, selective collisional ejection mechanism for inner-shell population inversion in laser-driven plasmas

A theoretical analysis of laser-driven collisional ejection of inner-shell electrons is presented to explain the previously observed anomalous kilovolt L-shell x-ray emission spectra from atomic Xe cluster targets excited by intense sub-picosecond 248nrn ultraviolet radiation. For incident ponderomotively-driven electrons photoionized by strong above threshold ionization, the collisional ejection mechanism is shown to be highly l-state and significantly n-state (i.e. radially) selective for time periods shorter than the collisional dephasing time of the photoionized electronic wavefunction. The resulting preference for the collisional ejection of 2p electrons by an ionized 4p state produces the measured anomalous Xe(L) emission which contains direct evidence for (i) the generation of Xe{sup 27+}(2p{sup 5}3d{sup 10}) and Xe{sup 28+}(2p{sup 5}3d{sup 9}) ions exhibiting inner-shell population inversion and (ii) a coherent correlated electron state collision responsible for the production of double 2p vacancies. For longer time periods, the selectivity of this coherent impact ionization mechanism is rapidly reduced by the combined effects of intrinsic quantum mechanical spreading and dephasing--in agreement with the experimentally observed and extremely strong {minus}{lambda}{sup {minus}6} pump-laser wavelength dependence of the efficiency of inner-shell (2p) vacancy production in Xe clusters excited in underdense plasmas.

Pump laser wavelength-dependent control of the efficiency of kilovolt x-ray emission from atomic clusters

An explanation is presented for the recently reported striking differences in the kilovolt Xe L-shell x-ray emission from Xe cluster targets excited by comparable terawatt ultraviolet (248 nm) and infrared (800 nm) femtosecond laser pulses under nearly identical experimental conditions (Kondo K et al 1997 J. Phys. B.: At. Mol. Opt. Phys. 30 2707-16). A classical analysis of these results, within the framework of the first Born approximation for electron-atom collisions producing inner-shell ionization, strongly suggests that both the times stronger Xe(L) emission under ultraviolet laser excitation and the observed differences in the x-ray spectra are caused primarily by the different ultraviolet and infrared pump laser wavelengths. The kinematics of photoionized electrons in the intense laser fields (-) and the Coulomb-driven expansion of the electron distribution photoionized from the atomic cluster both indicate that the strong pump-laser wavelength scaling in the production of kilovolt x-rays from Xe clusters results from the more localized and controlled electron-cluster interactions afforded by a shorter optical period.

Bifurcation mode of relativistic and charge-displacement self-channelling

Stable self-channeling of ultra-powerful (P{sub 0} - 1 TW -1 PW) laser pulses in dense plasmas is a key process for many applications requiring the controlled compression of power at high levels. Theoretical computations predict that the transition zone between the stable and highly unstable regimes of relativistic/charge-displacement self-channeling is well characterized by a form of weakly unstable behavior that involves bifurcation of the propagating energy into two powerful channels. Recent observations of channel instability with femtosecond 248 nm pulses reveal a mode of bifurcation that corresponds well to these theoretical predictions. It is further experimentally shown that the use of a suitable longitudinal gradient in the plasma density can eliminate this unstable behavior and restore the efficient formation of stable channels.

Dynamics of optimized stable channel formation of intense laser pulses with the relativistic/charge-displacement mechanism

Studies of the dynamics of stable relativistic/ponderomotive channel formation demonstrate that the use of an appropriate longitudinal gradient in the electron density can significantly enhance the efficiency of the power compression. A unidirectional stable zone locking rule, which allows the operating point of the system to enter the region of stable channelled propagation, but blocks departures from it, is established. These characteristics are extremely favourable for kilovolt x-ray amplification, charged-particle acceleration and the initiation of nuclear reactions.

Power scaling of the Xe(L) amplifier at λ~ 2.8 Å into the petawatt regime

Single-pulse and time-integrated spectral measurements of the characteristics of the Xe(L) amplifier at λ ~ 2.8 A indicate an efficiency of energy extraction of ~30% over a bandwidth of ~500 eV. These observations, together with data from prior studies, provide a basis for estimating a corresponding set of scaling limits for a laboratory sized ~4.5 keV Xe(L) system. Specifically, they are a peak power Px ~ 6.0 PW, an unfocused peak intensity Ix ~ 3.4 × 1021 W cm−2, peak brightness figures corresponding to B ~ 4.1 × 1034 photons s−1 mm−2 mrad−2 (0.1% bandwidth)−1 and Px/λ2 ~ 7.6 × 1030 W cm−2 sr−1, and an x-ray pulse length τx ~ 5–10 as.

Measurement of 160-fs, 248-nm pulses by two-photon fluorescence in fused-silica crystals.

Measurements of 160-fs, 248-nm ultrashort pulses are obtained through a two-photon fluorescence measurement based on the two-photon-induced color-center fluorescence in fused-silica crystals. The method proved to be reliable and advantageous in comparison with two-photon fluorescence techniques employing other materials, both solid state and gaseous.

High-resolution, flat-field, plane-grating, f /10 spectrograph with off-axis parabolic mirrors

A high-resolution, flat-field, plane-grating, f/10 spectrometer based on the novel design proposed by Gil and Simon [Appl. Opt. 22, 152 (1983)] is demonstrated. The spectrometer design employs off-axis parabolic collimation and camera mirrors in a configuration that eliminates spherical aberrations and minimizes astigmatism, coma, and field curvature in the image plane. In accordance with theoretical analysis, the performance of this spectrometer achieves a high spatial resolution over the large detection area, which is shown to be limited only by the quality of its optics and their proper alignment within the spatial resolution of a 13 microm x 13 microm pixelated CCD detector. With a 1500 lines/mm grating in first order, the measured spectral resolving power of lambda/Dlambda = 2.5(+/-0.5) x 10(4) allows the clear resolution of the violet Ar(I) doublet at 419.07 and 419.10 nm.

Spatially resolved observation of the spectral hole burning in the Xe(L) amplifier on single and double vacancy 3d → 2p transitions in the 2.62 Å < λ < 2.94 Å range

The analysis of spatially resolved Xe(L) spectra obtained with Z−λ imaging reveals two prominent findings concerning the characteristics of the x-ray amplification occurring in self-trapped plasma channels formed by the focusing of multi-TW subpicosecond 248 nm laser pulses into a high-density gaseous Xe cluster target. They are (1) strongly saturated amplification across both lobes of the Xe(L) hollow atom 3d → 2p emission profile, a breadth that spans a spectral width of ~600 eV, and (2) new evidence for the formation of x-ray spatial modes based on the signature of the transversely observed emission from the narrow trapped zone of the channel. The global characteristics of the spectral measurements, in concert with prior analyses of the strength of the amplification, indicate that the enhancement of the x-ray emission rate by intra-cluster superradiant dynamics plays a leading role in the amplification. This radiative interaction simultaneously promotes (a) a sharp boost in the effective gain, (b) the directly consequent efficient production of coherent Xe(L) x-rays from both single and double vacancy 3d → 2p transition arrays, estimated herein at ~30%, and (c) the development of a very short x-ray pulse width τx. In the limit of sufficiently strong superradiant coupling in the cluster, the system assumes a dynamically collective character and acts as a single homogeneously broadened transition whose effective radiative width approaches the full Xe(L) bandwidth, a breadth that establishes a potential lower limit of τx ~5–10 as, a value substantially less than the canonical atomic time ao/αc 24 as.