Laser Spectroscopy of nanodroplets

Nano Group at the University of Leicester



Laser spectroscopy of doped helium nanodroplets

Helium nanodroplets (HeNDs) can contain anything from dozens to many millions of helium atoms. The temperature of these droplets is close to 0.4 K, which is well below the temperature required to form superfluid helium (2.18 K). Atoms and molecules can be inserted into these superfluid droplets and are cooled to ca. 0.4 K by rapid evaporative loss of helium atoms.


Diagram illustrating the main components of the helium droplet apparatus


We are using HeNDs to learn new chemical information. In particular we form weakly bound complexes, which are then probed via laser spectroscopy. We can do this in the infrared region using an optical parametric oscillator or a dye laser for the visible/ultraviolet regions of the spectrum. Spectra are recorded by combining laser excitation with mass spectrometric detection, as illustrated in the diagram above. The ion signal recorded by a standard mass spectrometer declines when the complex inside a HeND absorbs radiation and this can be used as a means of detecting spectroscopic transitions, a procedure known as depletion spectroscopy.


We are currently using this methodology in several different experiments including:

  1. Ionic dissociation in solute-solvent systems. A classic example is the interaction of a salt with water, where the salt dissociates into ions in excess water. However, we are using HeNDs to learn about the interaction of a single salt molecule, such as NaCl, with a small number of water molecules. We form complexes of the type NaCl(H2O)n in order to try and understand how the salt interacts with the water and how much water is needed to achieve dissociation into separate Na+ and Cl- ions.
  1. To learn about weakly bound reaction intermediates that form when reactants collide and as products appear in chemical reactions. These so-called entrance and exit channel complexes affect the dynamics of chemical reactions but are difficult to study directly in experiments. Among the systems we are investigating are the complexes of Cl atoms with small organic molecules, such as alkanes, alkenes and alcohols.







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