Problem complexity in femtosecond quantum control

Adaptive femtosecond laser pulse shaping is employed to achieve active quantum control in selected gas phase photodissociation and photoionization processes of CpFe(CO)(2)Cl. An evolutionary optimization algorithm uses experimental feedback from the photofragment mass spectra in a learning loop setup to improve the applied spectral laser pulse phases iteratively. The significance of the resulting complex temporal electric-field profiles is investigated by artificial reduction of the optimization parameter space. The degree of problem complexity is further studied by correlation measurements of the molecular control objective versus second-harmonic generation (SHG). In the case of direct ionization, the previously unknown molecular control problem can be mapped one-to-one onto the search space topology of SHG, whereas in the case of photoproduct ratio optimization, no clear correlation is observed. The control mechanism in the latter example is therefore much more complex and cannot be explained by a simple laser pulse intensity variation. Correlation techniques in a general sense are considered to be very helpful in solving the so-called problem of inversion, i.e., obtaining information about the underlying photophysical and photochemical processes. (C) 2001 Elsevier Science B.V. All rights reserved.