Exploring and Controlling Fragmentation of Polyatomic Molecules with Few-Cycle Laser Pulses

Abstract

The removal of electrons from polyatomic molecules by ionization with intense, ultrashort laser pulses may trigger complex restructuring and fragmentation dynamics. Depending on the valence-shell from which the electrons are removed the molecular ion might be put into a certain dissociative or binding state by the ionization process. With control over the ionization process it might thus be possible to gain control over the subsequent restructuring and fragmentation process on a purely electronic level. Here we introduce two conceptually similar schemes that allow controlling the outcome of molecular restructuring and fragmentation processes in polyatomic molecules on sub-femtosecond time-scales. The first one involves recollision double ionization in few-cycle laser fields with a known carrier-envelope phase (CEP). We demonstrate experimentally CEP-control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene). As the recollision energy for a given intensity sensitively depends on the CEP, tuning of the CEP allows controlling the removal of inner-valence electrons and the controlled population of dissociative excited states. The second control scheme uses the strong preponderance of ionization from specific molecular orbitals to the alignment of the molecular axis with respect to the laser polarization direction for determining which valence level the electrons are removed from. We demonstrate experimental control over different two-body fragmentation and dissociation pathways from the cation and the dication of the acetylene molecule using the field-free alignment angle as a control knob. Finally, we turn from the demonstration of control schemes working at sub-femtosecond time-scales, for which the nuclear dynamics following the ionization are not essential, to the investigation of the coupled nuclear-electronic dynamics that in general take place for longer pulse durations. We explore experimentally the mechanism behind the surprisingly high charge states recently observed in the ionization of hydrocarbon molecules that have been explained by a multi-bond version of the well known enhanced-ionization (EI) mechanism taking place in parallel at many C–H bonds. Our experimental results are in agreement with the proposed multi-bond version of EI.