48 research outputs found
Orienting coupled quantum rotors by ultrashort laser pulses
We point out that the non-adiabatic orientation of quantum rotors, produced
by ultrashort laser pulses, is remarkably enhanced by introducing dipolar
interaction between the rotors. This enhanced orientation of quantum rotors is
in contrast with the behavior of classical paired rotors, in which dipolar
interactions prevent the orientation of the rotors. We demonstrate also that a
specially designed sequence of pulses can most efficiently enhances the
orientation of quantum paired rotors.Comment: 7 pages, 5 figures, to appear in Phys. Rev.
A quantitative theory-versus-experiment comparison for the intense laser dissociation of H2+
A detailed theory-versus-experiment comparison is worked out for H
intense laser dissociation, based on angularly resolved photodissociation
spectra recently recorded in H.Figger's group. As opposite to other
experimental setups, it is an electric discharge (and not an optical
excitation) that prepares the molecular ion, with the advantage for the
theoretical approach, to neglect without lost of accuracy, the otherwise
important ionization-dissociation competition. Abel transformation relates the
dissociation probability starting from a single ro-vibrational state, to the
probability of observing a hydrogen atom at a given pixel of the detector
plate. Some statistics on initial ro-vibrational distributions, together with a
spatial averaging over laser focus area, lead to photofragments kinetic
spectra, with well separated peaks attributed to single vibrational levels. An
excellent theory-versus-experiment agreement is reached not only for the
kinetic spectra, but also for the angular distributions of fragments
originating from two different vibrational levels resulting into more or less
alignment. Some characteristic features can be interpreted in terms of basic
mechanisms such as bond softening or vibrational trapping.Comment: submitted to PRA on 21.05.200
On negative higher-order Kerr effect and filamentation
As a contribution to the ongoing controversy about the role of higher-order
Kerr effect (HOKE) in laser filamentation, we first provide thorough details
about the protocol that has been employed to infer the HOKE indices from the
experiment. Next, we discuss potential sources of artifact in the experimental
measurements of these terms and show that neither the value of the observed
birefringence, nor its inversion, nor the intensity at which it is observed,
appear to be flawed. Furthermore, we argue that, independently on our values,
the principle of including HOKE is straightforward. Due to the different
temporal and spectral dynamics, the respective efficiency of defocusing by the
plasma and by the HOKE is expected to depend substantially on both incident
wavelength and pulse duration. The discussion should therefore focus on
defining the conditions where each filamentation regime dominates.Comment: 22 pages, 11 figures. Submitted to Laser physics as proceedings of
the Laser Physics 2010 conferenc
Spatial alignment of diatomic molecules in intense laser fields: II Numerical modelling
The angular distributions of ionic fragments produced by multi-electron dissociative ionization of diatomic molecules are calculated using a field-ionization Coulomb explosion model that includes dynamic rotation of the molecule in the laser field. The majority of dynamic alignment occurs on the leading edge of the laser pulse at low intensities before the laser intensity reaches the dissociative ionization threshold. This makes the degree of alignment sensitive to the precise value of the dissociative ionization threshold. Measuring the total ion signal for different pulse-lengths enables angle-dependent dissociative ionization to be distinguished from dynamic alignment. Increased alignment with long pulses is an unambiguous sign that the molecules are forced into alignment with laser field
Two-dimensional momentum imaging of Rydberg states using half-cycle pulse ionization and velocity map imaging
The influence of the half-cycle pulse (HCP) kick on the asymptotic velocity of the ejected electron has been studied for excited xenon atoms (n* = 34) in the presence of a static electric field (220 V cmâ1). We find that the HCP does not change the momentum distribution perpendicular to the kick direction. Therefore half-cycle pulse ionization and electron velocity map imaging can be used to obtain two-dimensional momentum distributions of atomic Rydberg states. Semiclassical and quantum mechanical calculations complement the experimental results
Two-channel competition of autoionizing Rydberg states in an electric field
We present experimental data on the decay of xenon Stark states converging to the upper spin limit. In an electric field the Rydberg electron has two qualitatively different decay paths. If the electron changes the core state from the upper spin state into the lower spin state, it gains sufficient energy to escape the ionic core and autoionizes. Moreover, if the electronic state is above the saddle point, created by the electric field, it can field ionize. The probability to autoionize is nearly constant around the saddle point whereas the probability to field ionize rapidly increases above the saddle point. With the velocity map imaging technique we monitor both ionization channels as a function of ÍincreasingÍ photoexcitation energy. We observe that the field ionization channel dominates the competition and gains yield at the expense of the autoionization channel. The spectra are explained both with full quantum calculations and with a relatively simple description for the overall behavior. These experiments show that the field ionization can be used in general as a clock for total core-dependent decay