Fatima Anis

Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields

We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

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.

Enhancing the intense field control of molecular fragmentation

We describe a pump-probe scheme with which the spatial asymmetry of dissociating molecular fragments-as controlled by the carrier-envelope phase of an intense few-cycle laser pulse-can be enhanced by an order of magnitude or more. We illustrate the scheme using extensive, full-dimensional calculations for dissociation of H(2)(+) and include the averaging necessary for comparison with experiment.

Rotational dynamics of dissociating H+2 in a short intense laser pulse

We present a fully quantum mechanical treatment of H+2 in intense laser pulses as short as 5 fs, focusing on the rotational dynamics of the dissociating fragments. The dependence of the dynamics on the pulse length and intensity is examined. We find that including nuclear rotation is essential for predicting the angular distribution of fragments even in these very short pulses. We further find that the axial recoil approximation holds for H+2 only when the laser pulse is longer than roughly 100 fs.

Comparison of theoretical analyses of intense-laser-induced molecular dissociation

We investigate the accuracy of different common methods for calculating the momentum distribution of the nuclei following dissociation of H+2 by a short, intense laser pulse. This analysis can be performed either exactly by projecting onto scattering states of the system or by using a Fourier transformation of the final wavefunction. When and how this Fourier transform is made determines the validity of the approximation. While the values of the total observables were comparable, the Fourier transform methods require further free propagation time to converge to the exact result.

Strong-field dissociation dynamics of molecular dications

Citation: Jochim, B., Severt, T., Zohrabi, M., Ablikim, U., Berry, B., Gaire, B., . . . Ben-Itzhak, I. (2015). Strong-field dissociation dynamics of molecular dications. 635(11). doi:10.1088/1742-6596/635/11/112044

Low kinetic energy release upon dissociation of benchmark molecular ions

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.

Vibrationally cold CO2+ probed by intense femtosecond laser pulses

Using a novel approach, we produce a vibrationally cold CO2+ beam for study in an intense ultrashort laser field. We observe perpendicular dissociation of the simple two-level CO v = 0 ion, and above-threshold dissociation peaks spaced by the photon energy.

Enhancement of carrier-envelope phase effects in photoexcitation of alkali atoms

We have conducted a detailed theoretical study of alkali atoms in ultrashort laser pulses to see carrier-envelope phase effects for photoexcitation. This effect has been noted previously by Nakajima and Watanabe (2006 Phys. Rev. Lett. 96 213001) for Cs atoms. The present study, however, includes results from Li, Na, K and Rb as well as from Cs for a range of intensities in an effort to identify a larger effect and thus enhance its chances to be seen experimentally.

Ionization and dissociation of molecular ion beams caused by ultrashort intense laser pulses

Studies of the simplest one-electron molecule, H2+, are the first step towards understanding the interaction of ultrashort intense laser pulses with molecules. We conduct coincidence 3D imaging measurements of H2+ beams following their exposure to intense ultrashort laser pulses. These measurements are compared with our time-dependent calculations as well as a simple model we recently proposed. Our findings include above threshold Coulomb explosion - a surprising structure in the energy spectrum near the ionization appearance intensity; above threshold dissociation (ATD) of the excited electronic states of H2+; and enhanced high-order ATD - involving the net absorption of at least 3 photons - brought about by closing the 2-photon channel.