Georg H. Schmid

Time-Resolved Measurement of Interatomic Coulombic Decay in Ne2

The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in Ne2 is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive Ne+(2p(-1))-Ne+(2p(-1)) state, which is probed with a second pulse, removing a further electron. The yield of coincident Ne+-Ne2+ pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the Ne2(+)(2s(-1)) state to be (150±50)  fs, in agreement with quantum calculations.

Entropic splitter for particle separation.

We present a particle separation mechanism which induces the motion of particles of different sizes in opposite directions. The mechanism is based on the combined action of a driving force and an entropic rectification of the Brownian fluctuations caused by the asymmetric form of the channel along which particles proceed. The entropic splitting effect shown could be controlled upon variation of the geometrical parameters of the channel and could be implemented in narrow channels and microfluidic devices.

Enzymic evidence for peroxisomes in a mutant of Chlorella vulgaris

SummaryIsopycnic sucrose density gradient centrifugation of cell-free extracts of a yellow mutant of Chlorella vulgaris and its green parent strain showed a distribution of catalase and glycollate oxidoreductase activity consistent with their association with a particle/organelle fraction. Gradient centrifugation starting from a pellet of cell-free material resulted in a concentration of enzyme activity in the 1.5 M to 2.0 M sucrose fractions which coincided with a microbody-containing fraction as determined by electron microscopy. The algal glycollate-oxidizing enzyme coupled to oxygen, oxidized both d- and l-lactate and was insensitive to cyanide in vitro, showing it to be similar to that of higher plants. The association of glycollate oxidase together with catalase, with the microbody fraction, may be taken as evidence for the presence of algal peroxisomes in these organisms.

Photooxidation reactions of diphenylcarbazide and their DCMU-sensitivity in thylakoids of the blue-green alga Oscillatoria chalybea.

Thylakoids of Oscillatoria chalybea are able to split water. The Hill reaction of these thylakoids is sensitive to DCMU. Diphenylcarbazide can substitute for water as the electron donor to photosystem II with these fully functioning thylakoids. However, the diphenylcarbazide photooxidation is completely insensitive to 3-(3,4-dichlorophenyl)-N-N′-dimethyl urea (DCMU) at high diphenylcarbazide concentrations. In with Tris-treated Oscillatoria thylakoids the water splitting capacity is lost and diphenylcarbazide restores electron transport through photosystem II as occurs with higher plant chloroplasts. However, also these photoreactions are insensitive to DCMU. If diphenylcarbazide acts in Oscillatoria as an electron donor to photosystem II the result suggests that diphenylcarbazide feeds in its electrons behind the DCMU inhibition site. This in turn indicates that in Oscillatoria the site of inhibition of DCMU is on the donor side of photosystem II.

Further enzymic studies and electron microscopy of the microbodies of a mutant of Chlorella vulgaris

Summary1.The presence of microbodies in the yellow mutant Chlorella vulgaris 211-2-11 h/20 has been demonstrated by means of electron microscopy.2.A microbody preparation, obtained from the algae by cell disruption and subsequent sucrose density gradient centrifugation, contained an enzyme pattern which suggests that the microbodies from glucose-grown cells are of the peroxisomal rather than of the glyoxysomal type. No malate synthetase and no isocitrate lyase were found.3.Microbodies of acetate-grown cells also did not contain isocitrate lyase or malate synthetase activities. In contrast to glucose-grown cells a supernatant fraction of acetate-grown cells exhibited low isocitrate lyase activity, but just as in the case of glucose-grown cells activity of malate synthetase was not detectable even if assayed for with a variety of methods, including the use of 14C-1-acetate.4.In acetate-grown cells malate was the major product of acetate assimilation as evidenced by the characterization of short-term products of 14C-acetate assimilation. Malate was not a primary product of 14C-acetate assimilation in glucose-grown cells.5.Since on the one hand the presence of malate synthetase could not be demonstrated in acetate-grown cells as well as in glucose-grown cells, on the other hand malate was the primary product of 14C-acetate assimilation in acetate-grown cells it must be concluded, either that the enzyme is so sensitive that it does not survive isolation, or that the mutant utilizes a pathway other than the glyoxylate by-pass for acetate assimilation when dependent upon acetate for growth.6.As previously demonstrated (Codd et. al., 1972) the isolated microbodies from glucose-grown cells contain high amounts of catalase. An attempt to localize catalase cytochemically by the diaminobenzidine technique failed: diaminobenzidine plus H2O2 stained the microbodies, the mitochondria, Golgi apparatus, and even the lamellar system of the chlorophyll-free plastid. Staining of both the microbodies and mitochondria was relieved by aminotriazole.

Photoregulation of β-d-glucose oxidase by blue light

Abstract Glucose oxidase, a FAD-enzyme is not inhibited by blue light in contrast to some FMN-enzymes. If the natural coenzyme is substituted by FMN, the enzymatic activity is low. Blue light, however, restores the enzymatic activity of this combination to almost the activity of the native enzyme. The experiments indicate that FMN plus blue light or the excited FMN molecule can substitute for the original coenzyme FAD of glucose oxidase. It is possible to photoregulate the FMN-combination by substances such as KI or NiSO 4 that quench the triplet state of excited flavin molecules.

Multiple Ionization of Free Ubiquitin Molecular Ions in Extreme Ultraviolet Free-Electron Laser Pulses

The fragmentation of free tenfold protonated ubiquitin in intense 70 femtosecond pulses of 90 eV photons from the FLASH facility was investigated. Mass spectrometric investigation of the fragment cations produced after removal of many electrons revealed fragmentation predominantly into immonium ions and related ions, with yields increasing linearly with intensity. Ionization clearly triggers a localized molecular response that occurs before the excitation energy equilibrates. Consistent with this interpretation, the effect is almost unaffected by the charge state, as fragmentation of sixfold deprotonated ubiquitin leads to a very similar fragmentation pattern. Ubiquitin responds to EUV multiphoton ionization as an ensemble of small peptides.

Multiple ionization and fragmentation dynamics of molecular iodine studied in IR-XUV pump-probe experiments

The ionization and fragmentation dynamics of iodine molecules (I(2)) are traced using very intense (∼10(14) W cm(-2)) ultra-short (∼60 fs) light pulses with 87 eV photons of the Free-electron LASer at Hamburg (FLASH) in combination with a synchronized femtosecond optical laser. Within a pump-probe scheme the IR pulse initiates a molecular fragmentation and then, after an adjustable time delay, the system is exposed to an intense FEL pulse. This way we follow the creation of highly-charged molecular fragments as a function of time, and probe the dynamics of multi-photon absorption during the transition from a molecule to individual atoms.

Two-Color Pump-Probe Experiments on Small Quantum Systems at the Free-Electron Laser in Hamburg

Within this thesis, the dynamical response of small quantum systems after the absorption of multiple extreme-ultraviolet (XUV) photons is studied via two-color pump-probe experiments at the free-electron laser (FEL) in Hamburg (FLASH) by employing many-particle recoil-ion momentum spectroscopy. The multi-photon ionization of argon atoms is investigated at a photon energy of 27 eV and FEL intensities of 10^13 - 10^14 W/cm^2. The sequential ionization channel is found to dominate and intermediate resonances are revealed by a delayed infrared (IR) laser pulse. Molecular hydrogen (H2) is studied at a photon energy of 28.2 eV. Dissociation via excited states and fragmentation by sequential two-photon ionization are observed. In addition, it is shown how the ground-state dissociation of H2+ can be used as a tool to determine the temporal overlap between an XUV and IR laser pulse. In the argon dimer, multiple interatomic relaxation processes are triggered by the absorption of several 27-eV-photons. Besides interatomic Coulombic decay (ICD), frustrated triple ionization and charge transfer at crossings of potential energy curves are observed. The lifetime of charge transfer is determined to be (531 +- 136) fs using an XUV-IR pump-probe scheme. The employed reaction microscope is upgraded by an in-line XUV split-delay and focussing optics, which was designed and commissioned as part of this thesis.

Clocking molecular fragmentation of N2 with XUV pump-probe experiments

The dynamics of multi-photon induced fragmentation of N2 has been investigated in XUV-pump/XUV-probe experiments at the Free-Electron Laser in Hamburg (FLASH) by recording the ion kinetic energy release(KER) and angular distributions for various dissociation- and Coulomb-explosion channels as a function of the pump-probe delay-time.