Our group works in the general areas of chemical reaction dynamics and new spectroscopic methods and applications. Some group members are using state-of-the-art imaging techniques to study chemical reactions initiated by absorption of a photon or collision of the molecule of interest with an electron. Others are investigating uses of optical microcavities in chemical sensing applications. We have also recently started working with clinicians at the John Radcliffe hospital to determine whether various types of spectroscopy can help with decision making during certain types of surgery.
  1. Photoinduced and electron-induced chemical reactions
  2. Ultra-fast detectors for imaging mass spectrometry
  3. Optical microcavities for chemical sensing
  4. Spectroscopic measurements for surgical decision making

Photoinduced and electron-induced chemical reactions

Chemical reactions initiated by light or by collisions with electrons play an important role in atmospheric chemistry, astrochemistry, synthetic chemistry, and biology, amongst other areas. Understanding the mechanisms of these reactions in detail offers new insight into a range of vital physical and chemical processes, ranging from the breaking of a single chemical bond all the way through to complex multistep processes such as photosynthesis.

We study photoinduced and electron-induced chemistry in the gas phase. The molecule of interest is prepared in a molecular beam so that its initial velocity and internal state distribution are well defined. Reaction is initiated by crossing the molecular beam with a laser or electron beam, the products to be detected are ionised if required by a second laser pulse, and the complete three-dimensional scattering distribution of the resulting ionised photoproducts is measured using a technique called velocity-map ion imaging (VMI). VMI combines time-of-flight mass spectrometry with imaging, separating ions of different mass and mapping their velocity distributions onto a position sensitive detector. You can see an animation of a velocity-map imaging experiment here. Having controlled the initial velocity of the reactants and measured the final velocities of the products, we can 'join the dots' to learn about the forces and energetics that drive the chemical reaction under study. Such experiments yield great insight into the physics underlying chemical reactivity.

Velocity-map imaging has been used for some time to study the photochemistry of small molecules, and we are currently exploring the capabilities of the technique for studying larger chemical systems of interest to the broader chemical community. Larger molecules have many more fragmentation pathways than smaller molecules, requiring the development of multimass imaging techniques. Multimass imaging requires a universal ionization technique (we use either a VUV 118 nm laser source or an electron beam) and an ultrafast imaging detector capable of recording multiple images on the microsecond timescale of a time-of-flight mass spectrum (see Pixel Imaging Mass Spectrometry below).

Examples of the types of photochemical systems we have studied recently include the photolysis of neutral and ionic ethyl bromide and ethyl iodide, which play a role in the marine boundary layer of the earth's atmosphere, and photolysis of N,N-dimethylformamide (DMF), a model for peptide bond fragmentation. We plan to extend the latter measurements to study fragmentation of a number of dipeptides and tripeptides in order to understand the role of different amino acid residues in determining the fragmentation dynamics. A multimass velocity-map imaging data set recorded for DMF using the PImMS ultrafast imaging sensor is shown below.

Our electron-molecule crossed-beam instrument and electron ionization cross-section instrument can be used to study a range of energy-dependent electron-impact ionization and fragmentation processes, all of which are interesting from a fundamental point of view. In addition, we are planning a variety of more applied studies, including investigations into the use of imaging data to separate contributions from different analytes in the mass spectrometric analysis of mixtures, for molecular fingerprinting, and to understand the role of low-energy electrons in radiation damage to DNA.

Ultra-fast detectors for time-of-flight imaging

We are part of the PImMS (Pixel Imaging Mass Spectrometry) consortium, a group of researchers who are working to develop ultra-fast imaging sensors suitable for applications in time-of-flight imaging. For example, the sensors allow velocity-map or spatial-map images to be acquired for each mass peak in a time-of-flight mass spectrum, opening up a range of new applications in mass spectrometry. The PImMS sensors are being used by several research groups within Oxford, Harwell, and Bristol, and have travelled around the world for experiments in Brookhaven, Ottawa, Hamburg, Aarhus, and numerous other locations.

Optical microcavities for chemical sensing

An optical cavity is a structure that traps light, either through repeated reflection between two or more mirrors, or through total internal reflection. Optical cavities are widely used for enhancing light-matter interactions through a variety of mechanisms. A simple way to think about the enhancement is that inside a cavity each photon has multiple chances to interact with a molecule when compared with conventional single-pass optical methods.

We are working with researchers in Oxford Materials to develop miniaturised cavities known as optical microcavities for applications in solution-phase chemical sensing and nanoparticle characterisation. Microcavities are only a few wavelengths in length, giving them interesting optical properties, and contain tiny quantities of liquid, often only a few tens of femtolitres. The trapped light forms standing waves or 'cavity modes' when the cavity length is a half-integer multiple of the wavelength. The resulting spectrum of light transmitted through the cavity consists of a series of sharp lines at well-defined wavelengths. We can perform chemical sensing in a number of ways by tracking the wavelength (or frequency) and intensity of the cavity modes:

  1. A shift in the position of a cavity mode reflects a change in refractive index of the cavity contents, providing a sensitive mechanism for measuring refractive index.
  2. An absorbing species within the cavity leads to an attenuation in the cavity modes at characteristic wavelengths. We can detect a few tens of molecules in this way.
  3. The cavity modes can be used as 'optical tweezers' to trap nanoparticles. As the nanoparticles undergo Brownian motion within the trap, the position of the cavity mode undergoes time-dependent fluctuations caused by the changing refractive index within the mode volume. These fluctuations can be analysed to extract detailed information on the shape, size, and composition of the nanoparticle.
  4. The microcavities can also be used to amplify signals from Raman scattering and fluorescence, which will form the basis of future work.

We are currently spinning out a company, Oxford HighQ, based on our microcavity technology.

Spectroscopic measurements in surgical decision making

We are currently working with two clinical groups at the John Radcliffe hospital on applications of various types of spectroscopy to clinical samples. In the first, we are investigating the use of Raman and fluorescence spectroscopy for delineating the borders of high grade and low grade gliomas during tumour resection. In the second, we are evaluating whether reflectance spectroscopy can be used to guide clinical decisions during stenting procedures in STEMI (heart attack) patients.