Light-waveform control of molecular processes

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.