We demonstrate that the use of time-dependent light polarization opens a new level of control over quantum systems. With potassium dimer molecules from a supersonic molecular beam, we show that a polarization-shaped laser pulse increases the ionization yield beyond that obtained with an optimally shaped linearly polarized laser pulse. This is due to the different multiphoton ionization pathways in K2 involving dipole transitions which favor different polarization directions of the exciting laser field. This experiment is a qualitative extension of quantum control mechanisms which opens up new directions giving access to the three-dimensional temporal response of molecular systems.

Interferences of free electron wave packets generated by a pair of identical, time-delayed, femtosecond laser pulses which ionize excited atomic potassium have been observed. Two different schemes are investigated: threshold electrons produced by one-photon ionization with parallel laser polarization and above threshold ionization electrons produced by a two-photon transition with crossed laser polarization. Our results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments.

The quantum control of the ionization of potassium atoms using shaped intense femtosecond laser pulses is investigated. We use sinusoidal phase modulation as a prototype for complex shaped pulses to investigate the physical mechanism of the strong-field quantum control by shaped femtosecond light fields. The influence of all parameters characterizing the sinusoidal phase modulation on strong-field-induced dynamics is studied systematically in experiment and theory. Our results are interpreted in terms of the selective population of dressed states ͑SPODS͒ which gives a natural physical picture of the dynamics in intense laser fields. We show that modulated femtosecond pulses in combination with photoelectron spectroscopy are a versatile tool to prepare and to probe SPODS. The decomposition of the excitation and ionization process induced by shaped pulses into elementary physically transparent steps is discussed. INTRODUCTIONOne of the intriguing aspects of quantum control is the ability to manipulate quantum systems with suitably tailored laser light fields almost at will. For instance, shaped pulses are employed to guide a quantum system from an initial state to a preselected final state with high efficiency. In recent years, numerous quantum control schemes have been proposed and successfully demonstrated which are reported in recent monographs and reviews ͓1-5͔. Besides quantum control schemes based on the detailed knowledge of the potentials and the use of simple pulse sequences, adaptive pulse manipulation using complex pulse shapes has opened new perspectives for quantum control ͓6͔. The combination of pulse-shaping techniques ͓7͔ with closed loop adaptive feedback learning algorithms allows us to optimize virtually any conceivable observable ͓8-18͔ with applications ranging from laser science, quantum optics, atomic and molecular physics, solid state physics, photochemistry to biophysics, or quantum computing. However, it is not always possible to deduce the underlying physical mechanism from the electrical fields obtained by this procedure. Therefore, the need to bridge the gap between the efficient "black box" closed loop optimal control methods and detailed understanding of the physical processes-especially in strong laser fields-is quite evident.Generally, weak-field control schemes are not applicable for intense laser fields where perturbation theory is no longer valid as exemplified on a generic example of ultrafast strongfield coherent control ͓19͔. However, some of the principles of weak-field excitation are applicable to nonperturbative control employing so-called real pulses ͓20͔. In this context real or complex pulses are defined by the properties of their temporal phase which determines whether the pulse envelope is real or complex valued. For example, in this context, linearly chirped pulses are complex whereas third order dispersion ͑TOD͒ produces real pulses. The implications of real or complex pulses with respect to the manipulation of the symmetry of photoelectron spectra was recently discu...

Coherent control beyond population control and spectral interferences is demonstrated on the interferences and intensity of the two Autler-Townes ͑AT͒ components in the photoelectron spectrum of K atoms, using a sequence of two intense time-delayed femtosecond laser pulses. Photoelectron spectra were taken at various delay times between the two laser pulses and at different laser intensities at a fixed delay time. With respect to the interferences in the AT doublet the role of time delay and laser intensity is interchangeable for (n ϩ0.5) excitation. Strong laser fields or the optical phase of the delayed laser pulse allow the quantum mechanical phase of an atomic state to be manipulated in a symmetrical fashion. The observations are discussed in terms of a two-level model coupled to the continuum. For suitable combinations of the laser intensity of the first pulse and the time delay, the second laser pulse leaves the excited state population unchanged.

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