We demonstrate a method to measure strong field laser ionization of aligned molecules. The method employs a macroscopic field-free dynamic alignment, which occurs during revivals of rotational wave packets produced by a femtosecond laser pulse. We investigate the dependence of strong field ionization of ${\mathrm{N}}_{\mathrm{2}}$ on molecular orientation. We determine that ${\mathrm{N}}_{\mathrm{2}}$ molecules are four times more likely to ionize when aligned parallel to the field than when aligned perpendicular to it.

We use linearly polarized 45 fs pulses to create rotational wave packets in ${\mathrm{N}}_2$ and ${\mathrm{O}}_2.$ We Coulomb explode molecules with a high-intensity circularly polarized pulse and use an ion imaging detector to measure a series of two-dimensional projections of the wave packet’s angular distribution in 27 fs increments. We highlight the evolving wave packet near the first, second, sixth, and tenth full revival times and also near the one-eighth, one-quarter, one-half, and three-quarter fractional revivals.

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.

Two-color (800 and 400 nm) short (45 fs) linearly polarized pulses are used to ionize and dissociate ${\mathrm{D}}_2$ into a neutral deuterium atom and a deuteron. The yields and energies of the ions are measured left and right along the polarization vector. As the relative phase of the two colors is varied, strong yield asymmetries are found in the ion-energy regions traditionally identified as bond softening, above-threshold dissociation and rescattering. The asymmetries in these regions are quite different. A model based on the dynamic coupling by the laser field of the gerade and ungerade states in the molecular ion accounts for many of the observed features.

The process by which a molecule in an intense laser field ionizes more efficiently as its bond length increases towards a critical distance $R_c$ is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for ${\mathrm{CO}}_2$, by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We find that for the $4+$ and higher charge states, 100 fs is the time scale required to reach the critical geometry $⟨R_{\mathrm{CO}}⟩\approx 2.1\AA$ and $⟨{\theta }_{\mathrm{OCO}}⟩\approx 163°$ (equilibrium ${\mathrm{CO}}_2$ geometry is $⟨R_{\mathrm{CO}}⟩\approx 1.16\AA$ and $⟨{\theta }_{\mathrm{OCO}}⟩\approx 172°$). The $\mathrm{CO}_2{}^{3+}$ molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than $3+$ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for ${\mathrm{CO}}_2$ and identify the electronic states responsible for CREI.