Abstract: 
We investigate the effects of a magnetic field on the dynamics of rotationally inelastic collisions of openshell molecules (Σ2, Σ3, and Π2) with closedshell atoms. Our treatment makes use of the Fraunhofer model of matter wave scattering and its recent extension to collisions in electric [M. Lemeshko and B. Friedrich, J. Chem. Phys. 129, 024301 (2008)] and radiative fields [M. Lemeshko and B. Friedrich, Int. J. Mass. Spec. 280, 19 (2009)]. A magnetic field aligns the molecule in the spacefixed frame and thereby alters the effective shape of the diffraction target. This significantly affects the differential and integral scattering cross sections. We exemplify our treatment by evaluating the magneticfielddependent scattering characteristics of the HeCaH (XΣ+2), HeO2 (XΣ–3), and HeOH (XΠΩ2) systems at thermal collision energies. Since the cross sections can be obtained for different orientations of the magnetic field with respect to the relative velocity vector, the model also offers predictions about the frontalversuslateral steric asymmetry of the collisions. The steric asymmetry is found to be almost negligible for the HeOH system, weak for the HeCaH collisions, and strong for the HeO2. While odd ΔM transitions dominate the HeOH [J=3/2,f→J′,e/f] integral cross sections in a magnetic field parallel to the relative velocity vector, even ΔM transitions prevail in the case of the HeCaH (X2Σ+) and HeO2 (XΣ−3) collision systems. For the latter system, the magnetic field opens inelastic channels that are closed in the absence of the field. These involve the transitions N=1,J=0→N′, J′ with J′=N′.
