Laser-pulse-induced molecular bond-forming reactions

Chemical reactions can be perceived as breaking of molecular bonds and subsequent formation of new bonds, resulting in the creation of compounds different from the initial reactants. While it is simple to break a molecular bond with an intense laser pulse, it is - in general - difficult to controllably trigger the formation of a molecular bond. However, ultrafast laser science has now reached a level of perfection where it is possible to produce intense single-cycle electric field transients of virtually arbitrary shape in a wide range of frequencies. And it has been shown, by the group of the applicant and others, that such transients can control the breakage of a chemical bond by directly driving the molecular valence electrons - that actually form a chemical bond - on their natural restructuring time-scale of sub-femtoseconds. As the formation of a chemical bond is probably the essential process in nature and ultimately determines the foundations of life, it is therefore almost surprising that this capability has so far not been applied to bond-forming reactions, especially since experiments on laser-driven plasma channels indicate that a strong laser pulse can indeed induce the formation of a chemical bond. The capability to laser-control the formation of bonds would open up a completely new research field in physics and chemistry with potential implications for a range of subjects. This project proposes to experimentally investigate the possibility of initiating and controlling the outcome of bond-making reactions between closely spaced but to a good approximation isolated molecules in small gas clusters by controllably steering molecular electronic processes with the strong force associated with the electric field of intense laser pulses. We will apply three different laser sources emitting light at the three disparate wavelengths 0.8 m, 1.5 m and 6.1 m, each of them capable of delivering trains of pulses with durations of only a few light oscillation cycles and with the same reproducible waveform from pulse to pulse. In particular the mid- infrared wavelength of 6.1 m allows making an exciting step into a thus far completely unexplored regime of laser molecule interaction. Bond-breaking and bond-making reactions induced by single pulses and double-pulse sequences from these laser systems will be studied using two complementary methods. The first one is detection of the generated molecular compounds by multi-hit capable detectors on a shot-to-shot basis, which allows filtering the measured data for the occurrence of certain created compounds. The other one is measuring the absorption of isolated soft X-ray attosecond pulses carried at photon energies covering the carbon K- absorption edge as a function of delay between the X-ray and laser pulses, which will allow particularly detailed insight into the laser-pulse-induced nuclear and also electronic dynamics.

In this project we were investigating laser-driven electronic processes in molecules using ultrashort, intense laser pulses. Intense laser fields are ideal tools for studying dynamics in molecules since their strong electric field-oscillations can not only serve as a temporal reference, but even offer the possibility to directly drive the intra-molecular valence electrons-that actually form a chemical bond-on their natural time-scale of attoseconds (1 as=10^-18 s). Using intense few-cycle laser pulses with controllable field-shape and multi-particle coincidence momentum imaging of electrons and ions we could in this project develop a number of methods for the measurement and control of molecular processes with applications in physics, chemistry and biology. For example, we have developed methods for the simultaneous measurement of attosecond electronic and femtosecond nuclear dynamics in molecules. The key idea of these methods is to use the rotation of the laser electric field vector of elliptically polarized light as a clock on the attosecond time-scale, and the pulse envelope as a timing reference for nuclear motion. The developed method allows tracing of molecular bond-breaking events with attosecond temporal and picometer-scale spatial resolution. We furthermore showed that this approach allows for the unambiguous assignment and extraction of the properties of two particular electrons emitted from molecular hydrogen. This opens up the possibility to measure otherwise inaccessible quantities such as the intramolecular electron localization dynamics on attosecond timescales. We were also investigating the possibility of triggering molecular reactions on attosecond times by the controlled population of excited quantum states via strong-field-driven ionization and recombination processes. This is a fundamentally different approach from the standard way of triggering molecular dynamics by resonant population through single/few-photon absorption, which is only sensitive to the pulse envelope rather than the sub-cycle field-shape. Using a two-dimensional two-color field we demonstrated control of the quantum properties of excited electrons through a field-driven process. Moreover, we could show that the position within the molecule to which the field-driven electron recombines during the laser interaction can determine the fragmentation pathway of a molecule. Both results together, thus, constitute a new method for sub-cycle control of molecular bond-breaking events. As a last example we mention our achievements of demonstrating field-controlled attosecond transfer of electrons across system boundaries. Using dimers, i.e., a quantum system where two entities are kept at mesoscopic distances by weak binding forces, we could demonstrate that an electron, freed at one site can be controllably driven across the system border to the other site, where it may recombine. These results are very promising, since electron transfer across system boundaries is a key process in many fields, for example in biology where it drives photoinduced biological processes, or in chemistry where it is responsible for photocatalytic reactions.