Publications (35)233.19 Total impact
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ABSTRACT: Deceleration of supersonic beams of SD radicals by a linear Stark decelerator was demonstrated for the first time. SD radicals produced by an electric discharge of supersonic beams mainly occupied the lowest two rotational states of (Formula presented.) and (Formula presented.) in the 2Π3/2 state. Laserinduced fluorescence of the decelerated beam showed that, due to the proximity of the Stark shift between these two rotational states, the final radical packet still contained both the (Formula presented.) and (Formula presented.) rotational states when the decelerator was operated at a constant phase below 50°. We show that by controlling the timing sequence of the Stark decelerator, it is possible to create a decelerated molecular packet containing only a single rotational state of either (Formula presented.) or (Formula presented.) of the SD radicals with a tunable velocity. The creation of a cold radical beam containing a single rotation–vibration state will be a useful tool for the study of cold collisions and reactions in the quantum regime.  [Show abstract] [Hide abstract]
ABSTRACT: The absolute density of SD radicals in a supersonic jet has been measured down to $(1.1\pm0.1)\times10^5$ cm$^{3}$ in a modestly specified apparatus that uses a crosscorrelated combination of cavity ringdown and laserinduced fluorescence detection. Such a density corresponds to $215\pm21$ molecules in the probe volume at any given time. The minimum detectable absorption coefficient was quantumnoiselimited and measured to be $(7.9\pm0.6)\times10^{11}$ cm$^{1}$, in 200 s of acquisition time, corresponding to a noiseequivalent absorption sensitivity for the apparatus of $(1.6\pm0.1)\times10^{9}$ cm$^{1}$ Hz$^{1/2}$.  [Show abstract] [Hide abstract]
ABSTRACT: We describe a novel experimental setup that combines the advantages of both laserinduced fluorescence and cavity ringdown techniques. The simultaneous and correlated measurement of the ringdown and fluorescence signals yields absolute absorption coefficients for the fluorescence measurement. The combined measurement is conducted with the same sample in a single, pulsed laser beam. The fluorescence measurement extends the dynamic range of a standalone cavity ringdown setup from typically three to at least six orders of magnitude. The presence of the cavity improves the quality of the signal, in particular the signaltonoise ratio. The methodology, dubbed cavityenhanced laserinduced fluorescence (CELIF), is developed and rigorously tested against the spectroscopy of 1,4bis(phenylethynyl)benzene in a molecular beam and density measurements in a cell. We outline how the method can be utilised to determine absolute quantities: absorption cross sections, sample densities and fluorescence quantum yields.  [Show abstract] [Hide abstract]
ABSTRACT: The production of a translationally cold (T < 1 K) sample of bromine atoms with estimated densities of up to 10(8) cm(3) using photodissociation is presented. A molecular beam of Br(2) seeded in Kr is photodissociated into Br + Br* fragments, and the velocity distribution of the atomic fragments is determined using (2 + 1) REMPI and velocity map ion imaging. By recording images with varying delay times between the dissociation and probe lasers, we investigate the length of time after dissociation for which atoms remain in the laser focus, and determine the velocity spread of those atoms. By careful selection of the photolysis energy, it is found that a fraction of the atoms can be detected for delay times in excess of 100 μs. These are atoms for which the fragment recoil velocity vector is directly opposed and equal in magnitude to the parent beam velocity leading to a resultant lab frame velocity of approximately zero. The FWHM velocity spreads of detected atoms along the beam axis after 100 μs are less than 5 ms(1), corresponding to temperatures in the milliKelvin range, opening the possibility that this technique could be utilized as a slow Br atom source. 
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ABSTRACT: We have demonstrated a new, accessible and economical technique, dubbed photostop, for producing high densities of trappable molecules. Direct measurements are presented of NO molecules produced with a narrow velocity distribution centered at zero in the laboratory frame. NO2, initially cooled in a pulsed molecular beam, is photodissociated such that the recoil velocity of the NO photofragments cancels out the velocity of the beam. NO(X^2Pi_3/2, v=0, J=1.5) molecules are observed up to 10 mircoseconds after the dissociation event in the probe volume at an estimated density of 1E7 cm3 per quantum state and at a translational temperature of 1.6 K. Through the choice of suitable precursors, photostop has the potential to extend the list atoms and molecules that can be slowed or trapped. It should be possible to accumulate density in a trap through consecutive loading of multiple pulses.  [Show abstract] [Hide abstract]
ABSTRACT: A method to reconstruct full threedimensional photofragment distributions from their twodimensional (2D) projection onto a detection plane is presented, for processes in which the expanding Newton sphere has cylindrical symmetry around an axis parallel to the projection plane. The method is based on: (1) onionpeeling in polar coordinates [Zhao et al., Rev. Sci. Instrum. 73, 3044 (2002)] in which the contribution to the 2D projection from events outside the plane bisecting the Newton sphere are subtracted in polar coordinates at incrementally decreasing radii; and (2) ideas borrowed from the basis set expansion (pBASEX) method in polar coordinates [Garcia et al., Rev. Sci. Instrum. 75, 4989 (2004)], which we use to generate 2D projections at each incremental radius for the subtraction. Our method is as good as the pBASEX method in terms of accuracy, is devoid of centerline noise common to reconstruction methods employing Cartesian coordinates; and it is computationally cheap allowing images to be reconstructed as they are being acquired in a typical imaging experiment.  [Show abstract] [Hide abstract]
ABSTRACT: Photoinduced Fetobpy charge transfer in [{Cp(dppe)Fe}(muC[triple bond]CC[triple bond]N){Re(CO)(3)(bpy)}]PF(6) has been observed by psTRIR spectroscopy, supported by UVVis/IR spectroelectrochemistry and DFT calculations.  [Show abstract] [Hide abstract]
ABSTRACT: Vibrationally inelastic scattering is a fundamental collision process that converts some of the kinetic energy of the colliding partners into vibrational excitation(,). The conventional wisdom is that collisions with high impact parameters (where the partners only 'graze' each other) are forward scattered and essentially elastic, whereas collisions with low impact parameters transfer a large amount of energy into vibrations and are mainly back scattered. Here we report experimental observations of exactly the opposite behaviour for the simplest and most studied of all neutralneutral collisions: we find that the inelastic scattering process H + D(2)(v = 0, j = 0, 2) > H + D(2)(v' = 3, j' = 0, 2, 4, 6, 8) leads dominantly to forward scattering (v and j respectively refer to the vibrational and rotational quantum numbers of the D(2) molecule). Quasiclassical trajectory calculations show that the vibrational excitation is caused by extension, not compression, of the DD bond through interaction with the passing H atom. However, the HD interaction never becomes strong enough for capture of the H atom before it departs with diminished kinetic energy; that is, the inelastic scattering process is essentially a frustrated reaction in which the collision typically excites the outwardgoing half of the HDD symmetric stretch before the HD(2) complex dissociates. We suggest that this 'tug of war' between H and D(2) is a new mechanism for vibrational excitation that should play a role in all neutralneutral collisions where strong attraction can develop between the collision partners.  [Show abstract] [Hide abstract]
ABSTRACT: The timedelayed forward scattering mechanism recently identified by Althorpe et al. [Nature (London) 416, 67 (2002)] for the H+D(2)(v=0,j=0)>HD(v(')=3,j(')=0)+D reaction was analyzed by using quasiclassical trajectory (QCT) methodology. The QCT results were found to match the quantum wavepacket snapshots of Althorpe et al., albeit without the quantum scattering effects. Trajectories were analyzed on the fly to investigate the dynamics of the atoms during the reaction. The dominant reaction mechanism progresses from hard collinear impacts, leading to direct recoil, toward glancing impacts. The increased time required for forward scattered trajectories is due to the rotation of the transient HDD complex. Forward scattered trajectories display symmetric stretch vibrations of the transient HDD complex, a signature of the presence of a resonance, or a quantum bottleneck state.  [Show abstract] [Hide abstract]
ABSTRACT: A quasiclassical trajectory study of the state specific H+D(2)(upsilon = 0,j = 0) > HD(upsilon' = 0,j' = 0) + D reaction at a collision energy of 1.85 eV (total energy of 2.04 eV) found that the scattering is governed by two unexpected and dominant new mechanisms, and not by direct recoil as is generally assumed. The new mechanisms involve strong interaction with the sloping potential around the conical intersection, an area of the potential energy surface not previously considered to have much effect upon reactive scattering. Initial investigations indicate that more than 50% of reactive scattering could be the result of these new mechanisms at this collision energy. Features in the corresponding quantum mechanical results can be attributed to these new (classical) reaction mechanisms.  [Show abstract] [Hide abstract]
ABSTRACT: Introduction Unwinding the Nuclear Wave Function Application to the HydrogenExchange Reaction Further Aspects of Topology Outlook and Conclusions 
Article: On the dynamics of the H++D2(v = 0,j = 0)→HD+D+ reaction: A comparison between theory and experiment
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ABSTRACT: The H++D2(v = 0,j = 0)→HD+D+ reaction has been theoretically investigated by means of a time independent exact quantum mechanical approach, a quantum wave packet calculation within an adiabatic centrifugal sudden approximation, a statistical quantum model, and a quasiclassical trajectory calculation. Besides reaction probabilities as a function of collision energy at different values of the total angular momentum, J, special emphasis has been made at two specific collision energies, 0.1 and 0.524 eV. The occurrence of distinctive dynamical behavior at these two energies is analyzed in some detail. An extensive comparison with previous experimental measurements on the Rydberg H atom with D2 molecules has been carried out at the higher collision energy. In particular, the present theoretical results have been employed to perform simulations of the experimental kinetic energy spectra. 
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ABSTRACT: A recent puzzle in nonadiabatic quantum dynamics is that geometric phase (GP) effects are present in the statetostate opacity functions of the hydrogenexchange reaction, but cancel out in the statetostate integral cross sections (ICSs). Here the authors explain this result by using topology to separate the scattering amplitudes into contributions from Feynman paths that loop in opposite senses around the conical intersection. The clockwiselooping paths pass over one transition state (1TS) and scatter into positive deflection angles; the counterclockwiselooping paths pass over two transition states (2TS) and scatter into negative deflection angles. The interference between the 1TS and 2TS paths thus integrates to a very small value, which cancels the GP effects in the ICS. Quasiclassical trajectory (QCT) calculations reproduce the scattering of the 1TS and 2TS paths into positive and negative deflection angles and show that the 2TS paths describe a direct insertion mechanism. The inserting atom follows a highly constrained "Sbend" path, which allows it to avoid both the other atoms and the conical intersection and forces the product diatom to scatter into high rotational states. By contrast, the quantum 2TS paths scatter into a mainly statistical distribution of rotational states, so that the quantum 2TS total ICS is roughly twice the QCT ICS at 2.3 eV total energy. This suggests that the Sbend constraint is relaxed by tunneling in the quantum system. These findings on H+H(2) suggest that similar cancellations or reductions in GP effects are likely in many other reactions.  [Show abstract] [Hide abstract]
ABSTRACT: The torsional motions of jetcooled 1,4bis(phenylethynyl)benzene (BPEB), a prototype molecular wire, were studied using cavity ringdown spectroscopy in the first UV absorption band (316321 nm). The torsional spectrum of 1,4bis(phenylethynyl)2,3,5,6tetradeuteriobenzene was also recorded in the gas phase. Both spectra were successfully simulated using simple cosine potentials to describe the torsional motions. The groundstate barrier to rotation was estimated to be 220235 cm(1), which is similar to that of diphenylacetylene (tolane). Complementary DFT calculations were found to overestimate the torsional barrier.  [Show abstract] [Hide abstract]
ABSTRACT: The crossing of two electronic potential surfaces (a conical intersection) should result in geometric phase effects even for molecular processes confined to the lower surface. However, recent quantum simulations of the hydrogen exchange reaction (H + H2 > H2 + H) have predicted a cancellation in such effects when product distributions are integrated over all scattering angles. We used a simple topological argument to extract reaction paths with different senses from a nuclear wave function that encircles a conical intersection. In the hydrogenexchange reaction, these senses correspond to paths that cross one or two transition states. These two sets of paths scatter their products into different regions of space, which causes the cancellation in geometric phase effects. The analysis should generalize to other direct reactions.  [Show abstract] [Hide abstract]
ABSTRACT: The scattering of highly excited hydrogen Rydberg atoms, H* (n = 36), with deuterium molecules in their rovibrational ground state, D2(v = 0, j = 0), has been investigated at a relative collision energy of 0.53 eV. Timeofflight distributions of elastically/inelastically scattered H* Rydberg atoms and reactively scattered D* Rydberg atoms have been measured at different laboratory angles. The extracted rovibrationally resolved state distributions of the HD product molecules from reactive collisions resemble closely those reported for the corresponding ionmolecule reaction, H+ + D2 > HD + D+. This similarity is rationalised using the free electron model which predicts that the Rydberg electron acts as a spectator while the ionic reaction takes place.  [Show abstract] [Hide abstract]
ABSTRACT: The photodissociation of jetcooled BrCl molecules has been investigated at many different excitation wavelengths in the range 330–570 nm by velocity map imaging of the ground state Br and ground and spin–orbit excited Cl atom products. Image analysis confirms literature values for the energies of the ground, Br(2P3/2)+Cl(2P3/2), and first excited [Br(2P3/2)+Cl(2P1/2)] dissociation asymptotes, and provides measures of the electronic branching into these two active product channels, and the recoil anisotropy of each set of products, as a function of parent vibrational level (v″ ⩽ 2) and excitation wavelength. The availability of such experimental data allows mapping of the partial crosssections for parallel (i.e., ΔΩ=0) and perpendicular (i.e., ΔΩ=±1) absorption as a function of excitation energy, and thus deconvolution of the room temperature parent absorption spectrum into contributions associated with excitation to the A 3Π(1), B 3Π(0+), and C 1Π(1) excited states of BrCl. This analysis of the continuous absorption spectrum of BrCl, when considered in conjuction with existing spectroscopic data for the ground electronic state and for the bound levels supported by the B state potential, allows determination of key regions of the potential energy curves for, and transition moments to, each of these three excited states. Further wave packet calculations, which reproduce the experimentally measured wavelength dependent product channel branching ratios and product recoil anisotropies very well, serve to validate the excited state potential energy functions so derived and allow estimation of the strength (∼80 cm−1) of the coupling between the bound (B) and dissociative (Y) diabatic states of 0+ symmetry. © 2003 American Institute of Physics.  [Show abstract] [Hide abstract]
ABSTRACT: The photodissociations of jetcooled IBr and Br2 molecules have been investigated using high resolution ion imaging methods, at excitation energies just above the thresholds for forming, respectively, I(2P3/2o)+Br(2P3/2o) and Br(2P3/2o)+Br∗(2P1/2o) products from parent molecules in their v″ = 0 levels. For such molecules, we observe in both cases, that fragments with larger recoil velocities have markedly reduced angular anisotropy, whereas those from photolysis of IBr molecules with v″ = 1 show an essentially constant, limiting anisotropy. Given the monochromaticity of the photolysis radiation, increased recoil velocity of fragments resulting from photolysis of v″ = 0 molecules can only be derived from increased parent internal (rotational) energy. The measurements thus provide a particularly clear and direct observation of the breakdown of the axial recoil approximation as applied to the photodissociation of a diatomic molecule, and have been modeled, quantitatively, using both quantum and semiclassical methods together with the best available potential energy curves for the relevant excited states of IBr and Br2.
Publication Stats
1k  Citations  
233.19  Total Impact Points  
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Institutions

20022015

Durham University
 Department of Chemistry
Durham, England, United Kingdom


2011

University of Oxford
 Chemical Research Laboratory
Oxford, England, United Kingdom


2008

Leiden University
Leyden, South Holland, Netherlands


19952005

Bielefeld University
 Faculty of Physics
Bielefeld, North RhineWestphalia, Germany


19982002

University of Bristol
 School of Chemistry
Bristol, ENG, United Kingdom
