Extensive control over relative product yields is shown to result from weak laser photodissociation in a two-color coherent control scenario. Photodissociation takes place from a superposition state prepared by stimulated emission pumping. The method allows for control in the presence of laser phase jumps by using four-wave mixing to prepare the required laser frequencies. The near UV photodissociation of Na2, resulting in the Na(3s) + Na(4s) and the Na(3s) + Na(3d) products, is used to illustrate the method.
Quantum wavepacket calculations have been performed to study ways of using chirped laser pulses to control (a) complete population transfer, (b) coherent Raman processes, and (c) vibrational cooling via photoassociation. It has been found that positively and negatively chirped pulses significantly influence the behavior of wavepackets for these elementary processes. Although some of the numerical results can be interpreted from the standpoint of intra-pulse pump-dump processes, we have discovered some novel characteristics of chirped pulses.
We show that the branching ratios into two or three channel photodissociation products can be controlled by a novel resonant two-photon vs two-photon coherent control scenario. The method allows for control even when the initial molecular system is in a mixed state, e.g., in thermal equilibrium, a significant advance for controlling reactions. Essential to the technique is the phase locking between different frequencies which results in cancellation of incoherences due to laser phase jumps. Reduction of uncontrolled background contributions also obtains. The scenario is demonstrated computationally on Na2 photodissociation, where a wide range of control is achieved over branching into two product channels [Na(3s)+Na(4s) vs Na(3s)+Na(3p)] at lower energies and over three product channels at energies above the Na(3s)+Na(3d) threshold.
We examine the attosecond electron recollision dissociation of
D2+ recently demonstrated experimentally [H. Niikura et al., Nature (London) 421, 826 (2003)] from a coherent control perspective. In this process, a strong laser field incident on
D2 ionizes an electron, accelerates the electron in the laser field to eV energies, and then drives the electron to recollide with the parent ion, causing
D2+ dissociation. A number of results are demonstrated. First, a full dimensional strong field approximation model is constructed and shown to be in agreement with the original experiment. This is then used to rigorously demonstrate that the experiment is an example of coherent pump-dump control. Second, extensions to bichromatic coherent control are proposed by considering dissociative recollision of molecules prepared in a coherent superposition of vibrational states. Third, by comparing the results to similar scenarios involving field-free attosecond scattering of independently prepared
D2+ and electron wave packets, recollision dissociation is shown to provide an example of wave-packet coherent control of reactive scattering. Fourth, this analysis makes clear that it is the temporal correlations between the continuum electron and
D2+ wave packet, and not entanglement, that are crucial for the subfemtosecond probing resolution demonstrated in the experiment. This result clarifies some misconceptions regarding the importance of entanglement in the recollision probing of
D2+. Finally, signatures of entanglement between the recollision electron and the atomic fragments, detectable via coincidence measurements, are identified.
When pump-probe measurements are performed using pulses originating from the same laser, it is necessary to consider the coherent interaction between the two pulses in determining any material relaxation rates on the time scale of the laser pulse.1-5 For pump and probe beams crossing at an angle, the source of the coherent artifact can be understood in the following manner.1 When the two beams are simultaneously present in the sample, they interfere to produce a spatial modulation of the dielectric constant. Light from the pump beam then scatters from this induced grating into the probe direction. This model has led to the suggestion that for the collinear, copropagating beams, where the spatial grating disappears, the coherent artifact should vanish.3 While the spatial modulation is eliminated by choosing this geometry, we demonstrate here that the coherent artifact, which results directly from the nonlinear polarizability of the material, still remains. We have, however, been able to develop a general method for isolating the incoherent response, thus permitting the material relaxation to be extracted without any knowledge of the coherence properties of the pulse.