Publications by authors named "Claire Vallance"

39 Publications

All-Optical Three-Dimensional Electron Momentum Imaging.

J Phys Chem A 2021 Jun 7;125(23):5220-5225. Epub 2021 Jun 7.

Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States.

We report a new implementation of three-dimensional (3D) momentum imaging for electrons, employing a two-dimensional (2D) imaging detector and a silicon photomultiplier tube (siPMT). To achieve the necessary time resolution for 3D electron imaging, a poly(-phenylene)-dye-based fast scintillator (Exalite 404) was used in the imaging detector instead of conventional phosphors. The system demonstrated an electron time-of-flight resolution comparable with that of electrical MCP pick-off (tens of picoseconds), while achieving an unprecedented dead time reduction (∼0.48 ns) when detecting two electrons.
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http://dx.doi.org/10.1021/acs.jpca.1c03445DOI Listing
June 2021

Multi-channel photodissociation and XUV-induced charge transfer dynamics in strong-field-ionized methyl iodide studied with time-resolved recoil-frame covariance imaging.

Faraday Discuss 2021 May;228(0):571-596

J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506, USA.

The photodissociation dynamics of strong-field ionized methyl iodide (CH3I) were probed using intense extreme ultraviolet (XUV) radiation produced by the SPring-8 Angstrom Compact free electron LAser (SACLA). Strong-field ionization and subsequent fragmentation of CH3I was initiated by an intense femtosecond infrared (IR) pulse. The ensuing fragmentation and charge transfer processes following multiple ionization by the XUV pulse at a range of pump-probe delays were followed in a multi-mass ion velocity-map imaging (VMI) experiment. Simultaneous imaging of a wide range of resultant ions allowed for additional insight into the complex dynamics by elucidating correlations between the momenta of different fragment ions using time-resolved recoil-frame covariance imaging analysis. The comprehensive picture of the photodynamics that can be extracted provides promising evidence that the techniques described here could be applied to study ultrafast photochemistry in a range of molecular systems at high count rates using state-of-the-art advanced light sources.
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http://dx.doi.org/10.1039/d0fd00115eDOI Listing
May 2021

Covariance-Map Imaging: A Powerful Tool for Chemical Dynamics Studies.

J Phys Chem A 2021 Feb 21;125(5):1117-1133. Epub 2021 Jan 21.

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, U.K.

Over the past decade or so, the state-of-the-art in the field of chemical reaction dynamics has progressed from studies of few-atom systems to wide-ranging investigations into a variety of photoinduced and collision-induced processes in much larger molecules. Many of these studies are of direct relevance to a wide audience of chemists, spanning fields such as atmospheric chemistry, astrochemistry, synthetic chemistry, and chemical biology. Key to this work has been the technique of velocity-map imaging, which allows complete product scattering distributions to be recorded for the process of interest. Recent advances in camera technology have enabled the development of multimass velocity-map imaging, in which the scattering distributions of all reaction products can be recorded in a single measurement. In addition to the scattering distributions of individual reaction products, the data set now contains information on correlations between the scattering distributions of two or more fragments. These correlations can be revealed using the technique of statistical covariance, yielding an approach known as covariance-map imaging. This review will introduce the reader to covariance mapping and will describe various applications of the technique within the field of chemical dynamics. The underlying concepts will be illustrated through a series of simple simulations, before moving on to describe a number of recent experimental studies in which covariance mapping has been used to obtain mechanistic insight and information on molecular structure on the femtosecond time scale.
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http://dx.doi.org/10.1021/acs.jpca.0c10038DOI Listing
February 2021

Coulomb explosion imaging for gas-phase molecular structure determination: An ab initio trajectory simulation study.

J Chem Phys 2020 Nov;153(18):184201

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd., Oxford OX1 3TA, United Kingdom.

Coulomb explosion velocity-map imaging is a new and potentially universal probe for gas-phase chemical dynamics studies, capable of yielding direct information on (time-evolving) molecular structure. The approach relies on a detailed understanding of the mapping between the initial atomic positions within the molecular structure of interest and the final velocities of the fragments formed via Coulomb explosion. Comprehensive on-the-fly ab initio trajectory studies of the Coulomb explosion dynamics are presented for two prototypical small molecules, formyl chloride and cis-1,2-dichloroethene, in order to explore conditions under which reliable structural information can be extracted from fragment velocity-map images. It is shown that for low parent ion charge states, the mapping from initial atomic positions to final fragment velocities is complex and very sensitive to the parent ion charge state as well as many other experimental and simulation parameters. For high-charge states, however, the mapping is much more straightforward and dominated by Coulombic interactions (moderated, if appropriate, by the requirements of overall spin conservation). This study proposes minimum requirements for the high-charge regime, highlights the need to work in this regime in order to obtain robust structural information from fragment velocity-map images, and suggests how quantitative structural information may be extracted from experimental data.
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http://dx.doi.org/10.1063/5.0024833DOI Listing
November 2020

Raman spectroscopy to differentiate between fresh tissue samples of glioma and normal brain: a comparison with 5-ALA-induced fluorescence-guided surgery.

J Neurosurg 2020 Oct 2:1-11. Epub 2020 Oct 2.

2Nuffield Department of Surgery, University of Oxford, John Radcliffe Hospital, Oxford.

Objective: Raman spectroscopy is a biophotonic tool that can be used to differentiate between different tissue types. It is nondestructive and no sample preparation is required. The aim of this study was to evaluate the ability of Raman spectroscopy to differentiate between glioma and normal brain when using fresh biopsy samples and, in the case of glioblastomas, to compare the performance of Raman spectroscopy to predict the presence or absence of tumor with that of 5-aminolevulinic acid (5-ALA)-induced fluorescence.

Methods: A principal component analysis (PCA)-fed linear discriminant analysis (LDA) machine learning predictive model was built using Raman spectra, acquired ex vivo, from fresh tissue samples of 62 patients with glioma and 11 glioma-free brain samples from individuals undergoing temporal lobectomy for epilepsy. This model was then used to classify Raman spectra from fresh biopsies from resection cavities after functional guided, supramaximal glioma resection. In cases of glioblastoma, 5-ALA-induced fluorescence at the resection cavity biopsy site was recorded, and this was compared with the Raman spectral model prediction for the presence of tumor.

Results: The PCA-LDA predictive model demonstrated 0.96 sensitivity, 0.99 specificity, and 0.99 accuracy for differentiating tumor from normal brain. Twenty-three resection cavity biopsies were taken from 8 patients after supramaximal resection (6 glioblastomas, 2 oligodendrogliomas). Raman spectroscopy showed 1.00 sensitivity, 1.00 specificity, and 1.00 accuracy for predicting tumor versus normal brain in these samples. In the glioblastoma cases, where 5-ALA-induced fluorescence was used, the performance of Raman spectroscopy was significantly better than the predictive value of 5-ALA-induced fluorescence, which showed 0.07 sensitivity, 1.00 specificity, and 0.24 accuracy (p = 0.0009).

Conclusions: Raman spectroscopy can accurately classify fresh tissue samples into tumor versus normal brain and is superior to 5-ALA-induced fluorescence. Raman spectroscopy could become an important intraoperative tool used in conjunction with 5-ALA-induced fluorescence to guide extent of resection in glioma surgery.
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http://dx.doi.org/10.3171/2020.5.JNS20376DOI Listing
October 2020

Reflectance spectral analysis for novel characterization and clinical assessment of aspirated coronary thrombi in patients with ST elevation myocardial infarction.

Physiol Meas 2020 05 7;41(4):045001. Epub 2020 May 7.

Oxford Heart Centre, NIHR Biomedical Research Centre, Oxford University Hospitals, John Radcliffe Hospital, Oxford, United Kingdom.

Objective: The visual appearance of coronary thrombi may be clinically informative in ST elevation myocardial infarction (STEMI) patients undergoing primary percutaneous coronary intervention (pPCI). However, subjective assessment is poorly reproducible and cannot provide an objective basis for treatment decisions or patient stratification. We have assessed the feasibility of a novel reflectance spectroscopy technique to systematically characterize coronary artery thrombi retrieved by aspiration during pPCI in patients with STEMI, and the clinical utility for predicting distal microvascular obstruction.

Approach: Patients with STEMI treated with pPCI and thrombus aspiration (n = 288) were recruited from the Oxford Acute Myocardial Infarction (OxAMI) Study. Of these, 158 patients underwent cardiac magnetic resonance imaging within 48 h for assessment of microvascular obstruction (MVO). Coronary thrombi were imaged by reflectance spectroscopy across wavelengths 500-800 nm.

Main Results: Spectral data were analysed using function fitting and multivariate models. The coefficient 'c ' determined from the fitting procedure correlated with the visually-assessed colour of thrombi ('red' or 'white') and with MVO. When applied to a reduced data set, consisting of spectra from 20 patients with the largest MVO and from 20 propensity-score-matched patients with no MVO, three multivariate analysis methods were able to discriminate spectra of thrombi from patients without MVO and with the largest MVO.

Significance: Reflectance spectral analysis of coronary thrombus provides new insights into the pathology of STEMI, with potential clinical implications for emergency patient care. Further studies are warranted for validation as a point-of-care stratification tool in predicting the degree of microvascular injury and clinical outcomes in STEMI.
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http://dx.doi.org/10.1088/1361-6579/ab81deDOI Listing
May 2020

Rapid intraoperative molecular genetic classification of gliomas using Raman spectroscopy.

Neurooncol Adv 2019 May-Dec;1(1):vdz008. Epub 2019 May 28.

Nuffield Department of Clinical Neurosciences, John Radcliffe Hospital, University of Oxford, UK.

Background: The molecular genetic classification of gliomas, particularly the identification of isocitrate dehydrogenase (IDH) mutations, is critical for clinical and surgical decision-making. Raman spectroscopy probes the unique molecular vibrations of a sample to accurately characterize its molecular composition. No sample processing is required allowing for rapid analysis of tissue. The aim of this study was to evaluate the ability of Raman spectroscopy to rapidly identify the common molecular genetic subtypes of diffuse glioma in the neurosurgical setting using fresh biopsy tissue. In addition, classification models were built using cryosections, formalin-fixed paraffin-embedded (FFPE) sections and LN-18 (IDH-mutated and wild-type parental cell) glioma cell lines.

Methods: Fresh tissue, straight from neurosurgical theatres, underwent Raman analysis and classification into astrocytoma, IDH-wild-type; astrocytoma, IDH-mutant; or oligodendroglioma. The genetic subtype was confirmed on a parallel section using immunohistochemistry and targeted genetic sequencing.

Results: Fresh tissue samples from 62 patients were collected (36 astrocytoma, IDH-wild-type; 21 astrocytoma, IDH-mutated; 5 oligodendroglioma). A principal component analysis fed linear discriminant analysis classification model demonstrated 79%-94% sensitivity and 90%-100% specificity for predicting the 3 glioma genetic subtypes. For the prediction of IDH mutation alone, the model gave 91% sensitivity and 95% specificity. Seventy-nine cryosections, 120 FFPE samples, and LN18 cells were also successfully classified. Meantime for Raman data collection was 9.5 min in the fresh tissue samples, with the process from intraoperative biopsy to genetic classification taking under 15 min.

Conclusion: These data demonstrate that Raman spectroscopy can be used for the rapid, intraoperative, classification of gliomas into common genetic subtypes.
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http://dx.doi.org/10.1093/noajnl/vdz008DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6777649PMC
May 2019

Multi-mass velocity-map imaging studies of photoinduced and electron-induced chemistry.

Authors:
Claire Vallance

Chem Commun (Camb) 2019 May;55(45):6336-6352

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford OX1 3TA, UK.

Multi-mass velocity-map imaging (VMI) is becoming established as a promising method for probing the dynamics of a variety of gas-phase chemical processes. We provide an overview of velocity-map imaging and multi-mass velocity-map imaging techniques, highlighting examples in which these approaches have been used to provide mechanistic insights into a range of photoinduced and electron-induced chemical processes. Multi-mass detection capabilities have also led to the development of two new tools for the chemical dynamics toolbox, in the form of Coulomb-explosion imaging and covariance-map imaging. These allow details of molecular structure to be followed in real time over the course of a chemical reaction, offering the tantalising prospect of recording real-time 'molecular movies' of chemical dynamics. As these new methods become established within the reaction dynamics community, they promise new mechanistic insights into chemistry relevant to fields ranging from atmospheric chemistry and astrochemistry through to synthetic organic photochemistry and biology.
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http://dx.doi.org/10.1039/c9cc02426cDOI Listing
May 2019

The 300 Faraday Discussion.

Faraday Discuss 2019 05 3;214:9-12. Epub 2019 May 3.

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford OX1 3TA, UK.

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http://dx.doi.org/10.1039/c9fd90015bDOI Listing
May 2019

Oral versus Intravenous Antibiotics for Bone and Joint Infection.

N Engl J Med 2019 01;380(5):425-436

From Oxford University Hospitals NHS Foundation Trust (H.-K.L., M.A.M., B.L.A., P.C.M., A.J.B., J.L., R.G., M.R., A.T., B.A., I.B., A.R.B., E.K., T.W., T.H.N.W., L.K.B., A.A., P.B., M.S.) and the Nuffield Departments of Orthopaedics, Rheumatology and Musculoskeletal Science (I.R., R.Z., C.C., A.A.) and Medicine (M.K., C.S., P.C.M., A.J.B., B.A., G.E.T., P.B., M.S.) and the Division of Medical Sciences (B.A.L.), University of Oxford, Oxford, the Division of Infectious Diseases, Imperial College London (H.-K.L., G.C.), Medical Research Council Clinical Trials Unit, University College London (A.S.W.), Royal Free London NHS Foundation Trust (S.W., D.J.F.M., S.H.), Guy's and St. Thomas' NHS Foundation Trust (C.J.H., K.B.), and Public Health England (J.P.), London, University Hospital of Wales, Cardiff (H.C.H.), University Hospital Birmingham NHS Foundation Trust (D.B.) and Heart of England NHS Foundation Trust (N.J., C.E.M., A.F.W., S.S.), Birmingham, Royal National Orthopaedic Hospital NHS Trust, Stanmore (S.W., F.E.F., D.J.F.M.), Royal Liverpool and Broadgreen University Hospitals NHS Trust, Liverpool (J.F., H.E.R.), Cambridge University Hospitals NHS Foundation Trust, Cambridge (E.M., J.M.), Queen Elizabeth University Hospital, NHS Greater Glasgow and Clyde (R.A.S., C.V.), and Health Economics and Health Technology Assessment, University of Glasgow (C.G., N.M., A.H.B.), Glasgow, Leeds Teaching Hospital NHS Trust, University of Leeds, Leeds (J.A.T.S.), Ninewells Hospital, NHS Tayside, Dundee (I.A.), Northumbria Healthcare NHS Foundation Trust, Northumberland (S.C.E., D.J.B.), Western General Hospital, NHS Lothian, Edinburgh (R.K.S.), and Hull and East Yorkshire Hospitals NHS Trust, Hull (G.B.) - all in the United Kingdom; University of Bern, Bern, Switzerland (P.S.); Oxford University Clinical Research Unit, Wellcome Trust, Ho Chi Minh City, Vietnam (G.E.T.); and Kenya Medical Research Institute (KEMRI)-Wellcome Trust Research Programme, Kilifi, Kenya (P.B.). C.F.O. and J.B. are patient representatives and do not have an institutional affiliation.

Background: The management of complex orthopedic infections usually includes a prolonged course of intravenous antibiotic agents. We investigated whether oral antibiotic therapy is noninferior to intravenous antibiotic therapy for this indication.

Methods: We enrolled adults who were being treated for bone or joint infection at 26 U.K. centers. Within 7 days after surgery (or, if the infection was being managed without surgery, within 7 days after the start of antibiotic treatment), participants were randomly assigned to receive either intravenous or oral antibiotics to complete the first 6 weeks of therapy. Follow-on oral antibiotics were permitted in both groups. The primary end point was definitive treatment failure within 1 year after randomization. In the analysis of the risk of the primary end point, the noninferiority margin was 7.5 percentage points.

Results: Among the 1054 participants (527 in each group), end-point data were available for 1015 (96.3%). Treatment failure occurred in 74 of 506 participants (14.6%) in the intravenous group and 67 of 509 participants (13.2%) in the oral group. Missing end-point data (39 participants, 3.7%) were imputed. The intention-to-treat analysis showed a difference in the risk of definitive treatment failure (oral group vs. intravenous group) of -1.4 percentage points (90% confidence interval [CI], -4.9 to 2.2; 95% CI, -5.6 to 2.9), indicating noninferiority. Complete-case, per-protocol, and sensitivity analyses supported this result. The between-group difference in the incidence of serious adverse events was not significant (146 of 527 participants [27.7%] in the intravenous group and 138 of 527 [26.2%] in the oral group; P=0.58). Catheter complications, analyzed as a secondary end point, were more common in the intravenous group (9.4% vs. 1.0%).

Conclusions: Oral antibiotic therapy was noninferior to intravenous antibiotic therapy when used during the first 6 weeks for complex orthopedic infection, as assessed by treatment failure at 1 year. (Funded by the National Institute for Health Research; OVIVA Current Controlled Trials number, ISRCTN91566927 .).
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http://dx.doi.org/10.1056/NEJMoa1710926DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6522347PMC
January 2019

C-I and C-F bond-breaking dynamics in the dissociative electron ionization of CFI.

Phys Chem Chem Phys 2019 Jul;21(26):14296-14305

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford OX1 3TA, UK.

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF3I at energies ranging from 20 to 100 eV. A beam-gas instrument has been used to measure the absolute total ionization cross-section for the molecule over the energy range from 0 to 300 eV. Coupled with data from an electron-molecule crossed beam velocity-map imaging instrument, this allows absolute partial ionization cross-sections to be determined for formation of each ionic fragment. These reveal a number of fragmentation channels involving both C-I and C-F bond cleavage, in some cases followed by further fragmentation of the resulting molecular ion. Velocity-map images have been recorded for the I+ and CF3+ products of C-I bond cleavage and the CF2I+ products of C-F bond cleavage. Analysis of fragment kinetic energy distributions extracted from the images reveals that CF3+ product of C-I bond cleavage appears to be formed via a statistical mechanism occurring over long timescales, while the CF2I+ products of C-F cleavage are formed via a much faster, more direct dissociation mechanism involving one or more repulsive states of the parent molecular ion. The I+ fragments arising from C-I bond cleavage display behaviour intermediate between the two extremes. For all fragments, the images show little or no dependence on the energy of the incident electron, implying that the initially excited ion state or states undergo rapid relaxation to the dissociative state(s) in all cases. Only a very small fraction of the incident electron's kinetic energy is released into kinetic energy of the recoiling atomic and molecular fragments, implying that most of the available energy remains with the two departing electrons. The kinetic energy distributions obtained for the various fragments of dissociative electron ionization are compared with the corresponding distributions reported from photoionization studies in order to gain insight into the electronic states involved.
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http://dx.doi.org/10.1039/c8cp06682eDOI Listing
July 2019

Coulomb explosion imaging of CHI and CHClI photodissociation dynamics.

J Chem Phys 2018 Nov;149(20):204313

Sorbonne Université, CNRS, Laboratoire de Chimie Physique-Matière et Rayonnement, LCPMR, F-75005 Paris, France.

The photodissociation dynamics of CHI and CHClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide / range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CHClI to be investigated. Furthermore, delay-dependent fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.
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http://dx.doi.org/10.1063/1.5041381DOI Listing
November 2018

Photodissociation of aligned CHI and CHFI molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization.

Struct Dyn 2018 Jan 25;5(1):014301. Epub 2018 Jan 25.

Department of Physics, Lund University, 22100 Lund, Sweden.

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon-iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.
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http://dx.doi.org/10.1063/1.4998648DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5785297PMC
January 2018

Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera.

J Chem Phys 2017 Jul;147(1):013933

Institut für Optik und Atomare Physik, Technische Universität Berlin, 10623 Berlin, Germany.

Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (CHFI) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with a pixel imaging mass spectrometry camera, a time-stamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. We show that we can obtain a high degree of one-and three-dimensional alignment and mixed-field orientation and compare the Coulomb explosion process induced at both wavelengths.
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http://dx.doi.org/10.1063/1.4982220DOI Listing
July 2017

Time-resolved multi-mass ion imaging: Femtosecond UV-VUV pump-probe spectroscopy with the PImMS camera.

J Chem Phys 2017 Jul;147(1):013911

National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada.

The Pixel-Imaging Mass Spectrometry (PImMS) camera allows for 3D charged particle imaging measurements, in which the particle time-of-flight is recorded along with (x, y) position. Coupling the PImMS camera to an ultrafast pump-probe velocity-map imaging spectroscopy apparatus therefore provides a route to time-resolved multi-mass ion imaging, with both high count rates and large dynamic range, thus allowing for rapid measurements of complex photofragmentation dynamics. Furthermore, the use of vacuum ultraviolet wavelengths for the probe pulse allows for an enhanced observation window for the study of excited state molecular dynamics in small polyatomic molecules having relatively high ionization potentials. Herein, preliminary time-resolved multi-mass imaging results from CFI photolysis are presented. The experiments utilized femtosecond VUV and UV (160.8 nm and 267 nm) pump and probe laser pulses in order to demonstrate and explore this new time-resolved experimental ion imaging configuration. The data indicate the depth and power of this measurement modality, with a range of photofragments readily observed, and many indications of complex underlying wavepacket dynamics on the excited state(s) prepared.
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http://dx.doi.org/10.1063/1.4978923DOI Listing
July 2017

Photofragmentation dynamics of N,N-dimethylformamide following excitation at 193 nm.

J Chem Phys 2017 Jul;147(1):013941

Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom.

N,N-dimethylformamide, HCON(CH), is a useful model compound for investigating the peptide bond photofragmentation dynamics. We report data from a comprehensive experimental and theoretical study into the photofragmentation dynamics of N,N-dimethylformamide in the gas phase at 193 nm. Through a combination of velocity-map imaging and hydrogen atom Rydberg tagging photofragment translational spectroscopy we have identified two primary fragmentation channels, namely, fission of the N-CO "peptide" bond and N-CH bond fission leading to the loss of CH. The possible fragmentation channels leading to the observed products are rationalised with recourse to CASPT2 calculations of the ground and first few excited-state potential energy curves along the relevant dissociation coordinates, and the results are compared with the data from previous experimental and theoretical studies on the same system.
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http://dx.doi.org/10.1063/1.4983704DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5446285PMC
July 2017

Physical chemistry: The fingerprints of reaction mechanisms.

Authors:
Claire Vallance

Nature 2017 06;546(7660):608-609

Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, UK.

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http://dx.doi.org/10.1038/546608aDOI Listing
June 2017

Evidence for concerted ring opening and C-Br bond breaking in UV-excited bromocyclopropane.

J Chem Phys 2016 Jun;144(24):244312

School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.

Photodissociation of gaseous bromocyclopropane via its A-band continuum has been studied at excitation wavelengths ranging from 230 nm to 267 nm. Velocity-map images of ground-state bromine atoms (Br), spin-orbit excited bromine atoms (Br(∗)), and C3H5 hydrocarbon radicals reveal the kinetic energies of these various photofragments. Both Br and Br(∗) atoms are predominantly generated via repulsive excited electronic states in a prompt photodissociation process in which the hydrocarbon co-fragment is a cyclopropyl radical. However, the images obtained at the mass of the hydrocarbon radical fragment identify a channel with total kinetic energy greater than that deduced from the Br and Br(∗) images, and with a kinetic energy distribution that exceeds the energetic limit for Br + cyclopropyl radical products. The velocity-map images of these C3H5 fragments have lower angular anisotropies than measured for Br and Br(∗), indicating molecular restructuring during dissociation. The high kinetic energy C3H5 signals are assigned to allyl radicals generated by a minor photochemical pathway which involves concerted C-Br bond dissociation and cyclopropyl ring-opening following single ultraviolet (UV)-photon absorption. Slow photofragments also contribute to the velocity map images obtained at the C3H5 radical mass, but the corresponding slow Br atoms are not observed. These features in the images are attributed to C3H5 (+) from the photodissociation of the C3H5Br(+) molecular cation following two-photon ionization of the parent compound. This assignment is confirmed by 118-nm vacuum ultraviolet ionization studies that prepare the molecular cation in its ground electronic state prior to UV photodissociation.
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http://dx.doi.org/10.1063/1.4954373DOI Listing
June 2016

Open-access microcavities for chemical sensing.

Nanotechnology 2016 Jul 31;27(27):274003. Epub 2016 May 31.

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford OX1 3TA, UK.

The recent development of open-access optical microcavities opens up a number of intriguing possibilities in the realm of chemical sensing. We provide an overview of the different possible sensing modalities, with examples of refractive index sensing, optical absorption measurements, and optical tracking and trapping of nanoparticles. The extremely small mode volumes within an optical microcavity allow very small numbers of molecules to be probed: our current best detection limits for refractive index and absorption sensing are around 10(5) and 10(2) molecules, respectively, with scope for further improvements in the future.
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http://dx.doi.org/10.1088/0957-4484/27/27/274003DOI Listing
July 2016

Gas-Phase Retro-Diels-Alder Reactions of Cyclohexene, 1-Methylcyclohexene, and 4-Methylcyclohexene following Photoexcitation at 193 nm: A Velocity-Map Imaging Study.

J Phys Chem A 2015 Dec 16;119(50):12218-23. Epub 2015 Sep 16.

Department of Chemistry, University of Oxford, Chemistry Research Laboratory , 12 Mansfield Road, Oxford OX1 3TA, U.K.

We present the results of a velocity-map imaging study into the retro-Diels-Alder (RDA) reactions of cyclohexene, 1-methylcyclohexene, and 4-methylcyclohexene following photoexcitation at 193 nm. Universal detection of all neutral fragments via vacuum-ultraviolet photoionization at 118 nm allows imaging of both RDA fragments in all cases. Fragment kinetic energy distributions reveal contributions from both dissociative ionization and retro-Diels-Alder reaction of the respective parent molecules, yielding reaction products with a high degree of internal excitation. Together with the observed isotropic product angular distributions, this is consistent with a mechanism in which the RDA reaction occurs from high vibrational levels of the electronic ground state following internal conversion from a higher-lying state initially populated in the photoexcitation process, as predicted by frontier molecular orbital theory. Velocity-map images and total translational energy distributions for the RDA products of 1-methylcyclohexene and 4-methylcyclohexene are very similar to those for unsubstituted cyclohexene, indicating that methyl substitution either adjacent to or far from the double bond has little effect on the dynamics of the RDA process.
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http://dx.doi.org/10.1021/acs.jpca.5b06185DOI Listing
December 2015

Dynamics of the A-band ultraviolet photodissociation of methyl iodide and ethyl iodide via velocity-map imaging with 'universal' detection.

Phys Chem Chem Phys 2015 Feb;17(6):4096-106

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford, OX1 3TA, UK.

We report data from a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at several wavelengths in the range 236-266 nm, within their respective A-bands. The use of non-resonant single-photon ionization at 118.2 nm allows detection and velocity-map imaging of all fragments, regardless of their vibrotational or electronic state. The resulting photofragment kinetic energy and angular distributions and the quantum yields of ground-state and spin-orbit excited iodine fragments are in good agreement with previous studies employing state-selective detection via REMPI. The data are readily rationalised in terms of three competing dissociation mechanisms. The dominant excitation at all wavelengths studied is via a parallel transition to the (3)Q0 state, which either dissociates directly to give an alkyl radical partnered by spin-orbit excited iodine, or undergoes radiationless transfer to the (1)Q1 potential surface, where it dissociates to an alkyl radical partnered by iodine in its electronic ground state. Ground state iodine atoms can also be formed by direct dissociation from the (1)Q1 or (3)Q1 excited states following perpendicular excitation at the shorter and longer wavelength region, respectively, in the current range of interest. The extent of internal excitation of the alkyl fragment varies with dissociation mechanism, and is considerably higher for ethyl fragments from ethyl iodide photolysis than for methyl fragments from methyl iodide photolysis. We discuss the relative advantages and disadvantages of single-photon vacuum-ultraviolet ionization relative to the more widely used REMPI detection schemes, and conclude, in agreement with others, that single-photon ionization is a viable detection method for photofragment imaging studies, particularly when studying large molecules possessing multiple fragmentation channels.
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http://dx.doi.org/10.1039/c4cp04654dDOI Listing
February 2015

An introduction to velocity-map imaging mass spectrometry (VMImMS).

Eur J Mass Spectrom (Chichester) 2014 ;20(2):117-29

This account introduces a new variant of time-of-flight mass spectrometry, termed velocity-map imaging mass spectrometry (VMImMS). While the ion abundances recorded in conventional ToF-MS measurements are highly useful for molecular quantification and structure determination, the final parent and fragment ion yields are Largely blind to the dynamics of the processes in which the ions were formed inside the mass spectrometer. By recording the velocity distribution of each ion in tandem with the mass spectrum, not only can the details of the dissociative ionisation dynamics be unravelled, but the extra dimensions of information can be used for enhanced molecular fingerprinting, separating contributions from ions with identical mass-to-charge ratio and resolving components within mixtures, to name but a few examples. Measuring ion-velocity distributions within a mass spectrometry measurement is not new, but incorporating imaging techniques developed within the reaction dynamics community provides vastly improved velocity resolution for all ions simultaneously in a single-stage instrument. This account provides an introduction to VMImMS, outlines the fundamental instrumentation and detector requirements and the challenges associated with developing the method further, and details proof-of-concept work from our Laboratory on a number of potential applications of the technique.
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http://dx.doi.org/10.1255/ejms.1264DOI Listing
June 2014

Absolute electron total ionization cross-sections: molecular analogues of DNA and RNA nucleobase and sugar constituents.

Phys Chem Chem Phys 2014 Jun;16(22):10743-52

Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, UK.

Accurate ionization cross-sections for DNA and RNA constituents in the condensed or aqueous phase are important parameters for models simulating radiation damage to genetic material in living cells. In this work, absolute gas-phase electron total ionization cross-sections (TICSs) have been measured for a series of six aromatic and eight non-aromatic cyclic species that can be considered as prototype functional group analogues for the nucleobases and sugar backbone constituents of DNA and RNA. TICSs for water, hexane, and ethylacetamide (a peptide bond analogue) are also reported. The experimental apparatus utilizes a cylindrical ion collector that surrounds the ionization region, providing essentially unit detection efficiency. Two theoretical models, the polarizability-correlation method and binary-encounter Bethe theory, are able to reproduce the measured maximum TICS well for all species studied. An empirical energy-dependent correction is found to yield improvement in the agreement between experimental energy-dependent cross sections and the predictions of the BEB model. Having characterised and optimised the performance of both models, they are then used to predict TICSs for gas-phase DNA and RNA nucleobases and sugars. Direct experimental determinations of TICSs for these species are difficult because of their low volatility, which makes it difficult to prepare suitable gas-phase samples for measurement.
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http://dx.doi.org/10.1039/c4cp00490fDOI Listing
June 2014

Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study.

Phys Chem Chem Phys 2014 Feb;16(5):2167-78

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford OX1 3TA, UK.

The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br(+) and C2H5I(+)) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin-orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin-orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC-Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br(+) cations undergo prompt C-Br bond fission to form predominantly C2H5(+) + Br* products with a near-limiting 'parallel' recoil velocity distribution. The observed C2H3(+) + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5(+) products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I(+), along with an additional energetically accessible C-I bond fission channel to form C2H5 + I(+) products. HX (X = Br, I) elimination from the highly internally excited C2H5X(+) cation is deemed the most probable route to forming the C2H4(+) fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C-C bond fission process to form CH2X(+) + CH3 products.
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http://dx.doi.org/10.1039/c3cp53970aDOI Listing
February 2014

Exploring surface photoreaction dynamics using pixel imaging mass spectrometry (PImMS).

J Chem Phys 2013 Aug;139(8):084202

Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, USA.

A new technique for studying surface photochemistry has been developed using an ion imaging time-of-flight mass spectrometer in conjunction with a fast camera capable of multimass imaging. This technique, called pixel imaging mass spectrometry (PImMS), has been applied to the study of butanone photooxidation on TiO2(110). In agreement with previous studies of this system, it was observed that the main photooxidation pathway for butanone involves ejection of an ethyl radical into vacuum which, as confirmed by our imaging experiment, undergoes fragmentation after ionization in the mass spectrometer. This proof-of-principle experiment illustrates the usefulness and applicability of PImMS technology to problems of interest within the surface science community.
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http://dx.doi.org/10.1063/1.4818997DOI Listing
August 2013

Fast sensors for time-of-flight imaging applications.

Phys Chem Chem Phys 2014 Jan;16(2):383-95

Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford OX1 3TA, UK.

The development of sensors capable of detecting particles and radiation with both high time and high positional resolution is key to improving our understanding in many areas of science. Example applications of such sensors range from fundamental scattering studies of chemical reaction mechanisms through to imaging mass spectrometry of surfaces, neutron scattering studies aimed at probing the structure of materials, and time-resolved fluorescence measurements to elucidate the structure and function of biomolecules. In addition to improved throughput resulting from parallelisation of data collection - imaging of multiple different fragments in velocity-map imaging studies, for example - fast image sensors also offer a number of fundamentally new capabilities in areas such as coincidence detection. In this Perspective, we review recent developments in fast image sensor technology, provide examples of their implementation in a range of different experimental contexts, and discuss potential future developments and applications.
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http://dx.doi.org/10.1039/c3cp53183jDOI Listing
January 2014

Quantification of ions with identical mass-to-charge (m/z) ratios by velocity-map imaging mass spectrometry.

Phys Chem Chem Phys 2013 Sep 16;15(33):13796-800. Epub 2013 Jul 16.

Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Oxford, UK.

By integrating a velocity-map imaging lens and position sensitive detector into an electron-impact time-of-flight mass spectrometer, it becomes possible to record ion kinetic energy release (KER) distributions for each fragment ion alongside the time-of-flight mass spectrum. The KER distributions allow ions of identical mass-to-charge ratio to be distinguished and quantified.
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http://dx.doi.org/10.1039/c3cp52219aDOI Listing
September 2013

Multimass velocity-map imaging with the Pixel Imaging Mass Spectrometry (PImMS) sensor: an ultra-fast event-triggered camera for particle imaging.

J Phys Chem A 2012 Nov 5;116(45):10897-903. Epub 2012 Nov 5.

Rutherford Appleton Laboratory, Chilton, Didcot, UK.

We present the first multimass velocity-map imaging data acquired using a new ultrafast camera designed for time-resolved particle imaging. The PImMS (Pixel Imaging Mass Spectrometry) sensor allows particle events to be imaged with time resolution as high as 25 ns over data acquisition times of more than 100 μs. In photofragment imaging studies, this allows velocity-map images to be acquired for multiple fragment masses on each time-of-flight cycle. We describe the sensor architecture and present bench-testing data and multimass velocity-map images for photofragments formed in the UV photolysis of two test molecules: Br(2) and N,N-dimethylformamide.
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http://dx.doi.org/10.1021/jp309860tDOI Listing
November 2012

A new detector for mass spectrometry: direct detection of low energy ions using a multi-pixel photon counter.

Rev Sci Instrum 2012 Jan;83(1):013304

Department of Chemistry, University of Oxford, Oxford, United Kingdom.

A new type of ion detector for mass spectrometry and general detection of low energy ions is presented. The detector consists of a scintillator optically coupled to a single-photon avalanche photodiode (SPAD) array. A prototype sensor has been constructed from a LYSO (Lu(1.8)Y(0.2)SiO(5)(Ce)) scintillator crystal coupled to a commercial SPAD array detector. As proof of concept, the detector is used to record the time-of-flight mass spectra of butanone and carbon disulphide, and the dependence of detection sensitivity on the ion kinetic energy is characterised.
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http://dx.doi.org/10.1063/1.3676164DOI Listing
January 2012

Absolute total electron impact ionization cross-sections for many-atom organic and halocarbon species.

J Phys Chem A 2012 Jan 22;116(1):767-77. Epub 2011 Dec 22.

Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom.

The experimental determination of absolute total electron impact ionization cross-sections for polyatomic molecules has traditionally been a difficult task and restricted to a small range of species. This article reviews the performance of three models to estimate the maximum ionization cross-sections of some 65 polyatomic organic and halocarbon species. Cross-sections for all of the species studied have been measured experimentally using the same instrument, providing a complete data set for comparison with the model predictions. The three models studied are the empirical correlation between maximum ionization cross-section and molecular polarizability, the well-known binary encounter Bethe (BEB) model, and the functional group additivity model. The excellent agreement with experiment found for all three models, provided that calculated electronic structure parameters of suitably high quality are used for the first two, allows the prediction of total electron-impact ionization cross-sections to at least 7% precision for similar molecules that have not been experimentally characterized.
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http://dx.doi.org/10.1021/jp210294pDOI Listing
January 2012
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