Molecular science with strong laser fields

The electric field of a laser pulse exerts a force on charged particles whi ch can be on the order of (or exceed) the forces that bind electrons to ato ms, molecules, solids or that bind atoms together in molecules or solids. W ith modern laser technology, this force can be applied with almost 1 fs, 1 mu m precision. Even if the field is lower than the field required to ionize atoms or molec ules, large nonresonant Stark shifts can be achieved. The Stark shift gives us a means to control molecules. The dependence of the Stark shift with re spect to the intensity profile of the laser focus determines the spatial fo rce exerted on the molecule. The dependence of the Stark shift on the orien tation of the molecule with respect to the laser polarization determines th e torque exerted on the molecule. Through these forces we can control posit ion, orientation, and linear and/or angular velocity. The Stark shift also depends on the internuclear co-ordinates, giving us so me degree of control over the structure of the potential energy surface in molecules. The ability to control these basic forces with precision depends on our ability to control optical pulses. Progress towards producing high power pulses with almost arbitrary time-dependent infrared fields will be d iscussed. In even stronger fields, where ionization occurs, the shifting and mixing o f states becomes extreme. Measurement in this complex spectroscopic environ ment is difficult. Intuition based on perturbation theory is of limited val ue. Yet strong field probing allows us to supply a lot of electronic energy to a molecule very rapidly and to localize measurements in space and time. We illustrate molecular alignment and strong field probing together in one experiment where we study femtosecond dissociative ionization.