Experimental testing of ab initio potential energy surfaces: Stereodynamics of NO(A2Σ+) + Ne inelastic scattering at multiple collision energies

Abstract

We present a crossed molecular beam velocity-map ion imaging study of state-to-state rotational energy transfer of NO(AΣ, v = 0, N = 0, j = 0.5) in collisions with Ne atoms. From these measurements, we report differential cross sections and angle-resolved rotational angular momentum alignment moments for product states N' = 3 and 5-10 for collisions at an average energy of 523 cm, and N' = 3 and 5-14 for collisions at an average energy of 1309 cm, respectively. The experimental results are compared to the results of close-coupled quantum scattering calculations on two literature ab initio potential energy surfaces (PESs) [Pajón-Suárez et al., Chem. Phys. Lett. 429, 389 (2006) and Cybulski and Fernández, J. Phys. Chem. A 116, 7319 (2012)]. The differential cross sections from both experiment and theory show clear rotational rainbow structures at both collision energies, and comparison of the angles observed for the rainbow peaks leads to the conclusion that Cybulski and Fernández PES better represents the NO(AΣ)-Ne interaction at the collision energies used here. Sharp, forward scattered (<10°), peaks are observed in the experimental differential cross sections for a wide range of N' at both collision energies, which are not reproduced by theory on either PES. We identify these as L-type rainbows, characteristic of attractive interactions, and consistent with a shallow well in the collinear Ne-N-O geometry, similar to that calculated for the NO(AΣ)-Ar surface [Kłos et al., J. Chem. Phys. 129, 244303 (2008)], but absent from both of the NO(AΣ)-Ne surfaces tested here. The angle-resolved alignment moments calculated by quantum scattering theory are generally in good agreement with the experimental results, but both experiment and quantum scattering theories are dramatically different to the predictions of a classical rigid-shell, kinematic-apse conservation model. Strong oscillations are resolved in the experimental alignment moments as a function of scattering angle, confirming and extending the preliminary report of this behavior [Steill et al., J. Phys. Chem. A 117, 8163 (2013)]. These oscillations are correlated with structure in the differential cross section, suggesting an interference effect is responsible for their appearance.

Probing the location of the unpaired electron in spin–orbit changing collisions of NO with Ar

“…23 Molecular orientation, in which a specific end or side of a molecule can be directed towards a collision partner, is most widely achieved through hexapole state selection coupled with adiabatic passage into a static electric [40][41][42]45,46 or magnetic 43,44,47 field. The good agreement between experimental and calculated steric preferences obtained in studies of inelastic collisions of state selected NO(X) with Ar and He atoms [40][41][42][48][49][50][51] have confirmed the accuracy of the NO(X) + He/Ar ground electronic PESs, [52][53][54] while (unoriented) scattering experiments of electronically excited NO(A) with Ne 17 pointed to shortcomings in the calculated PESs. 55,56 Electric field orientation requires the molecule to be oriented to posses a permanent dipole moment and, if only the dominant first-order Stark effect is taken into account, to be open-shell.…”

“…Calculations are needed, for instance, to assign transitions in high-resolution spectra, 1,2 to verify product distributions of reactions, [3][4][5] or to rationalize resonances observed in scattering experiments. [6][7][8][9][10][11] Experiments, in turn, are required to test calculated potential energy surfaces (PESs) [12][13][14][15][16][17][18] and to validate theoretical predictions [19][20][21] and approximations. [22][23][24] Increasingly refined methods to control molecules in collisions and reactions have provided particularly sensitive probes for testing the accuracy of PESs.…”

Stereodynamics of rotational energy transfer in NO(A2Σ+) + Kr collisions

“…25,27 There are larger discrepancies between QS calculations and experiment for NO(A) + Ne. 23,26 In particular, we observe a strong forward scattered peak in the 0-101 range in the experimental DCSs for a wide range of final rotational states, which is not reproduced by QS calculations. The experimental results are consistent with the presence of a shallow attractive well in a linear geometry, something seen in the NO(A)-Ar, Kr and Xe ab initio PESs, but absent from the literature NO(A)-Ne PESs.…”

“…However, following our initial demonstration of the combination of optical state preparation of NO(A) with crossed molecular beams and VMI detection, [20][21][22] we have performed a series of systematic experimental and theoretical studies of rotationally inelastic collisions of NO(A). [23][24][25][26][27] In these experiments we have measured both DCSs and angle-resolved rotational alignment moments for the NO(A) + He, Ne and Ar systems. We observed quantitative agreement between experiment and theory for NO(A) + He, and very good agreement for NO(A) + Ar.…”

“…In contrast to the relatively simple KA behaviour observed for the NO(X) + Rg scattering, all of the NO(A) + Rg systems studied to date show strong deviations from the KA model, in both experiment and theoretical results. 22,23,[25][26][27] A striking observation for collisions with Ne and Ar are strong, angle-dependent, deviations from the KA model, which are most pronounced at scattering angles close to the rainbow maxima in the DCS. Only in the backward scattering hemisphere, at angles greater than 1501, does the observed rotational alignment fit the KA model.…”