Steric Effects in the Inelastic Scattering of NO(X) + Ar: Side-on Orientation

Abstract

The rotationally inelastic collisions of NO(X) with Ar, in which the NO bond-axis is oriented side-on (i.e. perpendicular) to the incoming collision partner, are investigated experimentally and theoretically. The NO(X) molecules are selected in the |j = 0.5, Ω = 0.5, = −1, f state prior to bond-axis orientation in a static electric eld.The scattered NO products are then state selectively detected using velocity-map ion imaging. The experimental bond-axis orientation resolved dierential cross sections and integral steric asymmetries are compared with quantum mechanical calculations, and are shown to be in good agreement. The strength of the orientation eld is shown to aect the structure observed in the dierential cross sections, and to some extent 1 also the steric preference, depending on the ratio of the initial e and f Λ-doublets in the superposition determined by the orientation eld. Classical and quantum calculations are compared and used to rationalise the structures observed in the dierential cross sections. It is found that these structures are due to quantum mechanical interference eects, which dier for the two possible orientations of the NO molecule due to the anisotropy of the potential energy surface probed in the side-on orientation. Side-on collisions are shown to maximise and aord a high degree of control over the scattering intensity at small scattering angles (θ < 90 • ), whilst end-on collisions are predicted to dominate in the backward scattered region (θ > 90 • ).

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.…”

“…However, for spin-orbit changing transitions (and deviations from a pure Hund's case (a)), the off-diagonal matrix coupling elements, which are encoded in the difference potential, are required to obtain accurate scattering amplitudes. 78,79,81 As shown in our previous work, 50,51 the orientation dependent differential cross section (DCS) can be expressed in terms of the molecular bond-axis dependent polarization moments, or r-PDDCSs. The r-PDDCSs themselves are a function of the relevant scattering amplitudes, as defined in the ESI.…”

“…The experimental setup is identical to the one described previously for detection of the spin-orbit conserving (DO = 0) rotational transitions. 50,51 The NO molecules were expanded in a skimmed molecular beam containing about 15% NO in Ar. Using a hexapole, the molecules were state selected in their lowfield seeking f L-doublet state 64 and focused into the center of the scattering chamber where they were crossed with a neat argon beam at right angles.…”

“…From the fitted images, the relevant polarization moments were extracted and then used to calculate the DCSs. The method is described in detail in our earlier work 50,51 (see in particular Section 4 in ref. 51).…”

“…A consequence of the asymmetry of the side-on scattering process with respect to k is that the experimental velocity-map ion images contain the scattering distributions of both orientations to either side of k 50,51 (with k pointing roughly diagonal from the top left to the bottom right corner of the image, dividing it into two parts -this is indicated in the top left image in Fig. 2).…”