[en] This work is dedicated to the improvement of the near-field enhancement beneath the gold and silver tip apex due to plasmons excitation on a sub-wavelength grating engraved on the tip lateral surface. To study conditions of the maximal enhancement we have performed PSO-based optimization of intensity in search space of two parameters for gold and silver tip with different cone angles. Parameters of search space are period of the grating and its position in respect to the apex. The grating-assisted tip is illuminated with the incident light with wavelengths of 400 to 1000 nm in our model. All the simulations of electromagnetic waves scattering on the nanoantenna are based on the finite difference time domain method. (paper)

[en] This work is dedicated to the improvement of the near-field enhancement beneath the tip apex due to delocalized plasmons excitation on a sub-wavelength grating engraved on the tip mesoscopic surface. To study conditions of the maximal enhancement we have performed PSO-based optimization of intensity in search space of two parameters. Those parameters are period of the grating and its position in respect to the apex. The grating- patterned tip is illuminated with the incident light with wavelengths of 400 to 1000 nm in our model. All the simulations of electromagnetic waves scattering on the nanoantenna are based on the finite difference time domain method. (paper)

[en] We present a method for determining the detection efficiency of neutral atoms relative to keV ions. Excited D* atoms are produced by D₂ fragmentation in a strong laser field. The fragments are detected by a micro-channel plate detector either directly as neutrals or as keV ions following field ionization and acceleration by a static electric field. Moreover, we propose a new mechanism by which neutrals are detected. We show that the ratio of the yield of neutrals and ions can be related to the relative detection efficiency of these species

[en] In a recent Letter, Manschwetus et al.[Phys. Rev. Lett. 102, 113002 (2009)] reported evidence of electron recapture during strong-field fragmentation of H₂--explained using a frustrated tunneling ionization model. Unusually, the signature of this process was detection of excited H^* atoms. We report here an extensive study of this process in D₂. Our measurements encompass a study of the pulse duration, intensity, ellipticity, and angular distribution dependence of D^* formation. While we find that the mechanism suggested by Manschwetus et al. is consistent with our experimental data, our theoretical work shows that electron recollision excitation cannot be completely ruled out as an alternative mechanism for D^* production.

[en] Dissociation of a D⁺₂ (HD⁺) molecular ion beam is controlled by the time delay of a two-color field.

[en] Laser-induced dissociation of O₂⁺ is studied in the strong-field limit using two independent methods, namely a crossed laser-ion-beam coincidence 3D momentum imaging method and a supersonic gas jet velocity map imaging technique (790 and 395 nm, 8-40 fs, ∼10¹⁵ W/cm²). The measured kinetic energy release spectra from dissociation of O₂⁺ and dissociative ionization of O₂ reveal vibrational structure which persists over a wide range of laser intensities. The vibrational structure is similar for O₂⁺ produced incoherently in an ion source and coherently by laser pulses. By evaluation of the potential energy curves, we assign the spectral energy peaks to dissociation of the v=10-15 vibrational states of the metastable a ⁴Π_u state via the dissociation pathway |a ⁴Π_u>→|f ⁴Π_g-1ω>--a mechanism equivalent to bond softening in H₂⁺.

[en] The dissociation of CD⁺ induced by intense 30 fs pulses of central wavelengths 385 nm and 790 nm, over a range of peak intensities (5 × 10¹³ − 1 × 10¹⁶ W/cm²), was measured using a coincidence 3D momentum imaging technique. The observed kinetic energy release and angular distributions of the different products are presented and the possible dissociation pathways are discussed.

[en] A pertinent question in strong-field molecular physics is: what role does the permanent electric dipole moment of heteronuclear molecules play in their dissociation dynamics? Recently, Kiess et al (2008 Phys. Rev. A 77 053401) reported the first evidence for direct two-photon dissociation of an HD⁺ beam involving its permanent dipole moment, using 790 nm, 100 fs pulses. However, the measurement was convoluted by the fact that the H⁺ (H) and D⁺ (D) fragments could not be well resolved. Using high resolution coincidence 3D momentum imaging, which distinguishes all fragments, we find new evidence that challenges the previous findings. Specifically, we find that the small peak observed and assigned earlier to direct two-photon dissociation is instead due to one-photon dissociation of the v= 8 vibrational state by bond softening. Our vibrationally resolved spectra covering the intensity interval 3 x 10¹³ - 5 x 10¹⁴ W cm^-2 show that one-photon dissociation of HD⁺ dominates, with no clear evidence of permanent dipole transitions. (fast track communication)

[en] We explore very slow laser-induced dissociation processes of molecular ion beams employing an upgraded coincidence 3D momentum imaging technique. A selection of our studies, focused on zero-photon dissociation of H₂⁺, two-body breakup of D₃⁺, and vibrational structure in O₂⁺ dissociation, will be presented.

[en] An adaptive closed-loop feedback system is used to determine the optimal pulse shapes for manipulating the branching ratio of CO²⁺ following ionization by an intense laser pulse. For this target, selecting between the CO²⁺ and C⁺ + O⁺ final states requires control of the vibrational population distribution in the transient CO²⁺. The ability to both suppress and enhance CO²⁺ relative to C⁺ + O⁺ is observed, with shaped pulses surpassing a transform-limited pulse by factors of about 10 for suppression and 2 for enhancement. When optimizing small channels, such as non-dissociative CO²⁺, we demonstrate that a feedback signal obtained via a pulse counting technique is more robust than the more typical current mode signal collection. Furthermore, we demonstrate how the pulse counting technique allows control of a coincidence channel, specifically C⁺ + O⁺, by using logical electronic gates. Using these coincidence signals allows more specific final states to be incorporated into closed-loop control.