Showing papers by "Marc J. J. Vrakking published in 2009"

Impulsive orientation and alignment of quantum-state-selected NO molecules

Abstract: A technique that produces significant alignment of molecules in a beam should aid a wide range of experiments geared towards understanding and controlling molecular processes in the gas phase. Manipulation of the molecular-axis distribution is an important ingredient in experiments aimed at understanding and controlling molecular processes1,2,3,4,5,6. Samples of aligned or oriented molecules can be obtained following the interaction with an intense laser field7,8,9, enabling experiments in the molecular rather than the laboratory frame10,11,12. However, the degree of impulsive molecular orientation and alignment that can be achieved using a single laser field is limited13 and crucially depends on the initial states, which are thermally populated. Here we report the successful demonstration of a new technique for laser-field-free orientation and alignment of molecules that combines an electrostatic field, non-resonant femtosecond laser excitation14 and the preparation of state-selected molecules using a hexapole2. As a unique quantum-mechanical wavepacket is formed, a large degree of orientation and alignment is observed both during and after the femtosecond laser pulse, which is even further increased (to 〈cosθ〉=−0.74 and 〈cos2θ〉=0.82, respectively) by tailoring the shape of the femtosecond laser pulse. This work should enable new applications such as the study of reaction dynamics or collision experiments in the molecular frame, and orbital tomography11 of heteronuclear molecules.

Attosecond control of electron dynamics in carbon monoxide.

Abstract: Laser pulses with stable electric field waveforms establish the opportunity to achieve coherent control on attosecond time scales. We present experimental and theoretical results on the steering of electronic motion in a multielectron system. A very high degree of light-waveform control over the directional emission of ${\mathrm{C}}^{+}$ and ${\mathrm{O}}^{+}$ fragments from the dissociative ionization of CO was observed. Ab initio based model calculations reveal contributions to the control related to the ionization and laser-induced population transfer between excited electronic states of ${\mathrm{CO}}^{+}$ during dissociation.

Velocity map imaging using an in-vacuum pixel detector

Abstract: The use of a new type in-vacuum pixel detector in velocity map imaging (VMI) is introduced The Medipix2 and Timepix semiconductor pixel detectors (256×256 square pixels, 55×55 μm2) are well suited for charged particle detection They offer high resolution, low noise, and high quantum efficiency The Medipix2 chip allows double energy discrimination by offering a low and a high energy threshold The Timepix detector allows to record the incidence time of a particle with a temporal resolution of 10 ns and a dynamic range of 160 μs Results of the first time application of the Medipix2 detector to VMI are presented, investigating the quantum efficiency as well as the possibility to operate at increased background pressure in the vacuum chamber

Optimization of laser field-free orientation of a state-selected NO molecular sample

Abstract: We present a theoretical investigation of the impulsive orientation that can be induced by exposing a sample of state-selected NO molecules to the combination of a dc field and a short laser pulse. A strong degree of laser-field-free alignment and orientation is already achievable at moderate intensities and can be further improved by tailoring the temporal profile of the laser pulse. The alignment and orientation are only limited by the maximum value of the applied dc field and the pulse energy in the femtosecond laser pulse. Using an evolutionary algorithm coupled to a non-perturbative calculation of the time-dependent Schrodinger equation, the solutions that are obtained suggest that cos θ=0.964 is experimentally achievable.

Chemical physics: Electronic movies

Abstract: Strong laser fields can tear an electron away from a molecule, leaving a hole in the electronic wavefunction that races through the molecule. The ultrafast motion of such a hole has been traced at last.

Illuminating molecules from within

Abstract: Calculations show that with new short pulse x-ray light sources, it should be possible to use photoelectron emission to make movies of changes in molecular structure.

Attosecond electron interferometry

Abstract: The basic properties of atoms, molecules, and solids are governed by ultrafast electron dynamics. Attosecond pulses bear the promise to resolve these electronic dynamics on their natural time scale, the atomic unit of time, which is 24 attoseconds. The high frequency of the pulses, however, means that in most of the experiments performed so far the electrons that are excited by attosecond pulses are directly moved into the ionization continuum, where they rapidly disperse [1,2]. More interesting dynamics arise when electrons are excited into bound [3] or autoionizing states [4]. Here we present a method to determine the dynamics of a bound wave packet excited by an attosecond pulse, while - for the first time - keeping track of its spectral content with high precision. The key idea is that coincident with the creation of the bound wave packet, we also launch a broad continuum wave packet (Fig. 1). This free wave packet serves as a reference when, after a variable delay, the bound wave packet is ionized by a 7 fs infrared laser pulse, locked in phase with the bound wave packet. The interference fringes observed in the photoelectron spectrum enable precise determination of the bound electron wave packet. As in Ramsey spectroscopy, the spectral precision is here set not by the bandwidth of the excitation pulse, but by the delay between the pump and probe pulses as well as the experimental energy resolution of the photoelectron spectrometer used.