H atom Product Channels in the Ultraviolet Photodissociation of the 2-Propenyl Radical

Photodissociation of the trichloromethyl radical: photofragment imaging and femtosecond photoelectron spectroscopy

Halogen-containing radicals play a key role in catalytic reactions leading to stratospheric ozone destruction, thus their photochemistry is of considerable interest. Here we investigate the photodissociation dynamics of the trichloromethyl radical, CCl₃ after excitation in the ultraviolet. While the primary processes directly after light absorption are followed by femtosecond-time resolved photoionisation and photoelectron spectroscopy, the reaction products are monitored by photofragment imaging using nanosecond-lasers. The dominant reaction is loss of a Cl atom, associated with a CCl₂ fragment. However, the detection of Cl atoms is of limited value, because in the pyrolysis CCl₂ is formed as a side product, which in turn dissociates to CCl + Cl. We therefore additionally monitored the molecular fragments CCl₂ and CCl by photoionisation at 118.2 nm and disentangled the contributions from various processes. A comparison of the CCl images with control experiments on CCl₂ suggest that the dissociation to CCl + Cl₂ contributes to the photochemistry of CCl₃.

Photodissociation Dynamics of Vinoxy Radical via the B̃2A″ State: The H + CH2CO Product Channel

Photodissociation dynamics of the jet-cooled vinoxy radical (CH₂CHO) via the B̃²A″ state was studied in the near-ultraviolet (near-UV) region of 308-328 nm using high-n Rydberg H atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The vinoxy radical beam was produced by 193 nm photolysis of ethyl vinyl ether followed by supersonic expansion. The H + CH₂CO product channel was observed directly in the H atom TOF spectra. The H atom photofragment yield (PFY) spectra were obtained by integrating the H atom TOF spectra and measuring the H atom REMPI signals, and showed several vibronic bands of the B̃²A″ state. The translational energy distributions of the H + CH₂CO products, P(E_T)'s, were obtained at several vibronic transitions. The P(E_T) distributions were broad, peaking at a low energy of ∼3500 cm^-1. The product translational energy release was moderate; the average translational energy release in the maximum available energy, ⟨f_T⟩, was in the range of 0.24-0.27. The product angular distributions in this wavelength region were slightly anisotropic, with the β parameter in the range of 0.10-0.24. The near-UV photodissociation mechanism of the H + CH₂CO product channel of the vinoxy radical is consistent with unimolecular dissociation on the electronic ground state (X̃²A″) following internal conversion from the B̃²A″ state to the òA' state and then to the X̃²A″ state (although unimolecular dissociation from the first excited òA' may also contribute).

Multiple dehydrogenation of fluorene cation and neutral fluorene using the statistical molecular fragmentation model

The statistical molecular fragmentation (SMF) model was used to analyze the 306 fragmentation channels (containing 611 different species) that result from the fluorene (C13H10+) cation losing up to three hydrogen atoms (neutral radicals and/or a proton). Breakdown curves from such analysis permit one to extract experimentally inaccessible information about the fragmentation of the fluorene cation, such as the locations of the lost hydrogen atoms (or proton), yields of the neutral fragments, electronic states of the residues, and quantification of very low probability channels that would be difficult to detect. Charge localization during the fragmentation pathways was studied to provide a qualitative understanding of the fragmentation process. Breakdown curves for both the fluorene cation and neutral fluorene were compared. The SMF results match the rise and fall of the one hydrogen loss yield experimentally measured by imaging photoelectron-photoion coincidence spectroscopy using a VUV synchrotron.

Gas Phase Synthesis of the Elusive Trisilacyclopropyl Radical (Si3H5) via Unimolecular Decomposition of Chemically Activated Doublet Trisilapropyl Radicals (Si3H7)

The gas phase reaction of the simplest silicon-bearing radical silylidyne (SiH; X²Π) with disilane (Si₂H₆; X¹A_1g) was investigated in a crossed molecular beams machine. Combined with electronic structure calculations, our data reveal the synthesis of the previously elusive trisilacyclopropyl radical (Si₃H₅)-the isovalent counterpart of the cyclopropyl radical (C₃H₅)-along with molecular hydrogen via indirect scattering dynamics through long-lived, acyclic trisilapropyl (i-Si₃H₇) collision complex(es). Possible hydrogen-atom roaming on the doublet surface proceeds to molecular hydrogen loss accompanied by ring closure. The chemical dynamics are quite distinct from the isovalent methylidyne (CH)-ethane (C₂H₆) reaction, which leads to propylene (C₃H₆) radical plus atomic hydrogen but not to cyclopropyl (C₃H₅) radical plus molecular hydrogen. The identification of the trisilacyclopropyl radical (Si₃H₅) opens up preparative pathways for an unusual gas phase chemistry of previously inaccessible ring-strained (inorgano)silicon molecules as a result of single-collision events.

The statistical molecular fragmentation model compared to experimental plasma induced hydrocarbon decays

We compare the predictions of our recently developed statistical molecular fragmentation model with experimental results from plasma induced decay of propene and other small hydrocarbons.