Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy.

UV pump-extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C-Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C-Br fission along an approximately CS symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C3v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage

Angular momentum in rotating superfluid droplets

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated submicrometer superfluid 4He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host. In capsule-shaped droplets, vortices form a distorted triangular lattice, whereas they arrange along elliptical contours in ellipsoidal droplets. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity. The findings are corroborated by density functional theory calculations describing the velocity fields and shape deformations of a rotating superfluid cylinder

Velocity Map Imaging Studies Of Non-conventional Methanethiol Photochemistry

Velocity map imaging (VMI) in combination with state-selective resonance enhanced multiphoton ionization (REMPI) has been used to study the photodissociation dynamics of methanethiol following excitation to the first and second singlet electronically excited states. Formation of sulfur atoms, in both the singlet and triplet manifolds, is observed and can be attributed to primary dissociation of the parent molecule. We will report the nascent photofragment velocity distributions, and hence the internal energy of the methane co-fragment. Sulfur atom quantum yields are benchmarked against a known standard to evaluate the significance of this pathway. The role of non-conventional photochemical mechanisms such as roaming-mediated intersystem crossing, previously observed in methylamine photochemistry,\footnote{James O. Thomas, Katherine E. Lower, and Craig Murray, The Journal of Physical Chemistry Letters, 2012, 3 (10), 1341-1345.} will be discussed

Photodissociation Dynamics of CH2I2, OCS and CH3CHO

Chapter 1 outlines the focus of this thesis, understanding the mechanism of breaking achemical bond following absorption of light.In Chapter 2 the design, construction and calibration of a new velocity-map direct currentslice ion imaging (VMI) time-of-flight mass spectrometer is described. Wavelength tunable pulsed lasers are used to selectively pump (dissociate) a target molecule and probe (ionize) the fragments. Combing the techniques allows correlated photofragment quantum state distributions to be explored.Chapter 3 investigates the near-UV photodissociation dynamics of CH2I2 using ion imagingover a range of excitation wavelengths. Ground state I(2P3/2) and spin-orbit excited I*(2P1/2) atoms were probed using 2+1 resonance-enhanced multiphoton ionization (REMPI) or with single-photon VUV ionization. Analysis of the ion images shows that, regardless of iodine spin-orbit state, ~20% of the available energy is partitioned into translation ET indicating that the CH2I co-fragment is formed highly internally excited. A refined C–I bond dissociation energy of D0 = 2.155±0.008 eV is determined. In Chapter 4 the photoproducts of OCS after UV excitation have been followed with photofragment excitation spectroscopy (PHOFEX), using REMPI to state-selectively monitor S(1D) and S(3P2,1,0) products while the pump wavelength was scanned. Probing the major S(1D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast spectra obtained probing S(3P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C–S stretch, following direct excitation to quasi-bound singlet and triplet states. The results confirm a recent theoretical prediction that direct excitation to the 23A” state can occur in OCS.In Chapter 5 ion imaging measurements of CH3 fragments from photolysis of CH3CHO reveal multiple pathways to the same set of products. By systematically exploring product formation over a timescale of picoseconds to nanoseconds, and wavelengths between 265-328 nm, an evolving picture of the dynamics is found. Evidence to suggest that the three-body CH3+CO+H pathway remains closed at all wavelengths is presented