Biomedical subjects

J Ullrich

Publications and source records attributed to J Ullrich.

At least 37 records · Page 2Linked to original sources

PubMed2003

Rescattering of ultralow-energy electrons for single ionization of Ne in the tunneling regime.

Electron emission for single ionization of Ne by 25 fs, 1.0 PW/cm(2) laser pulses at 800 nm has been investigated in a kinematically complete experiment using a "reaction microscope." Mapping the complete final state momentum space with high resolution, a distinct local minimum is observed at P(e parallel )=0, where P(e parallel ) is the electron momentum parallel to the laser polarization. Whereas tunneling theory predicts a maximum at zero momentum, our findings are in good agreement with recent semiclassical predictions which were interpreted to be due to "recollision."

Journal Article↗

PubMed2003

Projectile-charge sign dependence of four-particle dynamics in helium double ionization.

Double ionization of helium by 6 MeV proton impact has been explored in a kinematically complete experiment using a "reaction microscope." For the first time, fully differential cross sections for positively charged projectiles have been obtained and compared with data from 2 keV electron impact. The significant differences observed in the angular distribution of the ejected electrons are attributed to the charge sign of the projectile, resulting in different dynamics of the four-particle Coulomb system, which is not considered in the first Born approximation.

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PubMed2003

Three-dimensional imaging of atomic four-body processes.

To understand the physical processes that occur in nature we need to obtain a solid concept about the 'fundamental' forces acting between pairs of elementary particles. It is also necessary to describe the temporal and spatial evolution of many mutually interacting particles under the influence of these forces. This latter step, known as the few-body problem, remains an important unsolved problem in physics. Experiments involving atomic collisions represent a useful testing ground for studying the few-body problem. For the single ionization of a helium atom by charged particle impact, kinematically complete experiments have been performed since 1969 (ref. 7). The theoretical analysis of such experiments was thought to yield a complete picture of the basic features of the collision process, at least for large collision energies. These conclusions are, however, almost exclusively based on studies of restricted electron-emission geometries. Here, we report three-dimensional images of the complete electron emission pattern for the single ionization of helium by the impact of C6+ ions of energy 100 MeV per a.m.u. (a four-body system) and observe features that have not been predicted by any published theoretical model. We propose a higher-order ionization mechanism, involving the interaction between the projectile and the target nucleus, to explain these features.

Journal Article↗

PubMed2002

Coincident fragment detection in strong field photoionization and dissociation of H2.

Electron-ion momentum spectroscopy is used to investigate the correlated electronic and nuclear motion in fragmentation of H2 in 4 x 10(14) W/cm(2), 25 fs laser pulses at 795 nm. Reaction channel dependent photoelectron spectra indicate that besides the main, stepwise H2 ionization H2(+) dissociation mechanism resulting in the products H(1s) + H(+) + e(-) a second new mechanism has to be assumed. The momentum distribution of H(+) ions in the dissociation channels H(1s) + H(+) + e(-) and 2H(+) + 2e(-) is found to be independent of the kinetic energy of the photoelectrons.

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PubMed2002

Simultaneous projectile-target ionization: a novel approach to (e, 2e) experiments on ions.

A kinematically complete experiment for simultaneous ionization of a projectile and target has been performed for 3.6 MeV/u C2+ on He collisions measuring the final vector momenta of the He1+ recoil ion and of two electrons (projectile, target) in coincidence with the emerging C3+ projectile. The feasibility of an event-by-event separation of the various reaction channels, among them the ionization of C2+ by the interaction with a quasifree target electron, is demonstrated in agreement with six-body classical trajectory Monte Carlo calculations, paving the way to kinematically complete electron-ion scattering experiments.

Journal Article↗

PubMed2002

[Evaluation of therapeutic success by patients with anxiety disorders after inpatient psychotherapy].

OBJECTIVE: To identify factors influencing the subjective assessment of patients regarding the outcome of in-patient psychotherapy for anxiety disorders. METHODS: A total of 231 patients were tested at admission, discharge and 1-year follow-up in a multi-center study. Subjective outcome assessment was measured with the revised German version of the Helping Alliance Questionnaire. The use of the term "outcome satisfaction" is discussed. RESULTS: Patient outcome assessment did not correlate with age, sex, or education, or with duration or severity of illness or duration of treatment. Outcome assessment was closely connected with therapy success. Reaching a low symptom level ("clinical significance") had a higher impact than a high pre-post-difference ("statistical significance"). CONCLUSIONS: Anxiety patients give a generally positive assessment of their therapy outcome, which is most determined by symptom-related treatment success. The differentiation of "clinical" and "statistical significance" of success is of importance for the patient's subjective view of the therapy outcome.

Adolescent↗

PubMed2002

Effect of reaction time on PCDD and PCDF formation by de novo synthetic reactions under oxygen deficient and rich atmosphere.

The effect of reaction time on formation of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) was studied under laboratory conditions in the system containing municipal waste incineration fly ash, activated carbon and copper chloride dihydrate at 300 degrees C in 99.999% N2 and N2 + 10% O2 atmosphere. The concentrations of tetra- to octa-chlorinated isomers as well as I-TEQ concentrations of toxic congeners are reported. The mechanism of PCDD and PCDF formation from chlorophenols and chlorinated biphenyls is discussed in the light of the time changes of PCDD/PCDF ratios.

Benzofurans↗

PubMed2001

Three-body Coulomb problem probed by mapping the Bethe surface in ionizing ion-atom collisions.

The three-body Coulomb problem has been explored in kinematically complete experiments on single ionization of helium by 100 MeV/u C(6+) and 3.6 MeV/u Au(53+) impact. Low-energy electron emission ( E(e)<150 eV) as a function of the projectile deflection theta(p) (momentum transfer), i.e., the Bethe surface [15], has been mapped with Delta theta(p)+/-25 nanoradian resolution at extremely large perturbations ( 3.6 MeV/u Au(53+)) where single ionization occurs at impact parameters of typically 10 times the He K-shell radius. The experimental data are not in agreement with state-of-the-art continuum distorted wave-eikonal initial state theory.

Journal Article↗

PubMed2001

Separation of recollision mechanisms in nonsequential strong field double ionization of Ar: the role of excitation tunneling.

Vector momentum distributions of two electrons created in double ionization of Ar by 25 fs, 0.25 PW/cm(2) laser pulses at 795 nm have been measured using a "reaction microscope." At this intensity, where nonsequential ionization dominates, distinct correlation patterns are observed in the two-electron momentum distributions. A kinematical analysis of these spectra within the classical "recollision model" revealed an (e,2e)-like process and excitation with subsequent tunneling of the second electron as two different ionization mechanisms. This allows a qualitative separation of the two mechanisms demonstrating that excitation-tunneling is the dominant contribution to the total double ionization yield.

Journal Article↗

PubMed2001

Double ionization of helium by electron-impact: complete pictures of the four-body breakup dynamics.

The dynamics of He double ionization by 2 keV electron impact is studied experimentally for a momentum transfer of 0.6 a.u. at excess energies of 10 and 40 eV. Complete sets of fivefold differential cross sections are presented for all electron emission angles in coplanar geometry. Contributions beyond the first Born approximation are identified comparing experimental data with first order convergent close-coupling calculations which are in considerably better agreement with the present experiment than with the earlier measurement of Kheifets et al. [J. Phys. B 32, 5047 (1999)].

Journal Article↗

PubMed2000

Electron correlations observed through intensity interferometry

Intensity interferometry was applied to study electron correlations in doubly ionizing ion-atom collisions. In this method, the probability to find two electrons emitted in the same double ionization event with a certain momentum difference is compared to the corresponding probability for two uncorrelated electrons from two independent events. The ratio of both probabilities, the so-called correlation function, is found to sensitively reveal electron correlation effects, but it is rather insensitive to the collision dynamics.

Journal Article↗

PubMed2000

Momentum distributions of ne(n+) ions created by an intense ultrashort laser pulse

Vector momentum distributions of Ne(n+) (n = 1,2,3) ions created by 30 fs, approximately 1 PW/cm(2) laser pulses at 795 nm have been measured using recoil-ion momentum spectroscopy. Distinct maxima along the light polarization axis are observed at 4.0 and 7.5 a.u. for Ne2+ and Ne3+ production, respectively. Hence, mechanisms based on an instantaneous release of two (or more) electrons can be ruled out as a dominant contribution to nonsequential strong-field multiple ionization. The positions of the maxima are in accord with kinematical constraints set by the classical "rescattering model."

Journal Article↗

PubMed2000

Dynamics of rab5 activation in endocytosis and phagocytosis.

Fluid-phase endocytosis is stimulated by H-ras-linked growth factor receptors and this stimulation requires activation of rab5. We utilized a GFP-rab5a:wt fusion protein to monitor GFP-rab5a:wt activation in living fibroblasts and in J774 macrophages. Control CHO cells that expressed GFP-rab5a:wt were cultured in serum-free conditions and showed GFP-rab5a:wt localized to endosomal vesicles with a mean diameter of 0.3 +/- 0.1 microm. Endosome fusion, membrane ruffling, and pinosome formation were rarely detected in these cells. Coexpression of H-ras:G12V, a constitutively active H-ras mutant that activates rab5a, in cells resulted in marked enlargement of labeled endosomes (mean diameter 0.7 +/- 0.2 microm) and large numbers of giant GFP-rab5a:wt-positive endosomes were present. Time-lapse recordings showed abundant fusion among giant labeled endosomes, and membrane ruffling and pinosome formation were commonly observed. Alterations in GFP-rab5a:wt endosome structure and activity in cells expressing H-ras:G12V were linked to rab5a activation because these changes were identical to those found in cells expressing GFP-rab5a:Q79L, a constitutively activated rab5a mutant. Furthermore, cells co-expressing H-ras:G12V and GFP-rab5a:S34N, an inactive rab5a mutant, exhibited no evidence of H-ras:G12V-induced endosome enlargement. To observe changes in endosome structure and activity that directly followed activation of GFP-rab5a:wt, we performed time-lapse recordings of cells cultured overnight in serum-free media after addition of EGF. EGF caused a rapid increase in endosome fusion and in membrane ruffling activity. Membrane ruffling was often associated with GFP-rab5a:wt-positive vesicle (pinosome) formation at the base of membrane ruffles. Endosome and pinosome fusion were common in EGF-stimulated cells. Phagocytosis is also regulated by GFP-rab5a:wt. J774 macrophages that expressed GFP-rab5a:wt showed transiently activation and recruitment of GFP-rab5a:wt to newly formed phagosomes that contained rhodamine-labeled Escherichia coli. These studies show that GFP-rab5a:wt activation results in dynamic alterations in the structure and activity of the early endosomal and early phagosomal elements.

Animals↗

PubMed1999

No evidence for olfactory blocking in honeybee classical conditioning

We use binary odorant compounds to investigate 'blocking' in honeybees which learn to associate an odorant (A-D) with a sucrose reward as the reinforcer (+). 'Blocking' means that learning about a stimulus B is reduced when trained in compound with a stimulus A that has previously been trained alone. Thus, reinforcement of B in these circumstances is not sufficient to induce learning. Such blocking is a frequently observed phenomenon in vertebrate learning and has also recently been reported in honeybee olfactory learning. To explain blocking, current models of conditioning include cognition-like concepts of attention or expectation which, consequently, seem also to apply to honeybees. Here, we first reproduce a blocking-like effect in an experimental design taken from the literature. We identify two confounding variables in that design and experimentally demonstrate their potential to support a blocking-like effect. After eliminating these confounding variables using a series of different training procedures, the blocking-like effect disappeared. Thus, convincing evidence for blocking in honeybee classical conditioning is at present lacking. This casts doubt on the applicability of cognition-like concepts to honeybees.

Journal Article↗

PubMed1997