We report on the use of the relative phase between a two-color optical field (1053 and 527 nm) to coherently control the photodissociation of the HD+ molecule. The field-induced spatial asymmetry in the fragment spectrum shows a strong phase dependence, while no selectivity is produced in nearly isoenergetic channels. The positively charged fragments are observed to preferentially dissociate in a direction counterintuitive to classical expectations.

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A numerical analysis using the resolvent-operator method shows that the space and time localization of ionization is in fact the key needed to understand the residual population that is left in the excited state in Stark-induced resonant multiphoton ionization. The two viewpoints recently opposed in the literature [G. N. Gibson, R. R. Freeman, and T. J. Mclllrath, Optics and Photonics News 3, 22 (1992)] are therefore compatible. However, the excited-state population can easily be destroyed if the resonance intensity or the pulse duration is increased. PACS number(s): 32.80.Rm Picosecond and subpicosecond pulses have revealed [1]the resonant nature of multiphoton ionization (MPI). In short, the optical Field required to observe MPI is usually strong enough to induce Stark resonances, which temporarily increases the ionization rate and dominates, in general, the nonresonant process. Due to averaging over the pulse space-time intensity distribution, the resonances are normally unobservable in the total ionization yield, but easily appear as sharp structures (typically 30 to 50 rnev wide) in the photoelectron energy spectrum produced by short pulses. Due to a ponderomotive shift of the ionization limit, the energy at which the structure appears is uniquely determined by the corresponding state's binding energy and the photon energy [1]. From the uncertainty principle, it may be argued that the sharp structures indicate that the atomic state remains in resonance for a time comparable to the inverse of its width (100 fs). It was therefore asserted [2] that the resonant structures originate only from the region of the interaction volume where the intensity (I") needed to bring the state into resonance is reached at the top of the pulse's temporal envelop where the shift rate is close to zero. This scenario results in a long-enough time scale for adiabatic passage to occur from the ground state to the excited state, which is then immediately ionized at a constant intensity, namely I". This model, which is based on a generalized Landau-Zener (LZ) level-crossing theory, led to the "shell" model of ionization, owing to the spatial shape of such a region in a Gaussian pulse. While the model was consistent with the photoelectron results, de Boer and Muller made the most interesting discovery that a population may survive in the Stark-shifted excited state after the end of the strong pulse [3]. From this observation, it was alternatively proposed that the excited state may indeed be populated when the resonance occurs during the rise of the pulse and subsequently ionized at various intensities during the rest of the pulse. The resulting photoelectron energy remains unchanged provided that the state has the same rate of shift as the ionization limit, i.e. , the ponderomotive shift. An obvious characteristic of this scenario is that it contradicts the shell model, since now the resonance intensity may be reached at any time during the pulse and therefore anywhere in the interaction volume. However, a recent experiment provided...