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Biomedical subjects

H G Muller

Publications and source records attributed to H G Muller.

10 recordsLinked to original sources
PubMed2004

Optimization of attosecond pulse generation.

The generation of attosecond pulses by superposition of high harmonics relies on their synchronization in the emission. Our experiments in the low-order, plateau, and cutoff regions of the spectrum reveal different regimes in the electron dynamics determining the synchronization quality. The shortest pulses are obtained by combining a spectral filtering of harmonics from the end of the plateau and the cutoff, and a far-field spatial filtering that selects a single electron quantum path contribution to the emission. This method applies to isolated pulses as well as pulse trains.

Journal Article↗
PubMed2004

Control of atomic ionization by two-color few-cycle pulses.

We have studied the ionization of Rydberg atoms by few-cycle radio-frequency pulses and used two-color fields to control the ionization dynamics. We show that the number of times that electrons are emitted during a pulse can be limited and that the duration of the electron emission can be shortened. These results, once they are transposed to the optical domain, may inspire new strategies for the production of single attosecond pulses.

Journal Article↗
PubMed2003

Attosecond synchronization of high-harmonic soft x-rays.

Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.

Journal Article↗
PubMed2003

Attosecond angle-resolved photoelectron spectroscopy.

We report experiments on the characterization of a train of attosecond pulses obtained by high-harmonic generation, using mixed-color (XUV+IR) atomic two-photon ionization and electron detection on a velocity map imaging detector. We demonstrate that the relative phase of the harmonics is encoded both in the photoelectron yield and the angular distribution as a function of XUV-IR time delay, thus making the technique suitable for the detection of single attosecond pulses. The timing of the attosecond pulse with respect to the field oscillation of the driving laser critically depends on the target gas used to generate the harmonics.

Photons↗
PubMed2003

Designing refoldable model molecules.

We report a numerical study of the design of lattice heteropolymers that can refold when the properties of only a few monomers are changed. If we assume that the effect of an external agent on a heteropolymer is to alter the interactions between its constituent monomers, our simulations provide a description of a simple allosteric transition. We characterize the free energy surfaces of the initial and the modified chain molecule. We find that there is a region of conformation space where molecules can be made to refold with minimal free energy cost. This region is accessible by thermal fluctuations. The efficiency of a motor based on such an allosteric transition would be enhanced by "borrowing" heat from the environment in the initial stages of the refolding, and "paying back" later. In fact, the power cycle of many real molecular motors does involve such a borrowing activation step.

Journal Article↗
PubMed2002

Rydberg state ionization by half-cycle-pulse excitation: strong kicks create slow electrons.

The asymptotic velocity distribution of electrons ionized in half-cycle-pulse excitation of high Rydberg states (n=34), placed in a static electric field, is studied using electron velocity-map imaging. At weak half-cycle pulse strengths, the electrons escape over the saddle point in the potential. For strong half-cycle pulses, the electrons are emitted in the direction of the field kick. The much slower and less intense half cycle of opposite polarity, which necessarily follows the main half-cycle pulse, strongly affects the momentum distribution and reduces the excess energy of the electrons significantly.

Journal Article↗
PubMed2001

Observation of a train of attosecond pulses from high harmonic generation.

In principle, the temporal beating of superposed high harmonics obtained by focusing a femtosecond laser pulse in a gas jet can produce a train of very short intensity spikes, depending on the relative phases of the harmonics. We present a method to measure such phases through two-photon, two-color photoionization. We found that the harmonics are locked in phase and form a train of 250-attosecond pulses in the time domain. Harmonic generation may be a promising source for attosecond time-resolved measurements.

Journal Article↗