Steric asymmetry and lambda-doublet propensities in state-to-state rotationally inelastic scattering of NO(2Π1/2) with He

Abstract

Relative integrated cross sections are measured for rotationally inelastic scattering of NO(2Pi(1/2)),hexapole selected in the upper lambda-doublet level of the ground rotational state (j = 0.5), in collisions with He at a nominal energy of 514 cm(-1). Application of a static electric field E in the scattering region, directed parallel or antiparallel to the relative velocity vector v, allows the state-selected NO molecule to be oriented with either the N end or the O end towards the incoming He atom. Laser-induced fluorescence detection of the final state of the NO molecule is used to determine the experimental steric asymmetry, [formula: see text], which is equal to within a factor of (- 1) to the molecular steric effect, S(i-->f) is identical with (sigma(He-->NO) - (sigma(He-->ON))/(sigma(He-->NO) + sigma(He-->ON)). The dependence of the integral inelastic cross section on the incoming lambda-doublet component is also observed as a function of the final rotational (j'), spin-orbit (omega'), and lambda-doublet (epsilon') state. The measured steric asymmetries are significantly larger than previously observed for NO-Ar scattering, supporting earlier proposals that the repulsive part of the interaction potential is responsible for the steric asymmetry. In contrast to the case of scattering with Ar, the steric asymmetry of NO-He collisions is not very sensitive to the value of omega'. However, the lambda-doublet propensities are very different for [omega=0.5(F1)-->omega'= 1.5(F2)] and [omega=0.5(F1)-->omega'=0.5(F1)] transitions. Spin-orbit manifold conserving collisions exhibit a propensity for parity conservation at low deltaj, but spin-orbit manifold changing collisions do not show this propensity. In conjunction with the experiments, state-to-state cross sections for scattering of oriented NO(2Pi) molecules with He atoms are predicted from close-coupling calculations on restricted coupled-cluster methods including single, double, and noniterated triple excitations [J. Klos, G. Chalasinski, M. T. Berry, R.Bukowski, and S. M. Cybulski, J. Chem. Phys. 112, 2195 (2000)] and correlated electron-pair approximation [M. Yang and M. H. Alexander, J. Chem. Phys. 103, 6973 (1995)] potential energy surfaces. The calculated steric asymmetry S(i-->f) of the inelastic cross sections at Etr= 514 cm(-1) is in reasonable agreement with that derived from the present experimental measurements for both spin-manifold conserving (F1-->Fl) and spin-manifold changing (F1 --F2) collisions, except that the overall sign of the effect is opposite. Additionally, calculated field-free integral cross sections for collisions at Etr = 508 cm(-1) are compared to the experimental data of Joswig et al. [J. Chem. Phys.85, 1904 (1986)]. Finally, the calculated differential cross section for collision energy Etr= 491 cm(-1) is compared to experimental data of Westley et al. [J. Chem. Phys. 114, 2669 (2001)] for the spin-orbit conserving transition F1 (j = 0.5) -F1f (j' = 3.5).

Differential cross sections for collisions of hexapole state-selected NO with He

“…In order to explore ⌳-state selected differential cross sections, a previously used crossed-beam apparatus 2,4,23,28 has been modified. The existing LIF detection system is replaced by newly designed ion optics for velocity-mapped ion-imaging.…”

“…Early work on He-NO and Ar-NO collisions 1 resulted in integral inelastic cross sections and showed that these exhibit an oscillatory structure as a function of the final rotational state. Large anisotropy effects in the Ar-NO and He-NO potential were found by De Lange et al [2][3][4] In their experiments NO molecules were oriented, after which the integral state-to-state collision cross section was determined using laser induced fluorescence ͑LIF͒. Typically, LIF provides only integral cross sections, while differential cross sections are nowadays usually obtained using ion-imaging techniques.…”

“…The methodology of scattering of the structureless partner with an open shell diatomic was described in detail before. 4,21,27,[32][33][34][35][36] We refer the reader to these references for details. The close-coupling calculations at a collision energy E col = 508.13 cm −1 employed all partial waves up to a total angular momentum quantum number J max = 101.5 and NO rotational levels up to jЈ = 17.5.…”

“…4 The HIBRIDON suite of codes 31 was used for the scattering calculations. The methodology of scattering of the structureless partner with an open shell diatomic was described in detail before.…”

Parity-dependent rotational rainbows in D2–NO and He–NO differential collision cross sections

“…28 Inspection of the V sum contour plot in Ref. 29 gives ͑at E = 500 cm −1 ͒ for a collision onto the N-end B = 4.40a 0 , A N = 5.65a 0 and for a collision onto the O-end A O = 5.25a 0 . To estimate the difference in anisotropy between the He-NO and D 2 -NO potentials, Eq.…”