I. Ben-Itzhak

Subfemtosecond steering of hydrocarbon deprotonation through superposition of vibrational modes

Subfemtosecond control of the breaking and making of chemical bonds in polyatomic molecules is poised to open new pathways for the laser-driven synthesis of chemical products. The break-up of the C-H bond in hydrocarbons is an ubiquitous process during laser-induced dissociation. While the yield of the deprotonation of hydrocarbons has been successfully manipulated in recent studies, full control of the reaction would also require a directional control (that is, which C-H bond is broken). Here, we demonstrate steering of deprotonation from symmetric acetylene molecules on subfemtosecond timescales before the break-up of the molecular dication. On the basis of quantum mechanical calculations, the experimental results are interpreted in terms of a novel subfemtosecond control mechanism involving non-resonant excitation and superposition of vibrational degrees of freedom. This mechanism permits control over the directionality of chemical reactions via vibrational excitation on timescales defined by the subcycle evolution of the laser waveform.

Carrier-Envelope Phase Control over Pathway Interference in Strong-Field Dissociation of H2+

The dissociation of an H2+ molecular-ion beam by linearly polarized, carrier-envelope-phase-tagged 5 fs pulses at 4×10(14) W/cm2 with a central wavelength of 730 nm was studied using a coincidence 3D momentum imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission direction of H+ fragments relative to the laser polarization were observed. These asymmetries are caused by interference of odd and even photon number pathways, where net zero-photon and one-photon interference predominantly contributes at H+ + H kinetic energy releases of 0.2-0.45 eV, and net two-photon and one-photon interference contributes at 1.65-1.9 eV. These measurements of the benchmark H2+ molecule offer the distinct advantage that they can be quantitatively compared with ab initio theory to confirm our understanding of strong-field coherent control via the carrier-envelope phase.

Steering Proton Migration in Hydrocarbons Using Intense Few-Cycle Laser Fields

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to manipulate the movement of nuclei with tailored light within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely, acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics is monitored using coincident 3D momentum imaging spectroscopy and described with a widely applicable quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wave packet by the intense off-resonant laser field.

Transition from SAMO to Rydberg State Ionization in C60 in Femtosecond Laser Fields

The transition between two distinct ionization mechanisms in femtosecond laser fields at 785 nm is observed for C60 molecules. The transition occurs in the investigated intensity range from 3 to 20 TW/cm2 and is visualized in electron kinetic energy spectra below the one-photon energy (1.5 eV) obtained via velocity map imaging. Assignment of several observed broad spectral peaks to ionization from superatom molecular orbitals (SAMOs) and Rydberg states is based on time-dependent density functional theory simulations. We find that ionization from SAMOs dominates the spectra for intensities below 5 TW/cm2. As the intensity increases, Rydberg state ionization exceeds the prominence of SAMOs. Using short laser pulses (20 fs) allowed uncovering of distinct six-lobe photoelectron angular distributions with kinetic energies just above the threshold (below 0.2 eV), which we interpret as over-the-barrier ionization of shallow f-Rydberg states in C60.

Quantum control of photodissociation using intense, femtosecond pulses shaped with third order dispersion

We demonstrate the ability to control the molecular dissociation rate using femtosecond pulses shaped with third-order dispersion (TOD). Explicitly, a significant 50% enhancement in the dissociation yield for the low lying vibrational levels (v ~ 6) of an ion-beam target was measured as a function of TOD. The underlying mechanism responsible for this enhanced dissociation was theoretically identified as non-adiabatic alignment induced by the pre-pulses situated on the leading edge of pulses shaped with negative TOD. This control scheme is expected to work in other molecules as it does not rely on specific characteristics of our test-case molecule.

Thick-lens velocity-map imaging spectrometer with high resolution for high-energy charged particles

A novel design for a velocity-map imaging (VMI) spectrometer with high resolution over a wide energy range surpassing a standard VMI design is reported. The main difference to a standard three-electrode VMI is the spatial extension of the applied field using 11 electrodes forming a thick-lens. This permits measurements of charged particles with higher energies while achieving excellent resolving power over a wide range of energies. Using SIMION simulations, the thick-lens VMI is compared to a standard design for up to 360 eV electrons. The simulations also show that the new spectrometer design is suited for charged-particle detection with up to 1 keV using a repeller-electrode voltage of -30 kV. The experimental performance is tested by laser-induced ionization of rare gases producing electrons up to about 70 eV. The thick-lens VMI is useful for a wide variety of studies on atoms, molecules and nanoparticles in intense laser fields and high-photon-energy fields from high-harmonic-generation or free-electron lasers.

Methods for measuring mean lifetimes of long lived molecular ions formed in fast collisions

Abstract The direct measurement of the mean lifetime of molecular ions in a fast-beam (0.1–1 MeV) which were produced in collisions with a thin target requires methods which are different from the ones used for metastable atomic cases. This is due to the nature of the molecular ions breakup. The kinetic energy released in the dissociation is sufficient to spread the fragments on a relatively large area on the detector plane, thus special care and appropriate detectors have to be used to ensure the detection of all fragments. Several experimental techniques to determine the mean lifetime of long lived molecular ions are described. They are based on evaluating the ratio of dissociated molecular ions to the non-dissociated ones as a function of the distance they traveled. Measurements of the mean lifetimes of the recently discovered long lived rare gas dimers, NeAr2+ and HeNe2+ are used to describe these methods.

Formation and mean lifetime measurements of the long-lived doubly charged HeNe2+ rare-gas dimer

Abstract The first observation of a long-lived doubly charged HeNe 2+ rare-gas dimer is reported. This dication was obtained in charge-stripping collisions of 900 keV HeNe + and Ar. The mean lifetime of HeNe 2+ , τ = 184 ± 32 ns, was determined by measuring the yield of its He fragments, which have dissociated after passing the analyzer, as a function of the distance between the target cell and analyzer exits. Theoretical calculations of the 1 Σ + ground state indicate no bound vibrational states for this shallow potential curve. Further calculations are needed to determine the long-lived electronic state which is most likely an excited state.

Long lived CH2+ and CD2+ dications

Abstract A search for long lived CH 2+ and CD 2+ dications formed in fast charge stripping collisions of CH + and CD + on Ar was conducted. An experimental method based on the detection of the H (or D) fragments of the dication was developed, in order to eliminate possible confusion with 13 C 2+ for the first and 14 N 2+ for the latter. The flight time of these dications through the apparatus is about 70 ns, well below the 3 μs time associated with earlier observations of CH 2+ . Our measurements indicate that no long lived states of either of these dications are formed in fast charge stripping collisions. However, this result does not exclude the possibility that long lived states, like the excited A 2 Σ + metastable state, are populated in slow charge stripping collisions.

Spectral splitting and quantum path study of high-harmonic generation from a semi-infinite gas cell

We have investigated the spectral splitting of high harmonics generated in a semi-infinite gas cell. By performing an EUV–IR cross-correlation experiment, we are able to use the phase behaviour of the different sub-peaks of each harmonic to identify them with different electronic trajectories. Both microscopic and macroscopic analyses of the spectra effects are made. The identification of a particular trajectory with a particular component of the splitting on the basis of a single-atom model is found to be incorrect, while the full macroscopic treatment is in agreement with the experiment.