Multielectron transitions induced by neutron impact on helium.

We explore excitation and ionization by neutron impact as a novel tool for the investigation of electron-electron correlations in helium. We present single- and double-ionization spectra calculated in accurate numerical ab initio simulations for incoming neutrons with kinetic energies of up to 150 keV. The resulting electron spectra are found to be fundamentally different from photoionization or charged particle impact due to the intrinsic many-body character of the interaction. In particular, doubly excited resonances that are strongly suppressed in electron or photon impact become prominent. The ratio of double to single ionization is found to differ significantly from those of photon and charged-particle impact.

Attosecond two-photon interferometry for doubly excited states of helium.

We show that the correlation dynamics in coherently excited doubly excited resonances of helium can be followed in real time by two-photon interferometry. This approach promises to map the evolution of the two-electron wave packet onto experimentally easily accessible noncoincident single-electron spectra. We analyze the interferometric signal in terms of a semianalytical model which is validated by a numerical solution of the time-dependent two-electron Schrödinger equation in its full dimensionality.

Observing the ultrafast buildup of a Fano resonance in the time domain.

Although the time-dependent buildup of asymmetric Fano line shapes in absorption spectra has been of great theoretical interest in the past decade, experimental verification of the predictions has been elusive. Here, we report the experimental observation of the emergence of a Fano resonance in the prototype system of helium by interrupting the autoionization process of a correlated two-electron excited state with a strong laser field. The tunable temporal gate between excitation and termination of the resonance allows us to follow the formation of a Fano line shape in time. The agreement with ab initio calculations validates our experimental time-gating technique for addressing an even broader range of topics, such as the emergence of electron correlation, the onset of electron-internuclear coupling, and quasi-particle formation.

Glucose homeostasis and endocrine pancreatic function in patients with chronic pancreatitis before and after surgical therapy.

In a prospective clinical-experimental study, 15 consecutive patients with chronic pancreatitis, operated on because of severe pain, were examined for the effects of a duodenum-preserving resection of the pancreas head on endocrine pancreas function. This was done by means of oral and intravenous glucose tolerance testing before the operation, on the 10th or 11th day postoperatively, and 3 months after the operation. In addition to glucose levels in the peripheral venous blood, levels of insulin, C-peptide, glucagon, and pancreatic polypeptide were determined. As indicated by the k value, glucose tolerance improved postoperatively in 10 patients (66.6%); three patients (19.9%) showed no change, and one patient (6.6%) was worse. Only one patient (6.6%) developed evident diabetes mellitus immediately postoperatively. Pre- and postoperative levels of insulin and C-peptide showed no significant differences. The fasting levels of glucagon were significantly lower postoperatively than before the operation (p < 0.01). The stimulation of pancreatic polypeptide after oral glucose was significantly lower postoperatively (p < 0.01). Duodenum-preserving pancreas head resection does not lead to an impairment of glucose tolerance in the majority of patients; a deterioration was observed only in few cases (13.3%).

Delay in photoemission.

Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of 21 +/- 5 attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100-electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.

Probing electron correlation via attosecond xuv pulses in the two-photon double ionization of helium.

Recent experimental developments of high-intensity, short-pulse extreme ultraviolet light sources are enhancing our ability to study electron-electron correlations. We perform time-dependent calculations to investigate the so-called "sequential" regime (variant Planck's over 2piomega > 54.4 eV) in the two-photon double ionization of helium. We show that attosecond pulses allow us not only to probe but also to induce angular and energy correlations of the emitted electrons. The final momentum distribution reveals regions dominated by the Wannier ridge breakup scenario and by postcollision interaction.

Dissociating sensory and cognitive contributions to visual persistence I. Photoreceptor response duration as a function of flash intensity, adaptation state, and candidate code.

Numerous careful behavioral studies of visual persistence have reported a variety of apparently contradictory effects. Variation of flash intensity has particularly been found to have both direct and inverse effects on subjective duration. This conflict has been addressed by theories which contain both sensory and cognitive components. Depending on the weight given to these components, one obtains theories which emphasize intensity dependence or task dependence. Few comparably detailed physiological studies of persistence exist. To clarify the issues raised by these theories, we examined the responses evoked in the model photoreceptor of the Limulus lateral eye. To explore the role of sensory variables, we manipulated adaptation state and flash intensity. To explore cognitive variables, the durations of the photoreceptor potentials (RPs) evoked in this model system were assessed by a mutually complementary and complete set of candidate sensory codes. Accordingly, sharp microelectrodes were used to record RPs intracellularly from single photoreceptor cells in response to 40-ms flashes whose intensity was varied over at least 3.6 log units. Two light adaptation states were used which differed in sensitivity by 3.5 log units. This model system made it possible to employ stringent objective assessments of data quality which ensured that only cells which had remained stable for several hours contributed to the present data. A variety of code-dependent trends were found: direct, inverse, invariant, and U-shaped trends related flash intensity to RP duration, while adaptation state interacted with some of these trends. Only some of the expectations which had generated this research were qualitatively corroborated and numerous quantitative discrepancies were found between data and theory. While caution is necessary when generalizing from neural responses to perception, these data indicate that two major gaps now exist in this field. First, both task and stimulus variables need to be exhaustively explored in more complete behavioral experiments. The present data make it more likely that sensory models and cognitive models simply address different aspects of the same phenomenon. Second, similarly detailed questions need to be posed to more central neural loci, particularly to those in the various visual cortices.

Dissociating sensory and cognitive contributions to visual persistence II. Photoreceptor integration of flash pairs as a function of interflash interval, intensity, integration site, and candidate code.

Perceived duration can be assessed behaviorally by adjusting the interval between two flashes so that an observer just perceives a certain relation between them. In such studies, the cognitive characteristics of the required relation necessarily interact with the sensory characteristics of the responses evoked by the two flashes. To dissociate the contributions of these two factors, we executed a physiological study which yielded more complete information on the role of each factor in two paradigms which have been used to characterize perceived duration behaviorally, namely the persistence-of-form design and the successive field design. The effect of sensory manipulations have yielded particularly problematic results in these two paradigms because opposite trends were found when intensity was varied. Intracellular recordings were therefore taken from photoreceptor cells exposed to procedural manipulations which match the sensory and cognitive variations employed in behavioral paradigms. The sensory variables of flash intensity, state of adaptation, and flash interval were explored with some completeness. Cognitive factors were assessed in two ways. First, the contribution of the neural site of sensory integration was determined by making a clear distinction between data collected when all stimuli affect the same receptors versus data collected when different stimuli affect different receptors. Second, the consequences of arbitrary choices of candidate code and dependent variable were also explored. When so organized, the physiological data provide a coherent basis for harmonizing apparently contradictory behavioral results because they qualitatively paralleled the behavioral data's complex dependence on intensity and interval. In particular, both direct and inverse dependencies of response duration on intensity exist in both physiology and behavior with the exact nature of the trend depending as much on the cognitive analysis of the neural responses as on their dynamics and energetics. Further, large quantitative differences were found which also were an expression of the different ways in which the two behavioral paradigms affect receptor potentials.

[The effect of duodenum-preserving pancreatic head resection on the endocrine pancreas function in patients with chronic head pancreatitis]. / Der Einfluss der duodenumerhaltenden Pankreaskopfresektion (DPKR) auf die endokrine Pankreasfunktion bei Patienten mit chronischer Kopfpankreatitis.

In a prospective clinical-experimental study, 15 patients with chronic pancreatitis operated consecutively due to severe pain were examined for the effects of a duodenum-preserving resection of the pancreas head on endocrine pancreas function. This was done by means of oral and intravenous glucose tolerance testing before the operation, on the 10th or 11th postoperative day, and three months after the operation. In addition to glucose levels in the peripheral venous blood, levels of insulin, C-peptide, glucagon, somatostatin, and pancreatic polypeptide were determined. As indicated by the k-value, glucose tolerance improved postoperatively in 11 patients; two patients showed no change, and one patient was worse. Only one patient developed evident diabetes mellitus immediately postoperatively. The pre- and postoperative levels of insulin and C-peptide showed no significant differences. The fasting levels of glucagon were significantly lower postoperatively than before the operation (2p less than 0.01). Duodenum-preserving pancreas head resection led to improvement of the glucose tolerance in the majority of patients; a deterioration was observed only in two cases.