Research Guides

We are currently working on projects in the following areas:

1. Chemical reaction dynamics and velocity-map imaging
2. Imaging mass spectrometry
3. Cavity ringdown spectroscopy for microfluidics applications
4. Supercontinuum light in cavity-enhanced spectroscopies

Chemical reaction dynamics and velocity-map imaging

The aim of experiments in chemical reaction dynamics is to understand the basic physics (i.e. the forces and energetics) that govern chemical reactivity. Reaction dynamics experiments are the chemist's version of the types of scattering experiments carried out by physicists at large facilities such as CERN. Breaking a chemical bond requires an energy of only a few electron volts (eV), an energy range that is easily accessible in the lab using molecular beam or laser techniques. We use a molecular beam to prepare internally cold molecules with a well-defined velocity, and a laser pump-probe cycle to initiate reaction and then to ionise one or more of the products a short time later. We are then able to use velocity-map imaging to record an image of the product scattering distribution.

Using this approach, we have looked at a wide range of elementary gas-phase photodissociation processes and bimolecular reactions. Many have involved small molecules of interest in atmospheric and combustion chemistry e.g. the unimolecular photodissociation of Cl₂, O₂, NO₂, N₂O, O₃, SO₂, NOCl and CH₃S₂CH₃, and a series of bimolecular reactions between atomic chlorine and the hydrocarbons methane, ethane and n-butane. We are now moving towards looking at larger molecules of interest in organic chemistry and mass spectrometry. We can probe several aspects of the collision dynamics:

1. Product quantum states: are there marked deviations from a thermal distribution?
2. Product velocity distributions: does the collision energy appear in product translation or internal excitation?
3. Product angular distribution: are the products forward, backward, or isotropically scattered?
4. Product angular momentum: does the product have 'frisbee', 'propeller' or 'cartwheel' type motion? Is the electronic angular momentum polarised?

Using unpolarised light, we are able to probe product quantum state and speed distributions. With polarised pump radiation we can access the product angular distribution, and with polarised pump and probe radiation we can also infer details of product angular momentum polarisation (for atomic products this yields the electron density distribution, essentially allowing us to 'image a wavefunction').

As an example, some images from a photodissociation experiment on the triatomic molecule NOCl are shown below. These images represent 2D projections of the scattering distribution, recorded in various experimental geometries defined by the propagation and polarisation directions of the pump and probe lasers, for ground and spin-orbit excited Cl atoms produced in the photolysis. The radial coordinate (product speed distribution) contains information on the energy released in the reaction and whether it ends up as translational kinetic energy or internal excitation of one or both fragments, while the angular coordinate contains information on the symmetries of the electronic states accessed during the dissociation.

 We collaborate with a number of other research groups within the UK and Europe, including the group of Mark Brouard in Oxford, and Dave Parker at the Radboud University of Nijmegen. We are also part of the ICONIC European network and the Bristol and Oxford chemical dynamics group.

Imaging mass spectrometry

Velocity-map imaging has truly captured the imagination of the reaction dynamics community, but has so far remained confined to this field. We are extending the imaging technology used in reaction dynamics studies to develop a new type of time-of-flight mass spectrometer, which, in addition to the conventional mass spectrum, will record the complete velocity or spatial distribution of each ionic species at its point of formation.

The computer simulations below show that for an initial ion distribution having both a spread of positions and a spread of velocities, the ion optics may be tuned either for spatial mapping (in which the positions of the ions are recorded, regardless of their initial velocities) or for velocity mapping (in which the velocities of the ions are recorded, regardless of their initial positions).

In spatial-map imaging mode, imaging mass spectrometry has the potential to identify, with high lateral resolution, the many molecular constituents that may be present at a surface. It therefore has potentially exciting applications in surface analysis or as a high-throughput detection method in the mass analysis of spatially-resolved arrays of samples. ‘Soft’ laser-based ionization methods such as MALDI (matrix assisted laser desorption ionization) and DIOS (desorption ionization on silicon) are prime candidates for our imaging modality. Both of these techniques yield gas-phase ions from molecular species on a surface without fragmentation, making spatial-map imaging mass spectrometry a promising avenue for the molecular analysis of biological samples.

In biological mass spectrometry, proteins and peptides are often studied via their fragmentation mass spectra following collision- or radiation-induced dissociation. We hope that the extra dimensions of information available in imaging mass spectrometry will yield significant structural and conformational information on the parent molecule and allow the mechanisms and dynamics of fragmentation processes to be probed, as well as providing a unique multi-dimensional ‘fingerprint’ that may be used in molecular identification. This has already been achieved for small molecules. The velocity distributions of fragment ions produced in photodissociation events are highly sensitive to the detailed dynamics of the fragmentation process, and in many ways provide a 'fingerprint' for the parent molecule. As an example, the figure below shows the highly distinctive velocity distributions recorded for several fragment ions arising from the 193 nm photodissociation of dimethyl disulphide, CH₃S₂CH₃.  

One of the challenges in implementing imaging mass spectrometry lies in developing fast cameras or image sensors capable of recording images of many molecular fragments on the microsecond time scale of a time-of-flight cycle. We are working together with Andrei Nomerotski of Oxford Physics and the CMOS imaging sensor group at the Rutherford Appleton Lab to develop this technology.

Cavity ringdown spectroscopy for microfluidics applications

Miniaturisation has revolutionised the world of electronics, with processing power that once required a room full of the latest in technology now existing on a small microchip. Such miniaturisation is also possible in chemistry, embodied in the fields of 'lab-on-a-chip' chemistry and micro total analysis systems (μ TAS). There is currently a great deal of research aimed at developing microfluidic chip devices, which integrate a raft of laboratory functions (sample preparation, mixing, reaction, separation etc) onto a plastic or glass chip a few centimetres in size. There are numerous advantages to scaling down chemistry in this way. For a start, only tiny sample volumes are required. This is of particular importance for biological samples, which can often only be prepared in very small amounts, but the small volumes of reagents also mean that chip-based chemistry is often inherently safer than larger-scale approaches. There are considerable financial savings associated with the low chip fabrication costs and minimal reagent use. Chemical tests can be miniaturised and made portable, and there is the potential for very high throughput, with many experiments or reactions being run in parallel. Such attributes mean that lab-on-a-chip approaches hold great promise for a range of applications in medical diagnostics, forensics, environmental monitoring, and synthetic chemistry. However, in addition to the positive qualities outlined above, there are also considerable technological challenges associated with chip design and implementation.

One of the key challenges is the development of detection techniques that can be interfaced to a microfluidic chip. At present, while reactions may be run on the chip, the products are often analysed 'off-chip' by standard techniques such as high pressure liquid chromatography (HPLC) or gas-chromatography mass spectrometry (GCMS). An 'on-chip' technique needs to be sensitive enough to detect and identify molecules in the tiny (picolitre or less) sample volumes found on a typical chip, and must also be capable of physically interfacing to the chip. We are investigating the application of cavity-ringdown spectroscopy (CRDS) to this problem. These techniques employ an optical cavity to achieve extremely long absorption path lengths, leading to exquisite detection sensitivities, within a compact experimental footprint. An optical cavity is an arrangement of optical components designed to 'trap' light for a period of time through multiple reflections. We are investigating both simple two-mirror cavities and fibre-loop cavities. Fiber-loop cavities rely on total internal reflection for their operation, and have the advantage of a much broader bandwidth than two-mirror cavities, in which the dielectric mirrors are generally optimised for ultra-high reflectivity over a narrow bandwidth. The basic setup for cavity ringdown spectroscopy using a two-mirror or fibre-loop optical cavity is shown below (with a very schematic microfluidic chip as the sample!):

In two-mirror CRDS, a pulse of laser light is incident on the first cavity mirror of reflectivity R, and a small fraction (1-R) is coupled into the cavity and undergoes repeated reflections between the mirrors. On each reflection, a small amount of light is transmitted through the mirror, and a photodetector situated behind the second mirror records the exponential decay of this 'leaking' light, which is proportional to the intensity decay of the light trapped in the cavity. The time constant of the exponential decay, often referred to as the 'ringdown time', τ, depends only on the reflectivity of the cavity mirrors and the length of the cavity. However, if an absorbing species is admitted to the cavity, this represents an additional source of loss, a correponding decrease in the time constant, and a means for measuring the absorptivity of the sample. Recording the ringdown time as a function of laser wavelength allows an absorption spectrum to be measured.

In fibre-loop ringdown (FLCRDS), light is coupled in and out through the side of the fibre and the sample is placed either in a small gap in the loop (direct absorption), or near a tapered region of the fibre (evanescent absorption). In this case the short duration of the laser pulse relative to the round-trip time of the fibre loop cavity means that instead of a continuous ringdown signal, individual light pulses are recorded as the light packets pass the detector on each circuit. Preliminary FLCRDS measurements on organic dyes have demonstrated a detection limit of around 1x10¹¹ molecules in a sample volume of approximately 50 pL. We are currently working on interfacing both FLCRDS and more conventional two-mirror CRDS with microfluidic chips in a range of experimental configurations.

This work is being carried out in collaboration with the group of Joao Cabral at Imperial College London, and with Elizabeth Farrant and Adrian Wright at Pfizer Research and Discovery in Sandwich, Kent.

Supercontinuum light in cavity-enhanced spectroscopies

Supercontinuum light is a fairly new development in optics, generated by passing monochromatic light from a pulsed laser through a non-linear medium. A supercontinuum light source combines a broad, flat spectral distribution with an intensity that is orders of magnitude higher than conventional thermal white light sources. It also has a high degree of spatial coherence that allows focusing to a tight spot or collimation into a narrow beam. In many ways, such sources essentially constitute 'white light lasers'.

In our supercontinuum source, shown schematically below, broadband light is generated in a length of non-linear photonic crystal (microstructured) fibre pumped by the focussed output of a microchip Nd:YAG laser. A variety of non-linear effects in the fibre broaden the 1064 nm pump radiation into a continuous spectrum spanning the range from around 450 to 1800 nm, while at the same time preserving the spatial coherence of the light. Two different pump laser configurations allow us to generate either 1 ns pulses with an energy of around 3 microjoules, or 7 ns pulses with an energy of around 30 microjoules. The image on the right hand side of the figure shows a supercontinuum pulse projected onto a screen after dispersion through a prism.

The availability of supercontinuum light opens the way to a wide range of applications in laser spectroscopy, either by using the entire broadband pulse or by combining the source with a monochromator or optical filters to create a simple and widely tuneable source of coherent light. We are exploring applications in the field of cavity-enhanced spectroscopy. For example:

1. Broadband cavity ringdown and cavity enhanced spectroscopy (CRDS and CEAS), in which we attempt to acquire a 'complete' absorption spectrum (i.e. spanning the range of wavelengths supported by the cavity) in a single laser shot.
2. CRDS and CEAS for online measurements in microfluidic systems.
3. Optimisation of light scattering measurements for particle detection.

•  M. Brouard, P. O'Keeffe and C. Vallance,'Feature article: The state resolved dynamics of elementary reactions' J. Phys. Chem. A., 106 3629 (2002).
• M. J. Bass, M. Brouard, A. P. Clark and C. Vallance, 'Fourier moment analysis of velocity-map ion images', J. Chem. Phys. 117(19) 8723 (2002).
• M. Brouard, A. P. Clark, C. Vallance, and O. S. Vasyutinskii, `Velocity-map imaging study of the O(³P) + N₂ product channel following 193nm photolysis of N₂O.', J. Chem. Phys. 119, 771, (2003).
• James E. Hudson, Michelle. L. Hamilton, Claire Vallance and Peter W. Harland, 'Absolute electron impact ionisation cross sections for the C1 to C4 alcohols', Phys. Chem. Chem. Phys., 5(15) 3162-3168 (2003).
• M. J. Bass, M. Brouard, C. Vallance, T. N. Kitsopoulos, P. C. Samartzis and R. L. Toomes., 'The dynamics of the Cl + n-C₄H₁₀^__> HCl(v',j') + C₄H₉ reaction at 0.3 eV', J. Chem. Phys. 121(15) 7175-7186 (2004).
• A. Karaiskou, C. Vallance, V. Papadakis, I. M. Vardavas, and T. P. Rakitzis, 'Absolute absorption cross-section measurements of CO₂ from 200 to 206 nm at 295 and 373 K', Chem. Phys. Lett. 400 30-34 (2004).
• Claire Vallance, 'Molecular photography: velocity-map imaging of chemical events' (review), Phil. Trans. A, 362 2591-2609 (2004).
• C. Vallance, 'Innovations in cavity ringdown spectroscopy' (review), New. J. Chem. 29(7) 867-874 (2005).
• M. J. Bass, M. Brouard, R. Cireasa, A. P. Clark and C. Vallance, 'Imaging photon-initiated reactions: a study of the Cl(²P_3/2) + CH₄ ^__> HCl + CH₃ reaction', J. Chem. Phys., 123 94301 (2005).
• J. E. Hudson, Z. F. Weng, C. Vallance and P. W. Harland, 'Absolute electron impact ionization cross-sections and polarizability volumes for the C₂ to C₆ methanoates and C₃ to C₇ ethanoates', Int. J. Mass Spectrom., 248 42-46 (2006).
• A. P. Clark, M. Brouard, F. Quadrini and C. Vallance, 'Atomic polarization in the photodissociation of diatomic molecules', Phys. Chem. Chem. Phys. 8 5591 (2006).
• M. Brouard, R. Cireasa, A. P. Clark, F. Quadrini and C. Vallance, 'Atomic polarization in the photodissociation of polyatomic molecules', in Molecular reaction and photodissociation dynamics in the gas phase, ed. P. Kleiber and K. C. Lin, p 267-332 (Research Signpost, 2007).
• M. Brouard, E. K. Campbell, A. J. Johnsen, C. Vallance, W. H. Yuen, and A. Nomerotski, 'Velocity-map imaging in time-of-flight mass spectrometry', Rev. Sci. Instrum. 79 123115 (2008).
• A. Boleininger, T. Lake, S. Hami and C. Vallance, 'A whispering gallery mode spectrometer for optical fibre profiling and chemical sensing', Sensors 10 1765-1781 (2010).
• S-M. Wu, D. Chestakov, G. Wu, X. Yang, C. Vallance, G. C. Groenenboom, W. J. van der Zande, and D. H. Parker, 'Highly polarised O(¹D₂) atoms from the photodissociation of O₂ via the B <-- X Schumann-Runge continuum', Molecular Physics 108(7) 1145 (2010).
• A. Nomerotski, M. Brouard, E. Campbell, A. Clark, J. Crooks, J. Fopma, J. J. John, A. J. Johnsen, C. Slater, R. Turchetta, C. Vallance, E. Wilman and W. H. Yuen, ‘Pixel imaging mass spectrometry with fast and intelligent pixel sensors’, J. Inst., 5 C07007 (2010).
• C. Vallance, 'A new generation of mass spectrometers', Projects Magazine, March 2010, p104-105 (British Publishers, www.projects.eu.com).
• Shiou-Min Wu, Dragana C. Radenovic, Wim. J. van der Zande, Gerrit C. Groenenboom, Claire Vallance, and Richard N. Zare, ‘Control and imaging of O(¹D₂) precession’, Nature Chemistry, in press October 2010.
• D. Chestakov, W. J. van der Zande, D. H. Parker, and C. Vallance, ‘Angular distributions and angular momentum alignment of O(³P_J) atoms formed in the photolysis of O₂ via the Herzberg continuum’, Phys. Chem. Chem. Phys., in press October 2010.