Orientation, rotation and solvation of ions in helium nanodroplets

Helium nanodroplets are liquid, finite size, nanoscale helium clusters that have been employed since the 1990s as a matrix for high-resolution spectroscopy of molecules. Spectroscopic studies of ionic species inside helium droplets were not however realised until 2010, even though the potential information about their structure is of special interest since they are fundamental reaction intermediates in biochemistry. Additionally, the interaction of these species with the helium environment can be determined from these experiments, which is interesting from a fundamental point of view.

To determine structural information and study product selectivity, we proposed using high electric fields to orient the molecular ions in helium nanodroplets. Pendular state spectroscopy of neutral polar molecules in helium nanodroplets in the presence of electric fields has previously given satisfactory results to achieve molecular orientation, allowing one to distinguish biomolecular isomers. Our first experiments validated the technique for neutral aniline since the polarisation difference ratios, which express the degree of orientation, obtained from the NH2-stretch transitions were well-reproduced by calculations. On the other hand, the measured degree of orientation of cationic aniline is noticeably smaller than calculations show. Even so, the variation of the degree of orientation with electric field lets us conclude that orientation of the ions in helium droplets was observed. The results of several experiments performed to investigate the possible effects influencing the apparent orientation of the cations did not lead to any straightforward explanation. Our study therefore shows that orientation of molecular ions indeed seems possible and leads the way to possible future experiments.

To examine the formation of ionic-alkali solvation complexes in helium droplets upon photoionisation of the alkali, sodium, and understand the helium-dopant interaction we have performed ion spectroscopy. We found that several scenarios are possible after ionisation including dissociation, solvation and complete solvent evaporation depending on the ionisation process, droplet size and number of water ligands considered. Direct ionisation of sodium leads to solvation in the droplet, collision with the water molecules and formation of the ionic complexes if the droplet size is sufficiently large. For these conditions, rovibrational spectroscopy of Na(OH2)+ was performed to obtain the A rotational constant in helium droplets and thus determine if the dopant-helium interaction affects the moment of inertia. From the results, we could confirm the latter but could not deduce the extent to which this was the case. This study shows how the spectral information of the solvated ions is useful to understand complexation dynamics in helium droplets and acquire further understanding about the dopant-helium interaction.