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Controlling the orientation of molecules inside liquid helium nanodroplets

Final Report Summary - DROPLETCONTROL (Controlling the orientation of molecules inside liquid helium nanodroplets)

The central subject of this project has been to induce rotation and alignment of molecules embedded inside helium nanodroplets and use the aligned molecules to extract information about their structure.
Sharply aligned molecules makes it possible to learn about the ubiquitous dependence on molecular alignment of interactions and reactions with other molecules, atoms or light. Controlling molecular alignment also provides the best possible view to the molecule and therefore maximizes the information content available from experiments.

Motivated by these two facts, one major goal of this project has been to develop a method to strongly aligning molecule for a broad range of species.
To achieve this goal, we applied a strong laser pulse to align molecules embedded in He nanodroplets. The laser pulse was switched on sufficiently slowly that the molecular rotation followed in pace and thus reached the highest degree of alignment at the peak of the laser pulse. This adiabatic alignment method provides the best alignment if the molecules are as cold as possible. Since the molecules in the droplets have a temperature of only 0.38 K, the degree of alignment observed was very high and, notably, better than what can be achieved for gas phase molecules. This is the first major advantage of using molecules in He droplets. For applications, it is, however, highly problematic that the alignment laser pulse is on. To solve this problem, we developed a method to turn-off the laser field rapidly at its peak. For gas-phase molecules, alignment is quickly lost due to free rotation but for the molecules in He droplets, the rotation is impeded, thereby creating a 10-20 picosecond window where the alignment remains sharp but now laser-free. This is the second major advantage of the He droplets. The third is the ability of droplets to host molecules and molecular complexes much larger than what is possible in the gas phase. We demonstrated that by studies on several different large molecules and their dimers.

We then applied the newly developed field-free alignment method to molecular complexes and showed that it becomes possible to determine the structure of molecular complexes by Coulomb exploding them with intense laser pulses and recording the angular distributions of the recoiling ionic fragments. The studies included dimers and trimers of CS2 and OCS as well as the homodimers of tetracene and heterodimer of iodine and bromobenzene. These studies open intriguing possibilities for time-resolved imaging of bimolecular chemical reactions and intermolecular processes underlying organic solar cells.

Another major goal of the project was to use femtosecond or picosecond laser pulses to initiate rotation of molecules in He droplets and explore how the environment influences the rotation. Among the many interesting results obtained, is that molecules can be set into coherent rotational dynamics provided they do not rotate too fast. This is the regime where the superfluidity of the He droplet is not distorted. In the opposite regime, where the laser pulse delivers a very large angular kick to the molecule, we showed that the fast rotation leads to a transient breaking-free from the local He solvation shell. The experimental observations were explained by a newly developed theoretical model, describing a quantum rotor dressed by the surrounding He droplet. Most recently, experiments have shown highly excited rotational states of molecules behave like a harmonic oscillator, i.e. completely different from gas phase molecules. Our results allow spectroscopy of these states and time-resolved information of how the rotational angular momentum transfers to the He droplet on picosecond time scales. Further studies may generalize this to create quantum vortices through an optical centrifuge constructed during the project.