Our research

Overview Reaction Dynamics Photoinduced Dynamics Imaging Mass Spectrometry Collaborations Funding

A Brief Overview

The research conducted by the group encompasses several different fields. Our lab currently contains 4 seperate experiments each involved in one of our key focus areas. These experiments fall under three main categories:

A panorama of the lab B2 in the Chemistry Research Laboratory. This laboratory is home to our group's experiments.

Reaction Dynamics

Our research in reaction dynamics is currently focussed on state-selected inelastic scattering.

• Hexapole State Selected Scattering

Our crossed-molecular beam machine, also known as the Blue Monster, combines hexapole state selection, electric field orientation, and velocity-map ion imaging to obtain an explicit picture of the scattering dynamics occurring between NO molecules and a collision partner (currently rare gas atoms). The hexapole focuses supersonically expanded NO molecules in their ground rotational state, and the electric field, located in the centre of the scattering chamber, enables the spatial orientation of the molecules. This technique provides control over the part of the molecule the collision partner impacts on and we can distinguish, for instance, between end-on collisions taking place at the N- and the O-end by switching the direction of the electric field. The rotationally excited NO are resonantly ionized and velocity mapped onto a detector and imaged with a camera to obtain angularly resolved scattering distributions. These experimentally measured angular distributions can then be compared to those predicted by exact quantum mechanical scattering calculations.

A schematic of the hexapole apparatus showing the crossed molecular beams, ionisation lasers and detection apparatus.

Our scattering studies allow for an extremely detailed description of the collision dynamics and, in particular, shed light on steric preferences associated with specific quantum states and collision geometries. The measurements provide valuable information about the underlying molecular forces and the potential energy landscapes that govern the dynamics, and are thus also a stringent test for the validation of theoretical models.


The latest experiments on the Blue Monster have investigated the three-vector correlation dynamics of end-on and  side-on oriented NO molecules with argon atoms. In these experiments, we detect rotationally excited NO molecules resulting from a collision with three well-defined vectors; the initial NO bond-axis vector, r, and the initial and final relative velocity vectors, k and k'. By recording the direction of the outgoing relative velocity vector of the system, we are able to distinguish between collisions taking place on either side of the molecule. Our results show striking differences in the differential scattering distributions (or differential cross sections , DCSs)  between the two ends and between the two sides, and a pronounced oscillation in steric preference as a function of the final rotational state being probed. These oscillations are a result of a quantum mechanical (QM) interference effect.

Experimental and QM simulated images for the N- and O-end orientations (first/second, and third/fourth row, respectively) for the final rotational states between 3.5e and 16.5e.

The Integral Steric Asymmetry (ISA) for end-on collisions is plotted as a function of the final rotational state, j'. A positive ISA indicates a preference for N-end collisions, while a negative ISA indicates a preference for O-end collisions. The QM calculations reproduce the experimental data very well, whereas the quasi-classical (QCT) calculations fail to capture the alternation in the sign of the ISA for adjacent j' transitions.

Photoinduced Dynamics

Our experiments in Photoinduced Dynamics are split into three different sub areas, described below.

• Coulomb Explosion Imaging
  

When exposed to intense laser fields, molecules may rapidly lose multiple electrons. The resulting polycations are unstable and rapidly fragment into smaller, mutually repulsive charged fragments - a so called "Coulomb explosion". Tracking the momenta of these fragments allows the original structure of the molecule to be determined. This is done by multi-mass velocity map ion imaging using the PImMS camera. The relative velocities of the different fragments can be determined through covariance analysis of the individual ion images, revealing great detail about the original molecular structure and the Coulomb explosion dynamics. This technique has been recently used in collaboration with Henrik Stapelfeldt's group at the University of Aarhus to identify structural isomers on a femtosecond time scale.

Identification of structural isomers of difluoroiodobenzene using Coulomb explosion imaging and covariance analysis. The covariance maps cov(A+,B+) show the velocity distribution of a given ion (B+) relative to a "reference" ion (A+).

The group has recently purchased and commisioned our own Ti:Sa femtosecond laser system. This will be used in the near future for a range of studies into Coulomb explosion dynamics and to explore potential applications of Coulomb explosion imaging.


• Femtosecond Reaction Dynamics
  

The use of femtosecond laser pulses allows the study of photoinduced dynamics as they happen in 'real time'. Here, a 'pump' laser pulse is used to induce the dynamics we want to study, and this is followed by a intense 'probe' pulse, which ionizes the system, and allows for the ionic fragments to be velocity mapped. Varying the delay between these two pulses allows the dynamics initiated by the 'pump' pulse to be followed on a femtosecond time scale, creating a 'molecular movie'.


The structural information provided through the measurement of ion trajectory correlations has also been used as a probe to follow the induced torsional motion between the two carbon rings in a biphenyl molecule. The torsional motion is first initiated using a femtosecond 'kick' laser pulse, followed a short time later (ps) by a much more intense 'probe' laser pulse, which Coulomb explodes the molecules. By measuring correlations between F+ and Br+ ion trajectories using the PImMS camera we were able to follow the motion of the molecule out to picosecond timescales, watching the two rings periodically rocking back and forth.


Imaging Coulomb explosion dynamics using PImMS. Correlating the trajectories of Br+ and F+ originating from the Coulomb explosion of 3,5-difluoro-3',5'-dibromo-4'-cyanobiphenyl (FBCBP) allows for measurement of the dihedral angle between the two rings.	Measurement of the dihedral angle as a function of time for (a) A single kick pulse, initiating torsional excitation (b) Two kick pulses separated by 1.22 ps, exhibiting enhanced torsional motion (c) Two kick pulses separated by 0.54 ps. At this time, the two benzene rings are rotating away from one each other, and so the second kick pulse greatly damps the torsional motion.

More recently, we have applied this technique to study photodissociation reactions in real time in a range of halomethane molecules. This work was done in collaboration with Daniel Rolles and coworkers at the FLASH free-electron laser in Hamburg. Here, a UV pump pulse initiated dissociation of a C-I bond, before a Coulomb explosion was induced by a probe laser, at a range of pump-probe delays. At longer pump-probe delays, the two neutral fragments originating from the photodissociation are further away from each other upon ionization by the probe pulse, and thus feel a smaller Coulomb repulsion. This channel therefore has a kinetic energy release which decreases as pump-probe delay increases. Analysis of this channel in the ion images allows us to determine information about the timescale of the dissociation, and how internal energy is partitioned following it. Applying covariance analysis allows us to determine the delay dependent relative ion momenta. In the near future we hope to use this to study more complex photochemistry, such as photoisomerisations, in real time.

The delay dependent kinetic energy of the I+ fragment following photodissociation and Coulomb explosion of CH2BrI. The measured kinetic energy decreases as pump-probe delay increases, due to the increased charge separation at the point of Coulomb explosion

Delay dependent recoil-frame ion-ion covariance maps of CH2Cl+ plotted with respect to I+ following the photodissociation and Coulomb explosion of CH2ClI. The ions recoil at 180 degrees to one another, as required by momentum conservation. Such covariance analysis allows isolation of the desired pump-probe feature (expected radius marked with white crosses) amidst background channels in which the two ions are not formed in coincidence.

• 3D Imaging

By using the capibilities of the PImMS camera, combined with home designed Newton-Sphere elongating ion optics we can not only slice the Newton sphere to gain higher resolution, but we can also achieve full 3D imaging of the molecular fragments. By slicing the Newton sphere, and centroiding in time and space, we are able to record full 3D velocity distributions without resorting to mathematical methods such as an Abel transform in order to obtain them. By using this powerful techniqe, blurring from pairwise correlations in diatom diatom inelastic scattering can be all but eliminated and 3D images obtained even in cases where the system lacks cylindrical symmetry.


A simulated image of a 3D Newton sphere obtained with the PImMS camera.	A simulated sliced ion image of the collision of NO+HD. Excitation of HD leads to multiple superimposed Newton spheres.

Imaging Mass Spectrometry

Imaging Mass Specrometry focuses on the efficient analysis of biological samples and developing technologies in order to perform this analysis. Our work in this area focusses on microscope mode mass spectrometry, in which spatially resolved chemical information about a surface can be obtained at very high throughput.


Imaging mass spectrometry is a powerful technique we develop in order to answer several questions: Which molecules are present in a tissue sample? Are there specific molecular markers which identify tumour cells? Where in a biological sample are drugs located a certain time after ingestion?

We are specialists in the development of new ion optic systems allowing imaging mass spectrometry to be performed with enhanced instrument specifications. By defocusing an ionisation laser onto a large section of a sample, molecular ions are emitted from the sample and spatially mapped onto a position sensitive detector. The observed ion image shows a molecular map of the surface. By employing self developed, ultrafast micro-channel-plate (MCP) scintillation detectors in combination with CMOS based imaging sensors (via the PImMS collaboration), we are able to detect multiple ion images of various masses within a single experimental cycle.


A schematic of the microscope-mode reflectron. Applied voltages: R - Repeller, E - Extractor, Pv - PEDA Pulse, Ei - Einzel lens, Dv - Deflector, Rv - Reflectron. Da denotes the detector angle. The blue trace shows a typical ion trajectory, originating in the ion optics, as it travelled through the instrument, terminating at the detector. The arrival position on the screen maps the position on the surface at which the ion was produced, while the arrival time at the detector is characteristic of the ion's m/z

Work in the group has focussed on developing instrumentation to push the resolution limits of miscroscope mode mass spectrometry in the spatial and mass domains. Recently, a novel microscope-mode reflectron instrument has been developed, as shown schematically above. This, in addition to new pulsed extraction schemes has greatly improved the resolution at which we can image, and the m/z range the technique can be succesfully applied to.

Microscope-mode ion images recorded using the PImMS1 camera on the reflectron instrument. Individual ion images below the mass spectra correspond to the labelled peaks in the recorded mass spectrum, all of which is recorded simultaneously by the PImMS sensor. Here, a spatial resolution of 34μm and a mass resolution(m/Δm) of 1600 is achieved. Using a CCD camera and a photomultiplier tube (PMT) to record the images and mass spectra, respectively, spatial and mass resolutions of 13μm and 8100 have been achieved.

Collaborations

Meet some of our frequent collaborators

Funding

We are grateful to the EU, STFC, and EPSRC for previous funding. We are currently funded through EPSRC Programme Grants in New directions in molecular scattering, and Ultrafast photochemical dynamics in complex environments.