Alex B Borisov

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.

Wavelength dependence of multiphoton-induced Xe(M) and Xe(L) emissions from Xe clusters

A direct comparative measurement of the dependence on the wavelength of irradiation of the kilovolt x-ray yields ( and ) multiphoton-induced from Xe clusters by excitation with intense femtosecond pulses at 248 and 800 nm has been made. The spectroscopic findings demonstrate that both the Xe(M) and Xe(L) emissions are strongly reduced with excitation at the longer wavelength (800 nm). The peak strengths of the Xe(M) and Xe(L) emissions are diminished by factors of and , respectively. Significant spectral differences are also observed. This sharp reduction in the amplitude of the excitation is in conflict with a thermal model for the production of kilovolt x-rays (Xe(M) and Xe(L)) from multiphoton 248 nm excited Xe clusters. These results are consistent with a dynamical mechanism of enhanced coupling which involves ordered many-electron motions in which a dephasing of the electrons can appreciably influence both the amplitude of excitation and the threshold intensity for inner-shell vacancy production. Within the framework of this picture, these experimental findings indicate an effective dephasing time for Xe clusters of - 2 fs, a range that is consistent with the measured k-space scattering dynamics of carriers in GaAs.

Z - imaging of Xe(M) and Xe(L) emissions from channelled propagation of intense femtosecond 248 nm pulses in a Xe cluster target

The spatial image of the spectrum of Xe(M) and Xe(L) emissions produced by the development of stable channelled propagation of intense femtosecond ( fs) 248 nm pulses propagating through a gas-phase Xe cluster target has been obtained. Analysis (i) shows that the observed channel length exceeds 20 Rayleigh ranges, (ii) reveals enhanced emission from the channel by very highly excited species such as Xe ions having double 2p-vacancies A), (iii) gives spectral evidence for a substantial departure of the excitation from conditions of thermal equilibrium in the channel, (iv) confirms earlier studies which indicated the development of intensities of in the channel, and (v) provides additional evidence for the role of an anomolously strong multiphoton coupling in the generation of the observed X-ray emission.

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.

Intensity dependence of the multiphoton-induced Xe(L) spectrum produced by subpicosecond 248 nm excitation of Xe clusters

Examination of the intensity dependence in the range of the multiphoton-induced Xe(L) spectrum from Xe clusters has demonstrated (i) a very high sensitivity for the production of ions having multiple vacancies and and (ii) a reduction of the average charge state of the emitting ions over the range of intensities in which the multiple-vacancy excitations are diminished. These results give evidence for a dynamical enhancement of the coupling strength arising from ordered many-electron motions induced by the incident 248 nm wave and indicate that the enhanced interaction can lead to a regime of strong coupling in which multiple electron ejection from inner shells can occur with substantial probability. Furthermore, the results are found to be in agreement with previous analyses of multiple electron processes, which involved comparisons with experimental data stemming from both electron (EBIT) and ion - atom collisional studies. The specific comparison with known properties of ion - atom ( + Ne) charge exchange collisions indicates that the multi-electron channels can dominate the multiphoton interaction if a threshold intensity is exceeded. The experimental results give evidence that this threshold falls in the 0.4 - range at 248 nm for the production of 2p vacancies in Xe clusters. These findings provide an experimental basis for the general expectation that a strengthened multi-electron coupling will govern the production of ions with deeply bound (>10 - 20 keV) multiple-vacancy states in clusters of heavy atoms irradiated at 248 nm with intensities above .

Explosive supersaturated amplification on 3d→2p Xe(L) hollow atom transitions at λ ∼ 2.7-2.9 Å

The Xe(L) system at ? ~ 2.9 ? 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 (??x200 ?rad), (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 go25?100 cm?1. Quantitative analysis shows that this result stands in clear conflict with the corresponding upper bound go40?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 ? = 2.71 ?, 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 ?2.71 ?, 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 ?2.93 ? 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.

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.

Realization of the conceptual ideal for x-ray amplification

The Xe(L) system is an amplifier with fundamentally different dynamic characteristics from all previously developed laser amplifiers; it represents the conceptual ideal through full utilization of the Kramers–Kronig relations that fundamentally couple the dispersive and absorptive components. The dispersive response of the system, through optimal governance of the power compression, rules the amplification and establishes a minimum gain for the amplifier. Accordingly, the amplification requires a minimum value of the dispersion to be surpassed; the corresponding gain follows automatically. As a leading consequence, since this minimum gain is sufficiently high, the key experimental observation is the uniform presence of saturated amplification signaled by strong spectral hole burning on all transitions exhibiting amplification, including double-vacancy lines. This cardinal signature demonstrates that the amplification is legislated by the saturated gain gs, not the corresponding small signal value g0. The chief outcome is that explosive dispersion yields perforce explosive amplification and the efficient generation of maximally bright coherent power.

Double optimization of Xe(L) amplifier power scaling at λ ~ 2.9 Å

The spectral and spatial characteristics of the Xe(L) amplifier at ? ~ 2.9 ? determine an optimum for the scaling of the peak power with channel length. The Xe31+ and Xe32+ (3d ? 2p) transition arrays represent two identical spectral optima for amplification, a property stemming from the extremum of spectral components (3245) characteristic of their electron configurations. Adroit matching of the spatial distribution of the intensity characteristic of the propagating 248 nm pulse dynamically generating the self-trapped plasma channel with the intensity required to excite selectively and efficiently the Xe31+ and Xe32+ arrays can also simultaneously maximize the spatial volume of the excitation. The net outcome of this double maximization is an amplifying channel for the optimal transitions that possesses high gain (~100 cm?1), low losses (<10?1cm?1) and a diameter of 15?20 ?m, a size sufficient to produce an x-ray pulse energy of ~50?100 mJ from a channel having an average xenon density of ~1020 cm?3 and a length of 1 cm. Since previous studies have experimentally demonstrated the ability to produce a saturated bandwidth of ~60 eV, a magnitude sufficient to support a pulse duration of ~30 as, peak powers Px 1 PW are clearly within the scaling limits of the Xe(L) system. The corresponding peak brightness scaling limit is accordingly bounded from below by Px/?2 1030 W cm?2 sr?1.