Laser Spectroscopy and Mass Spectrometry of Molecules, Ions and Clusters in Helium Nanodroplets
Helium nanodroplets are nanometre-sized droplets of liquid helium. They can contain anything from dozens to many billions of helium atoms. The temperature of these droplets is 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 droplets, and this where our interests lie.
One of our research interests is in using helium droplets as a tool to learn how molecules interact with each other. Take the case of a solute interacting with a solvent. Suppose the solute is the salt NaCl. When we dissolve this in water we expect it to separate into Na+ and Cl- ions. However, this does not always occur and in concentrated solutions the NaCl may remain close together as an ion-pair, Na+Cl-. We want to answer fundamental questions such as how many water molecules does it take to force the two ions apart, and how does this vary as the type of salt is changed. We do this by combining salt molecules with water molecules inside a helium nanodroplet. Once trapped within this cold environment, we can then learn about the way solutes and water interact by recording their spectra, and particularly their infrared spectra, using laser techniques.
Ion-Molecule Reactions in Liquid Helium Nanodroplets
We have carried out extensive studies of the behaviour of ions ejected from helium droplets, both cations and anions. Much of this work has been done in collaboration with Professor Paul Scheier at the University of Innsbruck. Helium droplets make it possible to explore some pretty exotic ion-molecule chemistry. For example, we have recently observed electron attachment to clusters of C60 molecules in helium nanodroplets, leading to the formation of dianions, a first in helium nanodroplets. Another example is the formation of crystalline salt structures triggered by the addition of electrons to helium droplets which contain a mixture of sodium atoms and SF6.
We have also recently reported the first observation of large negatively charged hydrogen cluster ions, which represent a new form of condensed hydrogen formed by injecting electrons into hydrogen-doped helium nanodroplets. These clusters adopt preferred, regular structures.
Growing Nanoparticles in Helium Nanodroplets: A Route to New Nanoscience and Nanotechnology
In a joint project with Dr Shengfu Yang, large liquid helium droplets are being used as a medium for growing new types of nanoparticles. A strength of the technique is the ability of the droplets to capture any material that can be produced at low pressure in gaseous form, whether a gas or a vapour from a liquid or heated solid. The gaseous atoms or molecules collide with the droplets and then enter, leading to coalescence inside the droplet.
We have seen many new and unusual nanoscale structures with this approach, including pure metal nanoparticles and core-shell (layered) nanoparticles. A particularly exciting and recent discovery is the formation of metallic nanoparticles that show much more extreme magnetic behavior than particles formed by other techniques. These particles can be removed from the helium droplets through collision with a solid target, which causes evaporation of the helium and leaves behind the particle. There are some potentially very exciting technological applications for such particles.
Beyond a certain size, nanowires rather than nanoparticles are formed in helium droplets. These are thought to reflect the presence of so-called quantum vortices within the superfluid helium, and these vortices act as a constraint to ensure growth in one dimension.
Proton Transfer Reaction Mass Spectrometry and its applications
Air contains a multitude of organic compounds, usually termed volatile organic compounds (VOCs). These can derive from natural and man-made sources and, although VOCs are usually present at very low concentrations, they can still have important implications for the environment and for human health. In addition, VOCs are the by-products of many industrial processes, ranging from the manufacture of beers and spirits through to the production of pharmaceuticals.
VOCs are also produced by the human body through normal biochemical processes and can be expelled from the lungs when breathing. There are therefore many reasons why someone might want to measure these VOCs. In a collaboration with Professor Paul Monks, we have been at the forefront in developing a particular technique, proton transfer reaction mass spectrometry (PTR-MS), for doing exactly that.
PTR-MS is a technique which is selective to VOCs and can detect, identify and quantify their presence down to levels as low as a few parts in one trillion (1012). Crucially, it is also fast and this opens up a whole range of applications. We are actively exploiting applications of PTR-MS in a variety of areas, ranging from forensic analysis through to medicine. A good example is our recent initiation of a research project to explore the growth of disease cells in real-time by monitoring the emission of VOCs and using these as measures of particular biochemical processes.