Articles you may be interested inFine-structure state resolved rotationally inelastic collisions of CH (A 2 Δ,v=0) with Ar: A combined experimental and theoretical study Quantum scattering studies of inelastic collisions of NH(A 3Π) with helium: Finestructure and Λdoublet propensities J. Chem. Phys. 95, 5036 (1991); 10.1063/1.461792Inelastic collisions of OH (2Π) with H2: Comparison between theory and experiment including rotational, fine structure, and Λdoublet transitions A comprehensive set of single-collision state-to-state rate constants for the relaxation of NH͑A 3 ⌸, vϭ0,N,F i ,e/ f ͒ levels in collisions with helium are presented. These rate constants were extracted from measured fluorescence intensities of isolated A→X rotational lines in the presence of He subsequent to laser excitation of individual levels in the A state. There is no indication of the collisional propensities predicted for a Hund's case ͑a͒ 3 ⌸ state, most likely because NH(A) represents an intermediate coupling case. With increasing N, NH(A) rapidly approaches the Hund's case ͑b͒ limit. For high initial N, rotational relaxation proceeds predominantly through ⌬NϭϪ1 fine-structure conserving transitions to ⌳ doublet levels with ⌸͑AЈ͒ reflection symmetry. In addition, a propensity to conserve the ⌸(AЈ)/⌸(AЉ) symmetry of the initial level is found for ⌬Nϭ0 fine-structure changing transitions. The observed propensities have been interpreted by comparison with full quantum close-coupling and coupled-states calculations of cross sections based on ab initio potential energy surfaces ͑PES's͒. The propensity for forming ⌸͑AЈ͒⌳ doublet levels in ⌬NϭϪ1 collisions of high N initial levels is found to be facilitated by approach in a ''helicopterlike'' orientation on the more attractive HeNH(A)AЈ PES, followed by curve crossing to the more repulsive AЉ PES which correlates adiabatically to the next lower rotational manifold. In addition, thermal rate constants derived from the calculated cross sections agree extremely well with those obtained from a deconvolution of the experimental spectra.

We report full quantum close-coupling and coupled-states calculations of cross sections and rate constants for the rotational relaxation of NH(c 1Π) in collisions with Ar, based on a new ab initio description of the potential energy surface for this system. Single-collision, state-to-state thermal rate constants were extracted from measured fluorescence intensities of relaxing NH(c 1Π,v=0,J) distributions in the presence of Ar starting with singly populated rotational states. Overall, the calculated and experimental rate constants agree well, although the dependence on the initial J is somewhat different. At high J the relaxation is dominated by J→J−1e/f changing transitions. This process is facilitated by approach on the more attractive ArNH(c)A′ PES followed by a curve crossing to the more repulsive A″ PES which correlates asymptotically to the next lower rotational level. For approach in a ‘‘helicopterlike’’ orientation, this process results in Jf→J−1,e transitions, while for approach in a ‘‘pinwheellike’’ orientation, this process results in Je→J−1,f transitions. Thus, similar to what we found earlier for the relaxation of high rotational levels of the CH(X) radical [M. H. Alexander and P. J. Dagdigian, J. Chem. Phys. 101, 7468 (1994)], a strong v, J correlation in the reactant channel results in a strong Λ, J correlation in the product channel. A full kinetic simulation, based on the calculated cross sections, reproduces nearly quantitatively the experimental observations of the pressure dependence of the equilibration of NH(c) Λ doublet populations monitored some years ago by Quinton and Simons.

Articles you may be interested inFate of isolated CH (B 2 Σ − ,v=0,J) states in inelastic collisions with CO

Quantum chemical ab initio calculations at the complete active space SCF level and with inclusion of correlation effects have been performed for the potential energy surfaces of PH in its X 3 ⌺ Ϫ ground state and its first excited triplet state, A 3 ⌸, colliding with He atoms. The PH distance was fixed at its experimental value ͑of the A 3 ⌸ state͒, the PH-He distance and the HePH angle were varied. All three potential energy surfaces ͓1 3 AЉ for PH(X) -He and 1 3 AЈ,2 3 AЉ for the two components of PH(A) -He͔ are purely repulsive, except for very shallow van der Waals minima with well depths of about 15-40 cm Ϫ1 . The interaction potentials decay approximately exponentially with increasing PH-He distance and show large angular anisotropies. Legendre expansions for the angular dependence of the potential surfaces converge slowly for V(1 3 AЉ) and the sum potential 1/2͓V(2 3 AЉ)ϩV(1 3 AЈ)͔, but rapidly for the corresponding difference potential 1/2͓V(2 3 AЉ)ϪV(1 3 AЈ)͔. The present PH(A) -He potentials have been used in the companion paper by Neitsch et al. ͓J. Chem. Phys. 106, 7642 ͑1997͔͒, for the calculation of thermal state-to-state rate constants for inelastic PH(A) -He collisions.

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