Showing papers by "Marc J. J. Vrakking published in 2002"

Application of a time-resolved event counting technique in velocity map imaging

Abstract: We illustrate the use of a three-dimensional (x,y,t) charge-coupled-device (CCD) camera detection system in an ion imaging experiment. The time measurement is based on the decay characteristics of the phosphor screen, which is recorded in two successive images by a double exposure CCD camera. The strength of the method is illustrated in a velocity map imaging experiment on iodine molecules that are ionized and dissociated by intense femtosecond laser pulses. Singly and doubly charged iodine fragments are detected and their (x,y) coordinates and arrival time are recorded in an event counting routine. We estimate the time resolution of the system to be 1.3 ns. We show that the fragment velocity distribution derived from the (x,y,t) data is similar and in some conditions more accurate than the distribution obtained by a mathematical inversion of the (x,y) data only. This principle of detection can be used in all situations in which inversion methods are impossible, for example, when the particle distribution does not have an axis of symmetry.

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

Abstract: 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.

Spectroscopie de photofragments résolue en temps. Manipulation d'interférences et codage d'information dans une molécule en cours de prédissociation

Abstract: Nous presentons une etude theorique de la possibilite de manipuler une molecule en cours de predissociation. Lors de la predissociation, une succession de paquets d'onde apparait sur la voie asymptotique. Lors de leur evolution, ces paquets d'onde s'etalent, se recouvrent et interferent. Nous decrivons comment l'utilisation d'impulsions effectuant des cycles de Rabi permettent de modifier la figure d'interference et de stocker de l'information.