12 November 1999 Ž . Chemical Physics Letters 313 1999 491–498 www.elsevier.nlrlocatercplett Steric asymmetry in state-resolved NO–Ar collisions M.J.L. de Lange a , M. Drabbels a,b , P.T. Griffiths a , J. Bulthuis a, ) , S. Stolte a , J.G. Snijders c a Laser Centre and Department of Physical Chemistry, Vrije UniÕersiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands b Department of Chemistry, Swiss Federal Institute of Technology Lausanne, CH-1015 Lausanne EPFL, Switzerland c Theoretical Chemistry, Materials Science Centre, RijksuniÕersiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received 30 July 1999; in final form 13 September 1999 Abstract New experimental results are reported on state-dependent steric effects in NO–Ar inelastic scattering. The NO molecules are selected in the J s1r2 y L-doublet state of the electronic ground state and oriented relative to the incident Ar atoms. Ž . Ž . X The steric asymmetry, S s s ys r s qs , has been measured as a function of the final rotational state J . In a NO ON NO ON previous study, quantum-mechanical scattering calculations were found to predict strong oscillations in S, but experimental evidence for this behaviour was not conclusive. The results of experiments presented here provide clear evidence of the qualitative correctness of the theoretical calculations. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction wx In a previous Letter 1 , we reported on the steric asymmetry in rotationally inelastic state-resolved NO–Ar collisions. Although earlier L-doublet propensities had been investigated in inelastic colli- w x sions of NO 2–8 , this was the first direct experi- mental evidence of the role of orientation in inelastic collisions. Moreover, the experiments were accom- panied by fully quantum-mechanical scattering cal- culations, adapted to take into account the preferen- tial orientation of the NO molecules and, conse- quently, the steric asymmetry. The NO molecules are selected in the 2 P , 1r2 y Ž J s 1r2 state the higher of the two L-doublet . states by an electrostatic hexapole field and oriented ) Corresponding author. Fax: q31-20-44-47643; e-mail: bulthuis@chem.vu.nl Ž in a uniform electric field by which the parity of the . selected state is broken , such that the incoming Ar atoms hit the NO molecules preferentially either at wx the N- end or at the O-end 1 . Ž NO The steric asymmetry S is defined by S s s ON . Ž NO ON . NO y s r s q s , where s denotes the cross-sections for collisions with Ar impinging at the N-end. Since the steric asymmetry is a ratio of cross-sections, the cross-section may be substituted by any quantity that is proportional to it. However, substitution by the intensity in a laser-induced fluo- Ž . rescence LIF detection scheme would require a density-to-flux transformation, and, consequently, knowledge of the differential cross-sections for the two NO orientations. Since the width of the laser beam exceeds the cross-section of the scattering volume, we expect that the error made by neglecting Ž the density-to-flux transformation is small see also w x. Refs. 3,8 . 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 01065-9