State-to-state chemistry and rotational excitation of CH^+ in photon-dominated regions (original) (raw)
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The Journal of chemical physics, 2014
Collisional energy transfer between the ground (X³B₁) and first excited (ã¹A₁) states of CH2 is facilitated by strong mixing of the rare pairs of accidentally degenerate rotational levels in the ground vibrational manifold of the [Formula: see text] state and the (020) and (030) excited bending vibrational manifolds of the X state. The simplest model for this process involves coherent mixing of the scattering T-matrix elements associated with collisional transitions within the unmixed ã and X states. From previous calculations in our group, we have determined cross sections and room-temperature rate constants for intersystem crossing of CH2 by collision with He. These are used in simulations of the time dependence of the energy flow, both within and between the X and ã vibronic manifolds. Relaxation proceeds through three steps: (a) rapid equilibration of the two mixed-pair levels, (b) fast relaxation within the ã state, and (c) slower relaxation among the levels of the X state. Col...
Chemical Physics Letters, 1988
A crossed-beam study of the collision-induced dissociation of CH: by Ar was carried out at a center-of-mass (c.m.) collision energy of 5.5 eV. The scattering shows three patterns for the formation of CH ;, (1) large-angle scattering at preferred impact parameters with little internal excitation of the products, (2) scattering near the cm. with nearly all collision energy transferred into products internal energy and (3) superelastic scattering, i.e. conversion of internal energy to translational energy, implying the reaction is Initiated by a long-lived excited state of CH: generated by electron impact ionization of methane. No previous evidence exists, to our knowledge, that excited states of CH: thus generated may have microsecond lifetimes.
Radiative properties of Ch and Ch+ molecular states
Nuclear Instruments and Methods, 1973
The lifetimes of the A2A, B 2 Z-, and C z Z'+ states of CH and the Aen state of CH + have been measured by the time-sampling technique. There is excellent agreement with previous investigators for the Az~I and Aln states of CH and CH +, respectively. Additional detailed studies must be made on the B2~ ' andC2Z +stateofCH.
Combined crossed beam and theoretical studies of the C(1D) + CH4 reaction
The Journal of Chemical Physics, 2013
The reaction involving atomic carbon in its first electronically excited state 1 D and methane has been investigated in crossed molecular beam experiments at a collision energy of 25.3 kJ mol −1 . Electronic structure calculations of the underlying potential energy surface (PES) and Rice-Ramsperger-Kassel-Marcus (RRKM) estimates of rates and branching ratios have been performed to assist the interpretation of the experimental results. The reaction proceeds via insertion of C( 1 D) into one of the C-H bonds of methane leading to the formation of the intermediate HCCH 3 (methylcarbene or ethylidene), which either decomposes directly into the products C 2 H 3 + H or C 2 H 2 + H 2 or isomerizes to the more stable ethylene, which in turn dissociates into C 2 H 3 + H or H 2 CC + H 2 . The experimental results indicate that the H-displacement and H 2 -elimination channels are of equal importance and that for both channels the reaction mechanism is controlled by the presence of a bound intermediate, the lifetime of which is comparable to its rotational period. On the contrary, RRKM estimates predict a very short lifetime for the insertion intermediate and the dominance of the H-displacement channel. It is concluded that the reaction C( 1 D) + CH 4 cannot be described statistically and a dynamical treatment is necessary to understand its mechanism. Possibly, nonadiabatic effects are responsible for the discrepancies, as triplet and singlet PES of methylcarbene cross each other and intersystem crossing is possible. Similarities with the photodissociation of ethylene and with the related reactions N( 2 D) + CH 4 , O( 1 D) + CH 4 and S( 1 D) + CH 4 are also commented on.
CH+ potential energy curves and photodissociation cross-section
Chemical Physics Letters, 2004
Accurate CH + ab initio potential energy curves were calculated. The quality of the potentials was checked by comparing experimental and calculated CH + absorption cross-sections for various rotational transitions. The relative importance of couplings and corrections to the Born-Oppenheimer approximation was studied. When relativistic corrections were included and core electrons also correlated, good agreement was achieved between the experiment and the theory for the photodissociation cross-section. The new potentials were used to calculate the radiative association cross-section.
State resolved rotational excitation cross-sections and rates in H2+H2 collisions
Chemical Physics, 2006
Rotational transitions in molecular hydrogen collisions are computed. The two most recently developed potential energy surfaces for the H 2 −H 2 system are used from the following works: 1) A.ibid. 112, 4465. Cross sections for rotational transitions 00→20, 22, 40, 42, 44 and corresponding rate coefficients are calculated using a quantum-mechanical approach. Results are compared for a wide range of kinetic temperatures 300 K ≤
Theoretical investigation of collisional energy transfer in polyatomic intermediates
International Reviews in Physical Chemistry, 2013
Quantum scattering calculations on collisional rotational and vibrational energy transfer in small hydrocarbon reactive intermediates are highlighted. This review focuses on recent theoretical studies of energy transfer in methylene (CH 2 ), in both its ground tripletX 3 B 1 and low-lying singletã 1 A 1 electronic states, and in the methyl (CH 3 ) radical. Propensities in the state-to-state cross sections are shown to depend upon the two types of anisotropies that are present in potential energy surfaces of systems involving nonlinear polyatomic molecules. Computed rate constants for rotational and vibrational relaxation are compared with available experimental data. In addition, collisional transfer between the CH 2X andã states is addressed. Collision-induced intersystem crossing is shown to be mediated by spectroscopically perturbed rotational levels of mixed electronic character.
Physical Chemistry Chemical Physics, 2015
We present a new set of potential energy surfaces (PESs) for the CH(X 2 Π)-He van der Waals system. Ab initio calculations of the CH-He PES were carried out using the open-shell single-and double-excitation coupled cluster approach with non-iterative perturbational treatment of triple excitations [RCCSD(T)]. The augmented correlation-consistent polarized valence quadruple-zeta aug-cc-pVQZ basis set was employed augmented by mid-bond functions. Integral cross sections for the rotational excitation in CH-He collisions were calculated using the new PES and compared with available experimental results. The newly constructed PES reproduces the available experimental results for CH(X 2 Π, v = 0)-He collisions better than any previously available PES. Differential cross sections (DCS) are presented for the first time for this system and discussed within the context of rotational rainbows. Finally, our work provides the first rate thermal coefficients for this system that are crucially needed for astrochemical modelling and future anticipated experiments in CH(X 2 Π, v = 0)-He collisions.
Ab initio potential energy surface of CH and reaction dynamics of H + CH+
Physical chemistry chemical physics : PCCP, 2011
This work presents a new ground state potential energy surface (PES) for CH. The potential is tested using quasi classical trajectory (QCT) and quantum reactive scattering methods for the H + CH(+) reaction. Cross sections and rate coefficients for all reaction channels up to 300 K are calculated. The abstraction rate coefficients follow the expected slightly decreasing behaviour above 90 K, but have a positive gradient with lower temperatures. The inelastic collision and exchange reaction rate constants are increasing monotonically with temperature. The rate coefficients of the exchange reaction differ significantly between QCT and quantum reactive scattering, due to intrinsic shortcomings of the QCT final state distributions.