Inelastic Scattering of He Atoms and NO(X²Π) Molecules: The Role of Parity on the Differential Cross Section

Abstract

Quasiclassical trajectory (QCT) and quantum mechanical (QM) close-coupling calculations have been used to study the state-resolved rotationally inelastic scattering of NO(X(2)Pi(1/2),v = 0,j = 1/2,e/f) by He on the most recent ab initio potential energy surface of J. Kłos et al. [J. Chem. Phys. 2000, 112, 2195.]. Opacity functions, and integral and differential cross sections are reported at collision energies of 63 and 147 meV and compared with previous theoretical calculations and experimental measurements on this and other systems. The existence of double peaks in the QCT and QM differential cross sections is examined in detail. While at a collision energy of 147 meV two rotational peaks appear in both the QCT and open-shell QM results, only a single peak is found in the QM calculations at the lower collision energy. The double peaks in the quantum-state-resolved differential cross sections (DCS) are found to be closely related to structure found in the corresponding state-resolved opacity functions. The structure in the QCT and QM DCSs is attributed to a flattening of the potential energy surface for sideways approach of He to the near-symmetric NO(X) molecule, and in both sets of calculations, it is shown to arise from a specific odd term in the expansion of the intermolecular potential. Although significant differences are found between the QCT and QM data in the forward scattered direction, and for higher final rotational levels, reflecting differences in the nature of the rotational rainbows observed in these two methods, in general, the QCT calculations are shown to give similar results to quantum theory. Furthermore, they provide valuable clues as to the mechanism of rotational energy transfer in this system.

The hyperfine excitation of OH radicals by He

“…This is in agreement with numerous previous studies of rotational cross sections in OH(X) + He [14,[27][28][29]. This spin-orbit propensity is not as important as in other systems such as NO(X) + He [30,31]. One can clearly see that there is a strong propensity in favor of Δj = ΔF transitions for spin-orbit conserving transitions.…”

Rotationally inelastic scattering of methyl radicals with Ar and N2

“…6 This body of work extended the scope of inelastic scattering of labile free radicals, which had previously concentrated on collisions of diatomic radicals with atomic species. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] A recent article by Dagdigian reviewed collisional energy transfer calculations for small hydrocarbon intermediates 26 and highlighted computational studies of integral cross sections for collisions of methylene (CH 2 ) 27,28 and methyl radicals. 5,28,29 The methyl radical is also the first polyatomic free radical to be slowed in a Zeeman decelerator using a pulsed magnetic field, 30 opening up new possibilities to study inelastic and reactive scattering at very low collision energies.…”

Collision dynamics of symmetric top molecules: A comparison of the rotationally inelastic scattering of CD3 and ND3 with He

“…[3][4][5][6][7][8][9][10] Some attention has also been paid to experimental and theoretical studies of inelastic collisions of nonlinear polyatomic molecules. [11][12][13][14][15][16][17][18][19][20][21] Collisions of nonlinear polyatomic molecules, such as the symmetric tops under study here, display a richer dynamics because the potential energy surface (PES) depends upon two angular coordinates, rather than just one as for diatomics, and the energy level structure is more complicated.…”

“…The particular symmetric tops selected for investigation here are deuterated methyl (CD 3 ) and ammonia (ND 3 ) molecules, and we will focus on collisions with helium. In our experimental studies of these molecules, 1, 2 resonanceenhanced multi-photon ionization detection (REMPI) allowed acquisition of VMI for methyl radicals in levels corresponding to a single rotational angular momentum quantum number n , but averaged over a subset of the projection quantum number k , whereas for ND 3 , images were acquired for molecules scattered into individual n ± k levels.…”