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:
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