B. Pons

Near-threshold high-order harmonic spectroscopy with aligned molecules.

We study high-order harmonic generation in aligned molecules close to the ionization threshold. Two distinct contributions to the harmonic signal are observed, which show very different responses to molecular alignment and ellipticity of the driving field. We perform a classical electron trajectory analysis, taking into account the significant influence of the Coulomb potential on the strong-field-driven electron dynamics. The two contributions are related to primary ionization and excitation processes, offering a deeper understanding of the origin of high harmonics near the ionization threshold. This Letter shows that high-harmonic spectroscopy can be extended to the near-threshold spectral range, which is in general spectroscopically rich.

High-order harmonic spectroscopy of the Cooper minimum in argon: Experimental and theoretical study

We study the Cooper minimum in high-order-harmonic generation from argon atoms by using long wavelength laser pulses. We find that the minimum in high-order-harmonic spectra is systematically shifted with respect to total photoionization cross section measurements. We use a semiclassical theoretical approach based on classical trajectory Monte Carlo and quantum electron scattering methods to model the experiment. Our study reveals that the shift between photoionization and high-order-harmonic emission is due to several effects: the directivity of the recombining electrons and emitted polarization, and the shape of the recolliding electron wave packet.

Common translation factor method

The basis and workings of a very useful technique in the treatment of atomic collisions is explained, which is the introduction of a common translation factor in the framework of close-coupling expansions. A historical review of the subject is presented, together with a description of the properties of the factor, and a detailed illustration of its performance.

Universality of photoelectron circular dichroism in the photoionization of chiral molecules

Photoionization of chiral molecules by circularly polarized radiation gives rise to a strong forward/backward asymmetry in the photoelectron angular distribution, referred to as photoelectron circular dichroism (PECD). Here we show that PECD is a universal effect that reveals the inherent chirality of the target in all ionization regimes: single photon, multiphoton, above-threshold and tunnel ionization. These different regimes provide complementary spectroscopic information at electronic and vibrational levels. The universality of the PECD can be understood in terms of a classical picture of the ionizing process, in which electron scattering on the chiral potential under the influence of a circularly polarized electric field results in a strong forward/backward asymmetry.

Relaxation Dynamics in Photoexcited Chiral Molecules Studied by Time-Resolved Photoelectron Circular Dichroism: Toward Chiral Femtochemistry

Unravelling the main initial dynamics responsible for chiral recognition is a key step in the understanding of many biological processes. However, this challenging task requires a sensitive enantiospecific probe to investigate molecular dynamics on their natural femtosecond time scale. Here we show that, in the gas phase, the ultrafast relaxation dynamics of photoexcited chiral molecules can be tracked by recording time-resolved photoelectron circular dichroism (TR-PECD) resulting from the photoionization by a circularly polarized probe pulse. A large forward–backward asymmetry along the probe propagation axis is observed in the photoelectron angular distribution. Its evolution with pump–probe delay reveals ultrafast dynamics that are inaccessible in the angle-integrated photoelectron spectrum or via the usual electron emission anisotropy parameter (β). PECD, which originates from the electron scattering in the chiral molecular potential, appears as a new sensitive observable for ultrafast molecular dynamics in chiral systems.

High-harmonic transient grating spectroscopy of NO2 electronic relaxation.

We study theoretically and experimentally the electronic relaxation of NO(2) molecules excited by absorption of one ∼400 nm pump photon. Semiclassical simulations based on trajectory surface hopping calculations are performed. They predict fast oscillations of the electronic character around the intersection of the ground and first excited diabatic states. An experiment based on high-order harmonic transient grating spectroscopy reveals dynamics occurring on the same time scale. A systematic study of the detected transient is conducted to investigate the possible influence of the pump intensity, pump wavelength, and rotational temperature of the molecules. The quantitative agreement between measured and predicted dynamics shows that, in NO(2), high harmonic transient grating spectroscopy encodes vibrational dynamics underlying the electronic relaxation.

Classical calculation of ionization and electron-capture total cross sections in H{sup +}+H{sub 2}O collisions

We report total cross sections for single ionization and electron capture in H{sup +} collisions with water molecules at impact energies 25 keV<E<5 MeV. Calculations have been carried out by applying the independent-particle model and the eikonal-classical trajectory Monte Carlo (CTMC) method. We have also estimated fragmentation cross sections by multiplying the partial cross sections by the branching ratios measured in the photoionization experiments of Tan et al. [Chem. Phys. Lett. 29, 299 (1978)].

Total and partial cross sections of electron transfer processes with hydrogen gas targets: Be4+ + H2

We report the implementation of programs to treat electronic transitions in ion-molecule collisions and the calculation of vibrationally-resolved partial cross sections. This includes the ab initio calculation of non-adiabatic couplings, the implementation of a block-diagonalization procedure, and the evaluation of vibrational transition probabilities. Using a two-state basis, we present some preliminary results on the Be4+ + H2(v = 0, L = 0) Be3+(3l) + H2+(v, ΣLM) reaction. These show that for nuclear velocities v < 0.2 a.u. vibration cannot be ignored, even though anisotropy effects are small. The reason is the strong dependence of the electronic transition amplitudes on the H–H distance.

Role of the ionic potential in high harmonic generation.

Recollision processes provide direct insight into the structure and dynamics of electronic wave functions. However, the strength of the process sets its basic limitations--the interaction couples numerous degrees of freedom. In this Letter we decouple the basic steps of the process and resolve the role of the ionic potential which is at the heart of a broad range of strong field phenomena. Specifically, we measure high harmonic generation from argon atoms. By manipulating the polarization of the laser field we resolve the vectorial properties of the interaction. Our study shows that the ionic core plays a significant role in all steps of the interaction. In particular, Coulomb focusing induces an angular deflection of the electrons before recombination. A complete spatiospectral analysis reveals the influence of the potential on the spatiotemporal properties of the emitted light.

Screening models for laser–cluster interactions

We study the applicability of various screening approaches within the context of laser–cluster interactions. Comparisons between well-known screening models, such as the static screened Coulomb, Debye–Huckel, Thomas–Fermi and ion–sphere ones, and a more elaborated approach based on the Lindhard dielectric function within the linear response theory (Gupta and Rajagopal 1982 Phys. Rep. 87 261) show that none of the simpler classical models can be used through the complete range of plasma situations encountered in laser–cluster interactions. Therefore, we have clearly specified the validity domains of the various models as well as the regions of electronic temperature and density where Friedel oscillations appear.