Professor Andrew M. Ellis
Professor of Physical Chemistry
Tel: +44 (0)116 252 2138
- B.Sc., Ph.D. (Southampton). Professor (Physical Chemistry)
- Research Group: Spectroscopy and Dynamics
The interests of my research group span a range of activities including spectroscopy, molecular structure and bonding, reaction dynamics, analytical chemistry, atmospheric chemistry, and nanoscience. Our work, much of which is laboratory based, uses purpose-built state-of-the-art instrumentation.
Probing solvation using laser spectroscopy
The conventional way to study solvation is via bulk solution properties. However, the weakness of this approach is that it is unable to provide a complete picture of the solvation process at the molecular level. Questions we might ask include the following: (i) what are the favoured arrangements of solvent molecules around a solute atom/molecule; (ii) are solvent shells formed; (iii) if the solute breaks into ions, how many solvent molecules are required before this happens? A classic and important example of this would be the following: how many water molecules does it take to dissolve a common salt molecule such as NaCl? We attempt to answer these questions in our laboratory in an unusual manner, by adding small numbers of solvent molecules to a solute in the gas phase and then probing its properties using laser spectroscopy. For example, our current work focuses on the solvation of atomic or molecular solutes by water or ammonia molecules. Solute-solvent clusters are formed in a supersonic expansion and probed using double resonance laser spectroscopy. This two-laser technique is coupled with mass spectrometry to furnish an IR spectrum of a complex with a specific mass (and therefore specific number of solvent molecules). A schematic of the apparatus is shown below.
Our initial work in this area has focused on alkali-ammonia clusters of formula M(NH 3)n, where M is an alkali metal atom such as Li or Na. Alkali metals readily dissolve in bulk liquid ammonia and yield strongly coloured solutions. At low concentrations the solutions are blue, the colour arising from solvated electrons which detach from the alkali and dissolve amongst the ammonia molecules. At relatively high concentrations (>10 mol per cent) the solution becomes bronze and the unpaired electron on the alkali is no longer able to completely escape from the alkali, creating a solution with very high electrical conductivity. We are using laser spectroscopy to try and understand these solutions and we adopt two approaches:
- Vibrational spectroscopy to determine solute-solvent structures
- Electronic spectroscopy to probe the loosely bound electron
For example, the infrared spectrum of Na(NH3)3 in the N-H stretching region is shown below along with two simulations derived from ab initio quantum chemical calculations.
Through collection of such spectra for various cluster sizes we have shown, for example, that the first solvation shell around the Na atom can hold a maximum of six ammonia molecules. Once the second solvation shell becomes occupied, the unpaired electron originally on the Na atom moves out into the solvent and starts to behave like a solvated electron.
We are now extending these investigations to involve other solvents, such as water and methanol. We are also exploring other solutes, such as alkaline earth metals and transition metals such as Cu and Au. Most exciting of all, we are planning to explore the solvation of common salts, such as NaCl, to try and determine definitively how many watermolecules are required to dissolve a salt molecule.
References for above topic
- Infrared spectroscopy of Li(NH3)n clusters for n = 4 - 7, T. E. Salter, V. A. Mikhailov, C. J. Evans and A. M. Ellis, J. Chem. Phys. 125, 034302 (2006).
- Microsolvation of lithium in ammonia: dissociation energies and spectroscopic parameters of small Li(NH3)n clusters (n = 1 and 2) and their cations, T. E. Salter and A. M. Ellis, Chem. Phys. 332, 132-138 (2007).
- Structures of small Li(NH3)n and Li(NH3)n+ clusters (n = 1-5): evidence from combined photoionization efficiency measurements and ab initio calculations, T. E. Salter and A. M. Ellis, J. Phys. Chem. A 111, 4922 (2007).
- Infrared photodissociation spectroscopy of Na(NH3)n clusters: probing the solvent coordination, T. E. Salter, V. Mikhailov, and A. M. Ellis, J. Phys. Chem. A 111, 8344-8351 (2007).
- Infrared photodepletion spectroscopy of Li(MeNH2)n clusters for n = 3, T. E. Salter and A. M. Ellis, J. Chem. Phys. 127, 144314/1-8 (2007).
Mass spectrometry and laser spectroscopy of molecules in helium nanodroplets
Helium nanodroplets are clusters composed typically of several thousand helium atoms held together by weak van der Waals forces. They can be formed by expanding helium gas at high pressure into vacuum through a pinhole nozzle cooled to ultra-low temperatures. They provide an extraordinary environment for studying molecules. The droplets are superfluid, and thus when a molecule is inserted into a helium nanodroplet, it is able to rotate and translate. Furthermore, the temperature within each droplet is exceedingly low, 0.38 K. These two properties combined make it possible to form and study unusual molecular complexes which are difficult, if not impossible, to study in any other environment.
To carry out these experiments, we have constructed the apparatus illustrated below. This incorporates, unusually, a pulsed helium droplet source.
One of the aims of our work is investigate how ionization processes take place within helium nanodroplets. Traditional electron impact ionization of molecules often leads to extensive cation fragmentation, but what happens in a helium nanodroplet? Is fragmentation avoided?
We have observed unusual phenomena, such as changes in fragmentation patterns of molecular ions in helium droplets and the ejection of ionic fragments with attached helium atoms. For example, part of the mass spectrum obtained from water clusters in helium nanodroplets is shown below. The dominant peaks are due to H+(H2O)n , which are also seen in standard gas phase work. More unusual are the (H2O)n+ ions, which are normally unstable but survive when cooled in helium nanodroplets. Weak peaks can also be seen due to He(H2O)n+ clusters, which reveal for the first time that the positive charge is located at the surface of the cluster ion.
Currently our work is now shifting towards using lasers to excite and probe molecules in superfluid helium. There are many potential experiments, including controlling chemical outcomes at ultra-low temperatures and exploring reaction mechanisms and dynamics using novel laser pump-probe procedures.
References for above topic
- Soft or hard ionization of molecules in helium nanodroplets? An electron impact investigation of alcohols and ethers, S. Yang, S. M. Brereton, M, D. Wheeler and A. M. Ellis, Phys. Chem. Chem. Phys. 7, 4082-4088 (2005).
- Electron impact ionization of haloalkanes in helium nanodroplets, S. Yang, S. M. Brereton, M, D. Wheeler and A. M. Ellis, J. Phys. Chem. A 110, 1791-1797 (2006).
- Controlled growth of helium nanodroplets from a pulsed source, S. Yang, S. M. Brereton and A. M. Ellis, Rev. Sci. Instrum. 76, 104102 (2005).
- Electron impact ionization of aliphatic alcohol clusters in helium droplets, S. Yang, S. M. Brereton, and A. M. Ellis, Int. J. Mass Spectrom. 253, 79-86 (2006).
- A model for the charge transfer probability in helium nanodroplets following electron impact ionization, A. M. Ellis and S. Yang, Phys. Rev. A 76, 032714 (2007).
- Electron impact ionization of water-doped helium nanodroplets: observation of He(H2O)n+ clusters, S. Yang, S. M. Brereton, S. Nandhra, A. M. Ellis, B. Shang, L.-F. Yuan, J. Yang, J. Chem. Phys. 127, 134303 (2007).
- Selecting the size of helium nanodroplets using time-resolved probing of a pulsed helium droplet beam, S. Yang, A. M. Ellis, Rev. Sci. Instrum. 79, 016106 (2008).
New types of nanoparticles: synthesis in helium droplets
The techniques and concepts employed in the helium nanodroplet spectroscopy experiment described above are also being put to use in a groundbreaking new way of making nanoparticles for technological use. Using ultra-large helium droplets, with billions of helium atoms, we can grow ‘designer' nanoparticles, layer-by-layer. In a joint project with Dr Shengfu Yang in Chemistry and Professor Chris Binns and Dr Klaus von Haeften in the Department of Physics and Astronomy, our current work is focused on making new types of magnetic nanoparticles. The layers in these magnetic ‘nano-onions' will be specially chosen to achieve the optimum magnetic properties for the application, which in the first instance is in new types of magnetic recording devices. The apparatus for making the nanoparticles has recently been assembled (see picture below) and is now being used to make the first series of nanoparticles.
The layer-by-layer growth of nanoparticles inside ultra-cold helium droplets offers some extraordinary possibilities, and will allow entirely new classes of nanoparticles to be synthesised. As well as providing new challenges to nanoscience, we envisage technological applications in many areas including catalysis, medicine, and sensors.
Proton transfer reaction mass spectrometry
Increasing concern about the impact of volatile organic compounds (VOCs) in a range of environmental issues is feeding a growing demand for devices to detect these compounds. Recently, a promising new technique has been developed, proton transfer reaction mass spectrometry (PTR-MS), and the first PTR-MS devices have appeared on the market. A limitation is the use of quadrupole mass spectrometers, which have modest mass resolution and can only monitor a single mass channel at any instant in time.
In a joint research programme with Professor Paul Monks, we are constructing the next generation of PTR-MS instruments. These involve replacement of the quadrupole mass spectrometer with a time-of-flight mass spectrometer (TOFMS). This allows far more rapid data acquisition and a first generation version is shown in the picture right. Applications are being explored in many areas and include environmental monitoring of a wide range of VOCs, the study of solvent emissions, crime-scene forensic analysis, and the diagnosis of illnesses through breath analysis. Collaborations with other scientists and industrial partners are an important part of this work. Another instrument is currently under development in which a Hadamard transform TOFMS replaces the standard reflectron TOFMS. Hadamard transform time-of-flight mass spectrometry is a new mass spectrometric technique pioneered by Zare and co-workers at Stanford University. It is capable of achieving a 100% duty cycle and will allow the fastest possible multi-component trace gas analysis at high mass resolution and with high sensitivity.
References for above topic
- “Demonstration of proton transfer reaction time-of-flight mass spectrometry for real-time analysis of trace volatile organic compounds”, R. S. Blake, C. Whyte, C. O. Hughes, A. M. Ellis, and P. S. Monks, Anal. Chem. 76, 3841-3845 (2004).
- “Differentiation of isobaric compounds using chemical ionization reaction mass spectrometry”, K. P. Wyche, R. S. Blake, K. A. Willis, P. S. Monks, and A. M. Ellis, Rapid Commun. Mass Spectrom. 19, 3356-3362 (2005).
- “Chemical ionization reaction time-of-flight mass spectrometry: multi-reagent analysis for determination of trace gas composition”, R. S. Blake, K. P. Wyche, A. M. Ellis and P. S. Monks, Int. J. Mass Spectrom. 254, 85-93 (2006).
- “Performance of chemical ionization reaction time-of-flight mass spectrometry (CIR-TOF-MS) for the measurement of atmospherically significant oxygenated volatile organic compounds”, K. P. Wyche, R. S. Blake, A. M. Ellis and P. S. Monks, Atmos. Chem. Phys. 7, 609-620 (2007).
- “Fast fingerprinting of arson accelerants by proton transfer reaction time-of-flight mass spectrometry”, C. Whyte, K. P. Wyche, R. S. Blake, P. S. Monks and A. M. Ellis, Int. J. Mass Spectrom. 263, 222-232 (2007).
- “Detection of chemical weapon agents and simulants using chemical ionization reaction time-of-flight mass spectrometry”, R. L. Cordell, K. P. Wyche, R. S. Blake, P. S. Monks and A. M. Ellis, Anal. Chem. 79, 8359-8366 (2007).
- “Intercomparison of oxygenated volatile organic (OVOC) measurements at the SAPHIR atmosphere simulation chamber”, E. C. Apel, T. Brauers, R. Koppmann, R. Tillmann, C. Holzke, R. Wegener, J. Bossmeyer, A. Brunner, T. Ruuskanen, M. Jocher, C. Spirig, R. Steinbrecher, R. Meier, D. Steigner, E. Gomez-Alverez, K. Müller, S. J. Solomon, G. Schade, D. Young, P. Simmonds, J. R. Hopkins, A. C. Lewis, G. Legreid, A. Wisthaler, A. Hansel, R. S. Blake, K. P. Wyche, A. M. Ellis, P. S. Monks, J. Geophys. Res - Atmospheres. 113, D20307 (2008).
- “Aldehyde and ketone discrimination and quantification using two-stage proton transfer reaction mass spectrometry”, R. S. Blake, M. Patel, P. S. Monks, A. M. Ellis, S. Inomata, H. Tanimoto, Int. J. Mass Spectrom. 278, 15-19 (2008).
- “Gas phase precursors to anthropogenic secondary organic aerosol formation: detailed observations of 1,3,5-trimethylbenzene photooxidation”, K. P. Wyche, Paul S. Monks, A. M. Ellis, R. L. Cordell, A. E. Parker, C. Whyte, A. Metzger, J. Dommen, J. Duplissy, A. S. H. Prevot, U. Baltensperger, A. R. Rickard, F. Wulfert, Atmos. Chem. Phys. 9, 635-665 (2009).
- “Proton transfer reaction mass spectrometry”, R. S. Blake, P. S. Monks, A. M. Ellis, Chem. Rev. 109, 861-896 (2009).
Above: probing solvation using laser spectroscopy
You can visit The Laser Spectroscopy Group web pages online.