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Richard Newland Dixon began his independent scientific career as a molecular spectroscopist in Sheffield, where he demonstrated new ways to measure, analyse and interpret the electronic spectra of small gas phase species. He later moved to the chair of theoretical chemistry at the University of Bristol, where he embraced new experimental methods, particularly techniques enabled by continuous wave and pulsed lasers, and honed his theoretical skills in angular momentum algebra and time-dependent quantum mechanics. This blend of experimental and theoretical expertise allowed him to make pioneering contributions in the field of gas phase molecular photodissociation dynamics. Richard will be remembered as a polite and modest man, and a very sharp scientist. Among his many lasting scientific achievements, he will be remembered for developing strategies for determining state-resolved correlations between the vectorial properties of the products of molecular photofragmentation processes, and for being one of the first to recognize the near ubiquitous importance of non-adiabatic couplings between potential energy surfaces in rationalizing the dynamics of such dissociations.

Photoinduced isomerization reactions, including ring-opening reactions, lie at the heart of many processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics unfolding on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free electron laser to trace the ultrafast ring opening of gas phase thiophenone molecules following photoexcitation at 265 nm. When combined with cutting edge ab initio electronic structure and molecular dynamics calculations of both the excited and ground state molecules, the results provide unprecedented insights into both electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state is shown to be driven by ballistic SC bond extension and to be complete within 350 femtoseconds. Theory and experiment also allow clear visualization of the rich ground-state dynamics involving formation of, and interconversion between, several ring opened isomers and the reformed cyclic structure, and fragmentation (CO loss) over much longer timescales., Comment: 40 pages, 21 figures Changes from the previous version: 1) Added theoretical calculations for explaining long timescale changes in shown in Figure 4(a). 2) Reworked on fitting (modelling) of the experimental data in Figure 2(b). Added another panel i.e. Figure 2(c). 3) Other minor changes and rewording in response to questions and suggestions by the referees

A microwave (MW) activated hydrogen plasma operating under conditions relevant to contemporary diamond chemical vapor deposition reactors has been investigated using a combination of experiment and self-consistent 2-D modeling. The experimental study returns spatially and wavelength resolved optical emission spectra of the d → a (Fulcher), G → B, and e → a emissions of molecular hydrogen and of the Balmer-α emission of atomic hydrogen as functions of pressure, applied MW power, and substrate diameter. The modeling contains specific blocks devoted to calculating (i) the MW electromagnetic fields (using Maxwell’s equations) self-consistently with (ii) the plasma chemistry and electron kinetics, (iii) heat and species transfer, and (iv) gas–surface interactions. Comparing the experimental and model outputs allows characterization of the dominant plasma (and plasma emission) generation mechanisms, identifies important coupling reactions between hydrogen atoms and molecules (e.g., the quenching of H(n > 2) atoms and electronically excited H2 molecules (H2*) by the alternate ground-state species and H3+ ion formation by the associative ionization reaction of H(n = 2) atoms with H2), and illustrates how spatially resolved H2* (and Hα) emission measurements offer a detailed and sensitive probe of the hyperthermal component of the electron energy distribution function.

We demonstrate experimentally that photodissociation of amorphous solid water at 100 K results in formation of H2 molecules with an ortho/para ratio of gOPR = 3. Two distinct mechanisms can be identified: endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields translationally and internally hot H2. These results are in accord with predictions by molecular dynamics calculations and their astrophysical implications are discussed.

The photoisomerisation dynamics following excitation to the S1 electronic state of two structurally related heterocyclic molecules, α-pyrone and coumarin, in acetonitrile solution have been probed by time-resolved vibrational absorption spectroscopy. Following irradiation at 310 nm, α-pyrone relaxes rapidly from its initially excited state, with a quantum yield for parent molecule reformation of 68%. Probing the antisymmetric ketene stretch region between 2100 cm−1 and 2150 cm−1 confirms the presence of at least two isomeric ring-opened photoproducts, which are formed highly vibrationally excited and relax on a picosecond timescale. Following vibrational cooling, a secondary, thermally driven, isomerisation is observed with a 1.8(1) ns time constant. In contrast, coumarin reforms the parent molecule with essentially 100% efficiency following excitation at 330 nm. The conical intersections driving the non-radiative relaxation of α-pyrone have been investigated using an automated search algorithm. The two lowest energy conical intersections possess remarkably similar structures to the two energetically accessible conical intersections reported previously for coumarin, suggesting that the differing photochemistry is the result of dynamical effects occurring after passage through these intersections.

We report a combined experimental/modeling study of microwave activated dilute N2/H2 and NH3/H2 plasmas as a precursor to diagnosis of the CH4/N2/H2 plasmas used for the chemical vapor deposition (CVD) of N-doped diamond. Absolute column densities of H(n = 2) atoms and NH(X3Σ–, v = 0) radicals have been determined by cavity ring down spectroscopy, as a function of height (z) above a molybdenum substrate and of the plasma process conditions, i.e., total gas pressure p, input power P, and the nitrogen/hydrogen atom ratio in the source gas. Optical emission spectroscopy has been used to investigate variations in the relative number densities of H(n = 3) atoms, NH(A3Π) radicals, and N2(C3Πu) molecules as functions of the same process conditions. These experimental data are complemented by 2-D (r, z) coupled kinetic and transport modeling for the same process conditions, which consider variations in both the overall chemistry and plasma parameters, including the electron (Te) and gas (T) temperatures, the electron density (ne), and the plasma power density (Q). Comparisons between experiment and theory allow refinement of prior understanding of N/H plasma-chemical reactivity, and its variation with process conditions and with location within the CVD reactor, and serve to highlight the essential role of metastable N2(A3Σ+u) molecules (formed by electron impact excitation) and their hitherto underappreciated reactivity with H atoms, in converting N2 process gas into reactive NHx (x = 0–3) radical species.

We report sub-diffraction limited patterning of Si substrate surfaces by laser-initiated liquid-assisted colloidal lithography. The technique involves exposing a two-dimensional lattice of transparent colloidal particles spin coated on the substrate of interest (here Si) immersed in a liquid (e.g. methanol, acetone, carbon tetrachloride, toluene) to a single picosecond pulse of ultraviolet laser radiation. Surface patterns formed using colloidal particles with different radii in the range 195 nm ≤ R ≤ 1.5 μm and liquids with differing indices of refraction (nliquid) are demonstrated, the detailed topographies of which are sensitively dependent upon whether the index of refraction of the colloidal particle (ncolloid) is greater or smaller than nliquid (i.e. upon whether the incident light converges or diverges upon interaction with the particle). The spatial intensity modulation formed by diffraction of the single laser pulse by the colloidal particles is imprinted into the Si substrate.

We report a combination of experimental (velocity map imaging measurements of the methyl (Me) radical products) and ab initio electronic structure studies that explore the influence of substituents (Y) on the dynamics of S-Me bond fission following excitation to the first excited S1 states of thioanisole and three 4-substituted thioanisoles (4-YPhSMe, with Y = H, Me, MeO and CN). In all bar the case that Y = CN, the resulting 4-YPhS products are found to be formed predominantly in their excited (Ã) electronic state. In all cases, the relative yield of X[combining tilde] state products increases upon tuning to shorter excitation wavelengths and, in the specific case of bare thioanisole (as found previously by Lim and Kim, Nat. Chem., 2010, 2, 627), jumps when exciting on the parent resonance assigned to the S1(v7a = 1) level. Two conical intersections (CIs) in the RS-Me stretch coordinate are crucial to rationalising all of the observed dynamics. The first, (CI-1, between the diabatic (1)ππ* and dissociative (1)πσ* potential energy surfaces (PESs) at RS-Me∼ 2 Å) lies above the S1(v = 0) level in energy, and the calculated minimum energy path through this barrier involves substantial deviations from planarity in all bar 4-CNPhSMe. Beyond this barrier, the potential is quite steeply repulsive, and Me + 4-YPhS(Ã) products are the inevitable products if the molecular framework is unable to re-planarise within the time it takes for the dissociating molecules to pass through the region of CI-2 (between the diabatic (1)πσ* and ground (S0) states) where the product electronic branching is determined. The gradual increase in the yield of 4-YPhS(X[combining tilde]) radicals upon tuning to shorter photolysis wavelengths, the much increased branching into PhS(X[combining tilde]) products when exciting the PhSMe (S1, v7a = 1) level and the dominance of 4-CNPhS(X[combining tilde]) products in the specific case that Y = CN can all be understood in terms of a (relative) lowering of the effective barrier associated with CI-1, thereby allowing access to the dissociative region of the PES at closer-to-planar geometries.

Velocity map imaging methods, with a new and improved ion optics design, have been used to explore the near ultraviolet photodissociation dynamics of gas phase 2-bromo- and 2-iodothiophene molecules. In both cases, the ground (X) and spin-orbit excited (X*) (where X = Br, I) atom products formed at the longest excitation wavelengths are found to recoil with fast, anisotropic velocity distributions, consistent with prompt C-X bond fission following excitation via a transition whose dipole moment is aligned parallel to the breaking bond. Upon tuning to shorter wavelengths, this fast component fades and is progressively replaced by a slower, isotropic recoil distribution. Complementary electronic structure calculations provide a plausible explanation for this switch in fragmentation behaviour-namely, the opening of a rival C-S bond extension pathway to a region of conical intersection with the ground state potential energy surface. The resulting ground state molecules are formed with more than sufficient internal energy to sample the configuration space associated with several parent isomers and to dissociate to yield X atom products in tandem with both cyclic and ring-opened partner fragments.

Methanol is typically observed within water-rich interstellar ices and is a source of interstellar organic species. Following the 157 nm photoexcitation of solid methanol at 90 K, desorbed CH3(v = 0) and OH(v = 0, 1) radicals have been observed in situ, near the solid surface, using resonance-enhanced multiphoton ionization (REMPI) detection methods. Time-of-flight and rotationally resolved REMPI spectra of the desorbed species were measured, and the respective fragment internal energy and kinetic energy distributions were obtained. Photoproduction mechanisms for CH3 and OH radicals from solid methanol are discussed. The formation of O(1D and 3P) atoms and H2O was investigated, but the yield of these species was found to be negligible. CH3 products arising following the photoexcitation of water-methanol mixed ice showed similar kinetic and internal energy distributions to those from neat methanol ice.