The scattering distributions of state-selected methyl radicals are measured for the O(¹D₂) reaction with methane using a crossed molecular beam ion imaging method at collision energies of 0.9 – 6.8 kcal/mol. The results are compared with the reaction with deuterated methane to examine the isotope effects. The scattering distributions exhibit contributions from both the insertion and abstraction pathways respectively on the ground and excited-state potential energy surfaces. Insertion is the main pathway, and it provides a strongly forward-enhanced angular distribution of methyl radicals. Abstraction is a minor pathway, causing backward scattering of methyl radicals with a discrete speed distribution. From the collision energy dependence of the abstraction/insertion ratio, the barrier height for the abstraction pathway is estimated for O(¹D₂) with CH₄ and CD₄, respectively. The insertion pathway of the O(¹D₂) reaction with CH₄ has a narrower angular width in the forward scattering and a larger insertion/abstraction ratio than the reaction with CD₄, which indicate that the insertion reaction with CH₄ has a larger cross section and a shorter reaction time than the reaction with CD₄. Additionally, while the insertion reaction with CD₄ exhibits strong angular dependence of the CD₃ speed distribution, CH₃ exhibits considerably smaller dependence. The result suggests that, although intramolecular vibrational redistribution (IVR) within the lifetime of the methanol intermediate is restrictive in both isotopomers, relatively more extensive IVR occurs in CD₃OD than CH₃OH, presumably due to the higher vibrational state density.

Collision cross section for transfer of anisotropy arising from collisions between electronically excited sodium dimer and ground level argon atoms has been examined. The experimental method is based on a polarization spectroscopy using a sophisticated resonant cw-pump-stimulated emission probe technique. Measurement of polarization from analysis of the emitted light is a very powerful method gaining information about the inelastic collision process between the electronically excited molecules and other collision partners. From the measurement, anisotropy-dependent polarization spectra of the Na^*₂ with Ar has been investigated.¹
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¹The author gratefully acknowledges financial support from the National Science Foundation (Grant No. NSF-PHY-1309571).

The spectroscopy of the nitrate cation NO₃⁺ remains poorly understood. It has a symmetric D_3h ground state with an IP = 12.55 eV and is predicted to have four low lying electronic states, three of E symmetry exhibiting the Jahn-Teller Effect. There have been two low resolution experiments - by photoionization spectroscopy and photoelectron spectroscopy, but evidence for the excited states is controversial. We report preliminary studies on the Threshold PhotoElectron-PhotoIon Coincidence (T-PEPICO) spectrum of the radical cation NO₃⁺ using the DELICIOUS3 coincidence spectrometer on the DESIRS beamline at the Soleil Synchrotron. Photoelectron and photoion imaging allow us to identify the transitions to the excited states, extract the NO₃⁺ signal from the ionization of the dominant background species NO₂ and N₂O₅, and observe vibronic spectra and dissociation dynamics of the electronic excited states.

The ultraviolet (UV) photodissociation dynamics of the cyclohexyl (c-C₆H₁1) radical was studied for the first time using the high-n Rydberg atom time-of-flight (HRTOF) technique. The cyclohexyl radical was produced by the 193 nm photodissociation of chlorocyclohexane and bromocyclohexane and was examined in the photolysis region of 232-262 nm. The H-atom photofragment yield (PFY) spectrum contains a broad peak centering around 250 nm, in good agreement with the UV absorption spectra of cyclohexyl. The translational energy distributions of the H-atom loss product channel, P(E_T)’s, show a large translational energy release peaking at   45 kcal/mol. The fraction of average translational energy in the total excess energy, < f_T > , is in the range of 0.45-0.57 from 232-262 nm. The H-atom product angular distribution is anisotropic with a positive β parameter in the range of 0.3-1.0, indicating a dissociation time scale faster than one rotation period of the radical. The translational energy release and anisotropy of the H-atom loss product channel are significantly larger than those expected for a statistical unimolecular dissociation of a hot radical, thus showing a non-statistical dissociation mechanism of this large radical. The dissociation mechanism is consistent with direct dissociation on a repulsive excited state surface or on the repulsive part of the ground state surface to produce cyclohexene + H, possibly mediated by conical intersection.

The ultraviolet (UV) photodissociation dynamics of the cyclohexenyl radical (c-C₆H₉) was studied for the first time in the photolysis region of 232-262 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The cyclohexenyl radical was produced by the 193 nm photodissociation of 3-chlorocyclohexene and 3-bromocyclohexene. The H-atom photofragment yield (PFY) spectrum contains a broad peak centering around 250 nm, in good agreement with the UV absorption spectra of the ²B₁ ← ²A₂ transition in cyclohexenyl. The translational energy distributions of the H-atom loss product channel, P(E_T)’s, for cyclohexenyl show a modest translational energy release peak at  ∼  10 kcal/mol. The fraction of average translational energy in the total excess energy, < f_T > , is  ∼  0.16 from 232-262 nm. The H-atom product angular distribution is isotropic with a β parameter  ∼  0. The dissociation mechanism is a statistical unimolecular dissociation of a hot radical following internal conversion from the excited electronic state to produce the lowest energy product, H + cyclohexadiene. The dissociation mechanisms of the cyclohexenyl radical and cyclohexyl radical will be compared.

The thermal decomposition of benzyl radical (C₆H₅CH₂) has been investigated using a combination infrared absorption spectroscopy in a neon matrix and 118.2 (10.487 eV) photoionization mass spectrometry. Both techniques are coupled with a heated tubular reactor to allow temperature control over the decomposition to indicate relative barrier heights of fragmentation pathways. Three possible chemical mechanisms have been considered. 1) Ring expansion to cycloheptatrienyl radical (C₇H₇) with subsequent breakdown to HCCH and C₅H₅, 2) isomerization to the substituted five-membered ring fulvenallene (C₅H₄=C=CH₂), which is of interest to kinetic theorists and finally 3) hydrogen shift to form methyl-substituted phenyl radical, which can then form ortho-benzyne, diacetylene and other fragments. Benzyl radical is generated from two precursors, C₆H₅CH₂CH₃ and C₆H₅CH₂Br, and both lead to the appearance of HCCH and C₅H₅. At slightly hotter temperatures peaks are observed at m/z 90, presumed to be C₅H₄=C=CH₂, and 89, potentially the substituted propargyl C₅H₄=C=CH. Additionally, decomposition of isotopically substituted parent molecules C₆H₅CD₂CD₃ and C₆D₅CH₂CH₃ indicates C₇H₇ as an intermediate due to H/D ratios in fragment molecules.

The rate constants for the dissociation of Ni⁺Pentanal at various internal energies (15000 cm⁻¹-18800 cm⁻¹) were measured using a custom time of flight instrument. Clusters are generated in a large source chamber by ablating the surface of a rotating nickel rod with an excimer and entraining the ablated metal atoms in a helium gas plume slightly doped with pentanal vapor. The molecular beam enters a Wiley-Mclaren type acceleration grid, and cations are accelerated along a 1.8 meter long time-of-flight mass spectrometer (TOFMS). At the other end of the TOF is a sector and a detector. The sector allows ions of a particular kinetic energy through to the detector. If an ion breaks apart in the time it takes to reach the sector, the mass (and therefore kinetic energy) is reduced, and the sector can be set to allow these fragment ions to reach the detector (fig 2). In our experiment, the energy required to initiate the breakdown is provided by a laser pulse between 15000 cm⁻¹ and 18800 cm⁻¹.

A table-top high harmonic XUV light source (50 eV to 70 eV) has been successfully utilized to explore the ultrafast dynamics of vinyl bromide (CH₂=CHBr) with electronic state specificity and elemental sensitivity. Strong-field ionization (SFI) provides a method to produce ions in different ionic states. The production and dissociation dynamics of these ionic states are investigated by femtosecond XUV transient absorption spectroscopy. The XUV photons probe the time-dependent spectroscopic features associated with transitions of the Br (3d) inner-shell electrons to vacancies in molecular and atomic valence orbitals. The experimental observation shows that two ionic states are produced by SFI. The first ionic excited state is dissociative, leading to C-Br bond dissociation which is observed in real time as a shift in the absorption energy. The results offer powerful new insights about orbital-specific electronic processes in high field ionization, coupled vibrational relaxation and dissociation dynamics, and the correlation of valence hole-state location and dissociation in polyatomic molecules, all probed simultaneously by ultrafast table-top XUV spectroscopy.

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,¹ will be discussed.
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¹James O. Thomas, Katherine E. Lower, and Craig Murray, The Journal of Physical Chemistry Letters, 2012, 3 (10), 1341-1345.