Interference structures in the differential cross-sections for inelastic scattering of NO by Ar C. J. Eyles 1 , M. Brouard 1 * , C.-H. Yang 2 , J. Klos 3 , F. J. Aoiz 4 , A. Gijsbertsen 5,6 , A. E. Wiskerke 5 and S. Stolte 5,7,8 Inelastic scattering is a fundamental collisional process that plays an important role in many areas of chemistry, and its detailed study can provide valuable insight into more complex chemical systems. Here, we report the measurement of differential cross-sections for the rotationally inelastic scattering of NO(X 2 P 1/2 , v 5 0, j 5 0.5, f ) by Ar at a collision energy of 530 cm 21 in unprecedented detail, with full L-doublet (hence total NO parity) resolution in both the initial and final rotational quantum states. The observed differential cross-sections depend sensitively on the change in total NO parity on collision. Differential cross-sections for total parity-conserving and changing collisions have distinct, novel quantum-mechanical interference structures, reflecting different sensitivities to specific homonuclear and heteronuclear terms in the interaction potential. The experimental data agree remarkably well with rigorous quantum-mechanical scattering calculations, and reveal the role played by total parity in acting as a potential energy landscape filter. I nelastic scattering is one of the simplest possible collisional pro- cesses, responsible for the transfer of energy from one molecule to another 1–3 . It plays an important role in areas as diverse as atmospheric, combustion and ultracold chemistry 4,5 , and can have a profound influence on chemical reactivity. The fully quantum state resolved angular distribution of the scattered products, which describes how the molecules are scattered in space after collision, constitutes one of the most detailed of all dynamical observations 6 . The measurement of such product angular distributions, which are proportional to the corresponding differential cross-sections (DCSs), can provide valuable insight into more complex chemical systems. Rotational inelastic scattering of the NO molecule is of particular interest, because it is an open shell species with both electronic spin and orbital angular momentum in its ground 2 P electronic state. The study of NO(X 2 P) þ Ar has attracted considerable attention, both experimentally and theoretically (see, for example, refs 4 and 7–14 and references therein), because it provides a model system on which to investigate the breakdown in the Born–Oppenheimer approximation 15 . The ground state of NO is split into two spin–orbit levels, with the V ¼ 3/2 level lying ≏123 cm 21 above the V ¼ 1/2 level (V is the projection of j onto the internuclear axis, and j is the total angular momentum of NO apart from nuclear spin, with the associ- ated quantum number, j ). Each rotational level is split further into two L-doublet sublevels, consisting of symmetric (labelled e ¼þ1 or e) and antisymmetric (e ¼ 21 or f ) combinations of þV and 2V wavefunctions. These sublevels differ only in total NO parity, p ¼ e (21) j21/2 , determined by the symmetry of the total NO wave- function with respect to space-fixed inversion, and are nearly degen- erate, being separated by ≏0.01 cm 21 for j ¼ 0.5. For a non-colinear approach of Ar, the degeneracy of the P-state is lifted, leading to two potential energy surfaces (PESs) of A ′ and A ′′ symmetry 15,16 . For Hund’s coupling case (a) molecules, in which the electronic orbital and spin angular momenta are both tied to the internuclear axis 17 , Alexander has shown that spin–orbit conserving (DV ¼ 0) transitions take place on a summed potential, V sum ¼ (A ′ þ A ′′ )/2, while spin–orbit changing (DV ¼ 1) collisions are governed by V diff ¼ (A ′′ 2 A ′ )/2 (refs 15,16). In the following, we report for the first time differential cross-sec- tions for the fully L-doublet quantum state selected and resolved scattering of NO(X 2 P 1/2 , v ¼ 0, j ¼ 0.5, f ) by Ar at a collision energy of 530 cm 21 , focusing exclusively on spin–orbit conserving transitions (differential cross-sections for spin–orbit changing collisions will be considered in a future publication). We show that the differential cross-sections are exquisitely sensitive to the NO L-doublet levels, despite being separated by only a tiny fraction of the experimental collision energy, and in particular sensitive to the change in total NO parity on collision. Results As described in the Methods, the experimental measurements were made using a crossed molecular beam apparatus 6 , which incorpor- ated a hexapole electric field before the interaction region to enable selection of the initial L-doublet state. By taking advantage of the Stark effect, only those molecules in the ( j ¼ 0.5, f ) L-doublet level of negative total parity are focused into the collision region 6 . The use of (1 þ 1 ′ ) resonantly enhanced multiphoton ionization on selected rotational branches, coupled with velocity-mapped ion imaging detection, then allows probing of the scattered NO in a specific final rotational ( j ′ ) and L-doublet (e or f ) state. The result- ing experimental ion images reflect the centre-of-mass (COM) velocity distribution of the NO(X) molecules after inelastic collision. Figure 1 presents a selection of such images, together with an 1 The Department of Chemistry, University of Oxford, The Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, UK, 2 Institute for Molecules and Materials, Radboud University Nijmegen, Heijendaalseweg 135, 6525 ED Nijmegen, The Netherlands, 3 Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, 20742, USA, 4 Departamento de Quı ´mica Fı ´sica, Facultad de Quı ´mica, Universidad Complutense, 28040 Madrid, Spain, 5 Laser Centre and Department of Physical Chemistry, Vrije Universiteit, Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, 6 FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands, 7 Atomic and Molecular Physics Institute, Jilin University, Changchun 130012, China, 8 Laboratoire Francis Perrin, Ba ˆtiment 522, DRECEM/SPAM/CEA Saclay, 91191 Gif sur Yvette, France. *e-mail: mark.brouard@chem.ox.ac.uk ARTICLES PUBLISHED ONLINE: 12 JUNE 2011 | DOI: 10.1038/NCHEM.1071 NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 1 © 2011 Macmillan Publishers Limited. 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