CORRELATED STATE DISTRIBUTIONS FOR H2_2 AND CO FROM FORMALDEHYDE PHOTODISSOCIATION USING DC SLICE IMAGING: THE "ROAMING H-ATOM MECHANISM"

Author Institution: Department of Chemistry, Wayne State University, Detroit, MI 48202We present a detailed experimental investigation of formaldehyde dissociation to H$_2$ and CO following excitation to a series of vibrational bands in S$_1$. The CO was detected by (2+1) REMPI at various rotational states of CO (J = 5-45) and the CO velocity distributions were measured using state-resolved DC Slice Imaging. These high-resolution measurements revealed the internal state distribution in the correlated H$_2$ cofragments. The results show that rotationally hot CO (J = 45) is produced in conjunction with vibrationally "cold" H$_2$ fragments ($\nu$ = 0-3): these products are formed through the celebrated skewed transition state. After excitation of formaldehyde above the threshold for the radical channel (H$_2$CO $\rightarrow$ H + HCO) we find formation of rotationally cold CO (J = 5-15) correlated to highly vibrationally excited H$_2$ ($\nu$ = 5-7). These products are formed through a novel roaming mechanism that involves intramolecular H-abstraction (D. Townsend et. al. Science \textbf{306}, 1158 (2004)), and avoids the region of the transition state entirely. The current measurements give us detailed insight into the energy dependence of the branching to these different reaction mechanisms

Complete state-resolved non-adiabatic dynamics of the O(³P) + D₂ → OD(X²π) + D Reaction

The first quantum-state-resolved distributions over the full range of available product levels are reported for any isotopic variant of the elementary reaction of O(P-3) with molecular hydrogen. A laser-detonation source was used to produce a hyperthermal oxygen-atom beam, which allowed for sufficient collision energy to surmount the reaction barrier. This beam was crossed by a supersonic beam of D-2. The nascent OD products were detected by laser-induced fluorescence. OD rotational distributions in vibrational states v' = 0, 1, and 2 at a collision energy of 25 kcal mol(-1) are reported, together with distributions for the dominant product vibrational level, v' = 0, at lower collision energies of 20 and 15 kcal mol(-1). The OD product is highly rotationally excited, to a degree that declines as expected for the higher vibrational levels or for reductions in the collision energy. The measured rovibrational distributions at the highest collision energy are in excellent agreement with previous theoretical predictions based on quantum scattering calculations on the triplet potential energy surfaces developed by Rogers et al. (J. Phys. Chem. A 2000, 104, 2308-2325). However, no significant OD spin-orbit preference was observed, in contrast to the predictions of most existing theoretical models of the non-adiabatic dynamics based on the widely used reduced-dimensional four-state model of Hoffmann and Schatz (J. Chem. Phys. 2000, 113, 9456-9465). Furthermore, a clear observed preference for OD II(A') A-doublet levels is not consistent with a simple extrapolation of the calculated relative reaction cross sections on intermediate surfaces of (3)A' and (3)A" symmetry

DYNAMICAL FINGERPRINTING IN FORMALDEHYDE DISSOCIATION

H. M. Yin, S. H. Kable, X. Zhang and J. M. Bowman ScienceL. R. Valachovic et alAuthor Institution: School of Chemistry, The University of Sydney, Sydney, NSW, Australia, 2006; Department of Chemistry, Wayne State University, Detroit, MI 48202Photolysis of formaldehyde to H + HCO, may occur on both the electronic ground state (S${_0}$) and the first excited triplet state (T${_1}$). A dynamical signature distinguishing the products of these two chemical channels has been previously established.} \underline{\textbf{311}}, 1443, 2006.} This criterion is now used to qualitatively investigate the branching ratios for these pathways at excitation energies immediately below the T${_1}$ potential energy barrier, and exceeding it. Comparisons are also made to the yield of the molecular product, CO, which is only generated on the S${_0}$ surface. Near the threshold for T${_1}$ participation, continual activity of both the S${_0}$ and T${_1}$ channels has been seen, with dominance fluctuating depending on the formaldehyde rovibrational state prepared.} \textit{J.~Chem.~Phys.} \underline{\textbf{112}}(6), 2752, Feb 2000.} This fluctuation is confirmed in the region of the T${_1}$ threshold, with the triplet channel dominating the majority of states. The fluctuations diminish as the energy of the system increases away from the barrier of the triplet surface. It is also demonstrated that both pathways significantly populate the first vibrationally excited state of the HCO product once the energetic threshold is reached. Reaction on the singlet surface, however, is seen to gives rise to slightly higher vibrational excitation

Pyrolysis of Phenolic Impregnated Carbon Ablator (PICA)

Molar yields of the pyrolysis products of thermal protection systems (TPSs) are needed in order to improve high fidelity material response models. The volatile chemical species evolved during the pyrolysis of a TPS composite, phenolic impregnated carbon ablator (PICA), have been probed in situ by mass spectrometry in the temperature range 100 to 935 °C. The relative molar yields of the desorbing species as a function of temperature were derived by fitting the mass spectra, and the observed trends are interpreted in light of the results of earlier mechanistic studies on the pyrolysis of phenolic resins. The temperature-dependent product evolution was consistent with earlier descriptions of three stages of pyrolysis, with each stage corresponding to a temperature range. The two main products observed were H₂O and CO, with their maximum yields occurring at ∼350 °C and ∼450 °C, respectively. Other significant products were CH₄, CO₂, and phenol and its methylated derivatives; these products tended to desorb concurrently with H₂O and CO, over the range from about 200 to 600 °C. H₂ is presumed to be the main product, especially at the highest pyrolysis temperatures used, but the relative molar yield of H₂ was not quantified. The observation of a much higher yield of CO than CH₄ suggests the presence of significant hydroxyl group substitution on phenol prior to the synthesis of the phenolic resin used in PICA. The detection of CH₄ in combination with the methylated derivatives of phenol suggests that the phenol also has some degree of methyl substitution. The methodology developed is suitable for real-time measurements of PICA pyrolysis and should lend itself well to the validation of nonequilibrium models whose aim is to simulate the response of TPS materials during atmospheric entry of spacecraft