Seemingly Anomalous Angular Distributions in H + D2 Reactive Scattering

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Science  29 Jun 2012:
Vol. 336, Issue 6089, pp. 1687-1690
DOI: 10.1126/science.1221329

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Spinning Backwards

When atoms and molecules collide, the energy embedded in the reaction products gets distributed among translations, vibrations, and rotations. Decades of meticulous experiments have mapped out the quantum mechanical rules underlying this distribution process, particularly in simple systems comprising just three light atoms. Now, Jankunas et al. (p. 1687; see the Perspective by Yang et al.) describe a previously unappreciated wrinkle in the elementary reaction of an H atom with deuterium. Typically, products with low vibrational and rotational excitation tend to scatter backwards from the collision, whereas the spinning products scatter sideways. Above a certain vibrational threshold, however, spinning HD products were observed to scatter backwards.


When a hydrogen (H) atom approaches a deuterium (D2) molecule, the minimum-energy path is for the three nuclei to line up. Consequently, nearly collinear collisions cause HD reaction products to be backscattered with low rotational excitation, whereas more glancing collisions yield sideways-scattered HD products with higher rotational excitation. Here we report that measured cross sections for the H + D2 → HD(v′ = 4, j′) + D reaction at a collision energy of 1.97 electron volts contradict this behavior. The anomalous angular distributions match closely fully quantum mechanical calculations, and for the most part quasiclassical trajectory calculations. As the energy available in product recoil is reduced, a rotational barrier to reaction cuts off contributions from glancing collisions, causing high-j′ HD products to become backward scattered.

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