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Znakovskaya, Irina (2012): Light-waveform control of molecular processes. Dissertation, LMU München: Faculty of Physics



The control of chemical reactions is of great interest from both a fundamental and an industrial perspective. Among the many different ways to control the outcome of chemical reactions, control with the electric field waveform of laser pulses offers the possibility to control dynamics on the femtosecond, or even attosecond, timescale. This thesis presents work on a recently developed approach to control molecular processes by guiding electron motion inside molecules with the waveform of light. The work presented in this thesis started right after the pioneering experiment on laser-induced electron localization in the dissociative ionization of molecular hydrogen with phase-stabilized few-cycle laser pulses. First, electron localization was studied for the different isotopomers H_2, HD, and D_2. The laser waveform driven strongly coupled electron and nuclear dynamics was investigated with single and two-color control schemes using near-infrared pulses as the fundamental. Furthermore, the subcycle control of charge-directed reactivity in D_2 at mid-infrared wavelengths (2.1 micrometers) was both observed experimentally and investigated quantum-dynamically. Two reaction pathways could be detected and controlled simultaneously for the first time. Extending the approach from the prototype hydrogen molecules, which contain only a single remaining electron after initial ionization, towards complex multielectron systems was a major goal of this thesis and first achieved for carbon monoxide. Experimental and theoretical results (by our collaborators from the de Vivie-Riedle group) on the waveform control of the directional emission of C^+ and O^+ fragments from the dissociative ionization of CO shed light on the complex mechanisms responsible for the waveform control in multielectron systems. In particular, it was found that not only the dissociation dynamics but also the ionization can lead to an observable asymmetry in the directional ion emission. In CO the contributions from these two processes could not be experimentally distinguished. Studies on another heteronuclear target, DCl, showed that for this molecule mainly the ionization step is responsible for an asymmetry in the fragment emission that can be controlled with the laser waveform. Another result of the studies on complex molecules was that the angular distributions of emitted ions from the breakup of the molecules in few-cycle laser fields showed the contributions of various orbitals in the ionization step. These results were supported by a new theoretical treatment by our collaborators from the de Vivie-Riedle group based on electronic structure theory for diatomic and larger systems, where multi-orbital contributions could be taken into account. Studies of the angle-dependent ionization of both homonuclear N_2, O_2 and heteronuclear CO and DCl molecules in few-cycle laser fields clearly show the importance of multi-orbital contributions (two HOMOs or HOMO+HOMO-1). Finally, waveform-controlled laser fields have been applied to orient molecules. Our findings on DCl suggested that samples of oriented molecular ions can be generated under field-free conditions, where the angle-dependent preferential ionization with a near single-cycle pulse is responsible for the orientation. The control of rotational wave packet dynamics by two-color laser fields was observed for CO and can be interpreted in the framework of two mechanisms: A) the hyperpolarizability orientation mechanism that dominates at low intensities, where the ionization probability is quite low and B) the ionization depletion mechanisms that prevails at high intensities, where substantial ionization occurs.