Biomedical subjects

M J J Vrakking

Publications and source records attributed to M J J Vrakking.

9 recordsLinked to original sources

PubMed2006

Control of electron localization in molecular dissociation.

We demonstrated how the subcycle evolution of the electric field of light can be used to control the motion of bound electrons. Results are presented for the dissociative ionization of deuterium molecules (D2 --> D+ + D), where asymmetric ejection of the ionic fragment reveals that light-driven intramolecular electronic motion before dissociation localizes the electron on one of the two D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and provide evidence of its usefulness in controlling reaction dynamics.

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PubMed2005

Single-shot measurement of revival structures in femtosecond laser-induced alignment of molecules.

We present a single-shot detection technique for field-free molecular alignment. The method is based on probing the time-varying birefringence of an aligned sample by use of a chirped probe pulse, thus encoding the dependence of the alignment on time onto the spectral domain. The technique is applied to alignment of O2. The recorded signals are well described by an analytical formula.

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PubMed2005

Response of polyatomic molecules to ultrastrong laser- and ion-induced fields.

The exposure of molecules to short, ultrastrong electric fields leads to multiple ionization and a subsequent Coulomb explosion. We present a comparative study where uracil molecules are exposed to fields generated by high-power laser pulses (tau approximately 75 fs, I > 10(16) W/cm2) or swift highly charged ions (0.5 MeV Xe25+) representing a half-cycle pulse of less than 10 fs duration. Molecular dynamics and structural information contained in the fragmentation pathways can be assessed separately. Despite the similar field strengths large differences in fragment kinetic energies are found which are related to field shape and duration with the aid of molecular dynamics simulations.

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PubMed2004

Observation of large quadrupolar effects in a slow photoelectron imaging experiment.

We have studied nondipolar effects in resonance-enhanced multiphoton ionization of Xe and have observed an azimuthal dependence of the photoelectron angular distribution on a quadrupole resonance, as well as a very large asymmetry with respect to the direction of the laser propagation close to the resonance, which is understood in terms of interference between dipole- and quadrupole-allowed ionization channels. The observed asymmetry in the photoelectron angular distribution provides insight into the ejection of slow photoelectrons near an ionization threshold.

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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.

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PubMed2004

Carrier phase dependence in the ionization of Rydberg atoms by short radio-frequency pulses: a model system for high order harmonic generation.

We report time-resolved electron emission in experiments on ionization of rubidium Rydberg atoms (n=90) by few-cycle radio-frequency (RF) (1-10 MHz) pulses. The electron emission occurs in multiple bursts and strongly depends on the carrier-envelope phase as well as the duration and amplitude of the RF pulses. Remarkably, ionization is observed during a series of cycles with the same amplitude. Even at the low RF frequencies, ionization is not completed in a single cycle. Remixing of the states at the zero crossing of the field is believed to play an essential role. Similarities with the ionization process leading to high order harmonic generation are discussed.

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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↗

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.

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PubMed2002

Photoionization microscopy.

We present the first experimental results of a technique called photoionization microscopy. Photoelectrons ejected in threshold photoionization of Xe are detected in a velocity map imaging apparatus, and interferences between various trajectories by which the electron moves from the atom to the detector are observed. The structure of the interference pattern, which contains the transverse component of the electronic wave function, evolves smoothly with the excess energy above the saddle point. The main observed features are interpreted within the framework of the semiclassical approximation.

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