Transition between mechanisms of laser-induced field-free molecular orientation

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 x 10^13 W/cm^2, the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrate absolute rates of non-perturbative strong-field molecular ionization.

Field-free orientation of CO molecules by femtosecond two-color laser fields

We report the first experimental observation of non-adiabatic 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 photoemission.

Attosecond control of electron dynamics in carbon monoxide

Laser pulses with stable electric field waveforms establish the opportunity to achieve coherent control on attosecond timescales. We present experimental and theoretical results on the steering of electronic motion in a multi-electron system. A very high degree of light-waveform control over the directional emission of C+ and O+ fragments from the dissociative ionization of CO was observed. Ab initio based model calculations reveal contributions to the control related to the ionization and laser-induced population transfer between excited electronic states of CO+ during dissociation.