Abinitiopotential energy surfaces for C2H→C2+H photodissociation

Multiphoton breakdown of acetylene; formation of organic building block fragments

Mass resolved REMPI spectra and electron and ion velocity map images were recorded for REMPI of acetylene in the case of two-photon resonant excitations to low lying 3p and 4p Rydberg states. Combined data analysis of ion signal intensities and electron and ion kinetic energy release distribution revealed multiphoton-fragmentation processes in terms of photodissociation and photoionization channels to form the molecular ion, C₂H₂⁺ and the fragment ions H⁺, C⁺, CH⁺, CH₂⁺, C₂⁺ and C₂H⁺. The ratio of fragment ion formation over the parent ion formation increases with excitation energy. To a large extent, multiphoton-fragmentation involves the initial breakdown of the molecule into ground and excited state neutral fragments by two-, three- and four-photon dissociation processes prior to multiphoton ionization. The three-photon dissociation processes via superexcited molecular state(s) are found to be the most important and electronically excited fragment species playing a significant role in the overall multiphoton-fragmentation. Furthermore, the data are indicative of the involvement of secondary photodissociation processes and provide information on fragment energetics as well as state interactions. The question, whether acetylene could be an important source of building block fragments for the formation of organic molecules in interstellar space, is addressed.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

We survey and assess current knowledge regarding the primary photochemistry of hydrocarbon molecules and radicals.

The chemistry of disks around T Tauri and Herbig Ae/Be stars

CONTEXT

Infrared and (sub-)mm observations of disks around T Tauri and Herbig Ae/Be stars point to a chemical differentiation between both types of disks, with a lower detection rate of molecules in disks around hotter stars.

AIMS

To investigate the underlying causes of the chemical differentiation indicated by observations we perform a comparative study of the chemistry of T Tauri and Herbig Ae/Be disks. This is one of the first studies to compare chemistry in the outer regions of these two types of disks.

METHODS

We developed a model to compute the chemical composition of a generic protoplanetary disk, with particular attention to the photochemistry, and applied it to a T Tauri and a Herbig Ae/Be disk. We compiled cross sections and computed photodissociation and photoionization rates at each location in the disk by solving the FUV radiative transfer in a 1+1D approach using the Meudon PDR code and adopting observed stellar spectra.

RESULTS

The warmer disk temperatures and higher ultraviolet flux of Herbig stars compared to T Tauri stars induce some differences in the disk chemistry. In the hot inner regions, H2O, and simple organic molecules like C2H2, HCN, and CH4 are predicted to be very abundant in T Tauri disks and even more in Herbig Ae/Be disks, in contrast with infrared observations that find a much lower detection rate of water and simple organics toward disks around hotter stars. In the outer regions, the model indicates that the molecules typically observed in disks, like HCN, CN, C2H, H2CO, CS, SO, and HCO+, do not have drastic abundance differences between T Tauri and Herbig Ae disks. Some species produced under the action of photochemistry, like C2H and CN, are predicted to have slightly lower abundances around Herbig Ae stars due to a narrowing of the photochemically active layer. Observations indeed suggest that these radicals are somewhat less abundant in Herbig Ae disks, although in any case the inferred abundance differences are small, of a factor of a few at most. A clear chemical differentiation between both types of disks concerns ices. Owing to the warmer temperatures of Herbig Ae disks, one expects snowlines lying farther away from the star and a lower mass of ices compared to T Tauri disks.

CONCLUSIONS

The global chemical behavior of T Tauri and Herbig Ae/Be disks is quite similar. The main differences are driven by the warmer temperatures of the latter, which result in a larger reservoir or water and simple organics in the inner regions and a lower mass of ices in the outer disk.

QUASI-CLASSICAL STUDY OF STEREO-DYNAMICS FOR THE REACTION C + CH → C2 + H ON THE 12A′ POTENTIAL ENERGY SURFACE

The quasi-classical trajectory (QCT) method and the 12A′ potential energy surface (PES) [Boggio-Pasqua et al., Phys Chem Chem Phys2:1693, 2000] have been employed to study the stereo-dynamics of the reaction C + CH (v = 0, j) → C2 + H at different collision energies over the range of 0.01–0.6 eV and for different rotational quantum number j = 0 - 3. The reactive total cross section with initial revibrational state of v = 0 and j = 0 as a function of collision energy is presented and compared with the quantum mechanics results. The forward-backward asymmetry phenomenon has been found in the angular distribution of the products. The calculated distribution of P(θr) indicates a strong product alignment perpendicular to k, but this kind of product alignment is found to be rather insensitive to the collision energy. The calculated distribution of P(ϕr) revealed that at low collision energy the products tend to be oriented along the negative direction of the y-axis, while at high collision energy, this product orientation tends to be pointed to the positive direction of the y-axis. Such product orientation tends generally to become stronger with the increase of collision energy. Further, product polarization (i.e. orientation and alignment) becomes weak with high rotational excitation of the reagent CH molecule.

THEORETICAL STUDY OF STEREO-DYNAMICS FOR THE REACTION C(3P) + CH(2II) → C2 + H ON THE THREE LOWEST POTENTIAL ENERGY SURFACES

Based on the global three-dimensional adiabatic potential surfaces (PESs) 12 A ′, 22 A ′, 12 A ″ [Boggio-Pasqua et al., Phys Chem Chem Phys2:1693, 2000], the stereo-dynamics of the reaction C + CH → C 2 + H has been investigated by using the quasi-classical trajectories (QCT) method. The four polarization-dependent differential cross sections (PDDCSs) and the angular distributions of P(θr), P(ϕr), P(θr, ϕr) have been calculated at the collision energy 0.1 eV on the three PESs, respectively. The calculated results indicate that the distribution of the product C2 is backward-forward scattering on the 12 A ′ and 12 A ″ PESs and backward scattering on the 22 A ′ PES. The product rotational angular momentum is strongly aligned along the perpendicular direction to the reagent relative velocity k on the three PESs. The orientation of the product C 2 rotational angular momentum tends to point to the positive direction of the y-axis on the 12 A ′ PES but the negative direction on the 22 A ′ and 12 A ″ PESs.

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

Symmetric and nonsymmetric hydrogen abstraction reactions are studied using state-of-the-art ab initio electronic structure methods. Second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] methods with large correlation consistent basis sets (cc-pVXZ, where X = D,T,Q) are used in determining the transition-state geometries, activation barriers, and thermodynamic properties of several representative hydrogen abstraction reactions. The importance of basis set, electron correlation, and choice of zeroth-order reference wave function in the accurate prediction of activation barriers and reaction enthalpies are also investigated. The ethynyl radical (*CCH), which has a very high affinity for hydrogen atoms, is studied as a prototype hydrogen abstraction agent. Our high-level quantum mechanical computations indicate that hydrogen abstraction using the ethynyl radical has an activation energy of less than 3 kcal mol(-1) for hydrogens bonded to an sp(2) or sp(3) carbon. These low activation barriers further corroborate previous studies suggesting that ethynyl-type radicals would make good tooltips for abstracting hydrogens from diamondoid surfaces during mechanosynthesis. Modeling the diamond C(111) surface with isobutane and treating the ethynyl radical as a tooltip, hydrogen abstraction in this reaction is predicted to be barrierless.

Photodissociation of CCH: Classical trajectory calculations involving seven electronic states

The photodissociation dynamics of ethynyl radical, C(2)H, involving seven electronic states is studied by classical trajectory calculations. Initial values of the trajectories are selected based on relative absorption intensities calculated by Mebel et al. The energies and the derivatives are interpolated by three-dimensional cubic spline interpolator using an extended data pool. Mean square errors and standard deviations in interpolation of energies for 450 data points are found to be in the range 3.1 x 10(-6)-1.4 x 10(-5) and 1.7 x 10(-3)-3.8 x 10(-3) hartrees, respectively. The photofragments of C(2) and H are produced mainly in the X (1)Sigma(g) (+), a (3)Pi(u), b (3)Sigma(g) (-), c (3)Sigma(u) (+), A (1)Pi(u), B (1)Delta(g) electronic states of C(2) as product. The avoided crossings do not appear to be in the main dissociation pathways. The internal distributions are in good accord with the experimental results where comparison is possible, suggesting that the fragmentation mechanism of C(2)H(2) into C(2) and H is a two step process involving C(2)H radical as an intermediate with a life time long enough to allow complete collection of the phase space in the experiments.