Michael Burt Research Group

Department of Chemistry University of Oxford

The Burt group uses ion imaging mass spectrometry and ultrafast laser pulses to study molecular structures and dynamics in the gas phase. To do this, we use laser-induced Coulomb explosion as a means to break molecules up into ionic fragments, which are then recorded using velocity-map imaging mass spectrometry. By judicious selection of the ionic fragments observed, we are able to learn about the structure and dynamics of the parent molecule - which is intrinsically related to a range of chemical phenomena. 

Developments of tabletop amplified femtosecond laser systems in the last two decades enable researchers to easily and reliably produce high energy (~5 mJ) ultrashort (~40 fs) laser pulses in a conventional laboratory setting. The high peak power of these laser pulses (~1016 W cm-2) means that they can be used to induce a "Coulomb explosion" in a molecular target. The strength of the laser field is sufficient to strip away multiple valence electrons from the target molecule, creating a highly-charged parent cation which rapidly fragments due to Coulombic repulsion. The resulting cationic fragments can subsequently be imaged (see below), a process akin to taking a "snapshot" of the molecular geometry at the point of ionisation. Click here to see a video made by former group postdoc James Pickering during his PhD explaining this process and how it can be used experimentally.

This process occurs on a femtosecond timescale, so by inducing slower molecular dynamics (e.g. photodissociation, or vibration) it is possible to use Coulomb explosion as a "probe" of the time-dependent behaviour. For example, dissociation of a molecule typically occurs over nanoseconds, so probing the molecular geometry during such processes allows them to be followed in real-time.

The same method has also been used to determine the structure of static molecules, especially molecules that are difficult to study using conventional spectroscopic methods. A major advantage of the Coulomb explosion technique is that it is a strong field non-resonant process, so can be applied to systems which are optically dark and not accessible via conventional spectroscopic methods. In the Burt lab we apply this process to fundamental photochemical processes (such as charge migration) in both small molecules and large biomolecules.

The ionic fragments produced via the Coulomb Explosion process are subsequently focussed onto a detector through a velocity-map imaging spectrometer. This type of spectrometer is an array of electrostatic lenses which will focus ions onto a charged-particle detector. The focussing is optimised such that each ion created with a specific velocity in the detector plane is focussed to a unique point on the detector, irrespective of where in the spectrometer it was ionised. This allows an image to be built up, where high-velocity fragments are seen towards the edges of the detector, and low-velocity fragments are found nearer to the center. If the explosion produces an anisotropic distribution of the direction of the ion velocities (in the detector plane), this will also be reflected in the image. In this way, detailed structural information can be obtained from the resulting images, especially when the imaging is coupled with covariance analysis.

Figure adapted from https://doi.org/10.1063/1.5023441The figure to the right illustrates  some of these concepts. The data shown is from the Coulomb explosion of a halogenated benzene (d) (iodine is purple and fluorine is blue). The time-of-flight spectrum of all the ions that explosion of this molecule produces is shown at the bottom of the figure. We are then able to produce velocity-mapped images of specific ion fragments - here shown are H+, F+, and I+. Focussing on the image of I+ (c), several illustrative points can be made. Firstly, the asymmetry of the image in the vertical direction is due to the I+ ion either recoiling upwards or downwards, as in this instance the target molecule is aligned in space pointing upwards or downwards by an additional laser field. Both the upward and downward ions are detected as the image is a sum of several thousand explosion events. Secondly, focussing on only the uppermost signal in image (c), it is clear that it is split into two parts - this is due to the I+ ion being formed with two different kinetic energies due to the presence of two different Coulomb explosion channels (I+ being formed with a different charged partner in each case). The higher energy fragment appears towards the edge of the detector, while the lower energy fragment is closer to the middle, at a smaller radius. 

This illustrates some of the structural information that can be obtained using Coulomb explosion imaging. This information can be further enhanced by analysis of the correlations between different ion fragments using covariance analysis.

In a typical Coulomb explosion experiment, a large number of charged fragments are produced during each explosion event, as in general multiple molecules will explode simultaneously. Straightforward analysis of the images of these fragments can be illuminating, but much more detailed information can be gained by analysing the correlations between different fragments. For example, in explosion of a large molecule producing several different ionic fragments, it may be interesting to know which fragments are generally produced together, more than it is interesting to know just what fragments are produced. It is possible to select a specific fragment and, via analysis of the correlations, learn with which fragments it generally appears. In this way, a more detailed understanding of the mechanics of the explosion process can be gained.
 

In the Burt group, we gFigure adapted from https://doi.org/10.1063/1.5023441enerally use covariance analysis, a type of correlation analysis applicable when the number of charged fragments produced during each experimental acquisition cycle is high. Covariance analysis performed between the velocity vectors of recoiling ion fragments can lead to very deep understanding of both the physics of the explosion process, and of the structure and dynamics of the molecule undergoing Coulomb explosion. Covariances calculated in this way can be presented as a recoil-frame covariance map, where the velocity vectors of recoiling ion fragments are presented relative to a chosen 'reference fragment'. The figure to the left shows how a recoil-frame covariance map (bottom right) is constructed from two individual ion images (bottom left and top right), and how it can then be used to measure the bond angle of a specific halogenated benzene molecule (top left). 
 

Figure adapted from https://doi.org/10.1063/1.5023441

The figure to the right shows recoil-frame covariance maps for an experiment where different structural isomers of a certain halogenated benzene were exploded and imaged. Each map shows the velocity distribution of a certain ion, plotted relative to the velocity vector of a given reference ion. Each column headed cov(A+, B+) is showing the covariance of ion B plotted relative to ion A. Looking along each row, it should be evident that the structure of each isomer (shown in the leftmost column) can be inferred from the corresponding covariance maps. In this particular experiment, the molecules were aligned in space using an additional laser field, so that the C-I axis was aligned along a space-fixed axis. Free rotation around this axis is possible, which accounts for the symmetry about a vertical axis observed in many of the images. By analysing the covariances between different pairs of ions, structural parameters (such as bond angles) can be inferred, and the different structures can be discriminated.