| Résumé | The electric field of a laser pulse exerts a force on charged particles which can be on the order of (or exceed) the forces that bind electrons to atoms, molecules, solids or that bind atoms together in molecules or solids. With modern laser technology, this force can be applied with almost 1 fs, 1 µm precision. Even if the field is lower than the field required to ionize atoms or molecules, large nonresonant Stark shifts can be achieved. The Stark shift gives us a means to control molecules. The dependence of the Stark shift with respect to the intensity profile of the laser focus determines the spatial force exerted on the molecule. The dependence of the Stark shift on the orientation of the molecule with respect to the laser polarization determines the torque exerted on the molecule. Through these forces we can control position, orientation, and linear and/or angular velocity. The Stark shift also depends on the internuclear co-ordinates, giving us some 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 discussed. In even stronger fields, where ionization occurs, the shifting and mixing of states becomes extreme. Measurement in this complex spectroscopic environment is difficult. Intuition based on perturbation theory is of limited value. 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. |
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