Seemingly Anomalous Angular Distributions in H + D ₂ Reactive Scattering

Abstract

When a hydrogen (H) atom approaches a deuterium (D(2)) 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 + D(2) → 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.

Simultaneous Measurement of Reactive and Inelastic Scattering: Differential Cross Section of the H + HD → HD(v′, j′) + H Reaction

“…Actual recoil values are tabulated in Table 1. In our earlier studies this inherent uncertainty in the speed of HD(ν , j ) product was taken into account by convoluting the theoretical calculations with a ∼ 200 m/s wide Gaussian, i.e., typical HD + recoil values, see Table 1 [7][8][9]. Here we add the appropriate error bars on experimentally measured points rather than further blurring theoretical calculations.…”

“…Indeed, similar conclusions were reached by Fernandez-Alonso et al [40] who measured the D 2 (v = 0, j) reactant rotational state distribution and found that about 10% of the population resides in D 2 (v = 0, j > 2) states. Subsequently, all theoretical calculations on H + D 2 reaction were performed assuming only three initial reactant states, i.e., D 2 (v = 0, j ≤ 2), and the agreement in most cases was found to be very good [7][8][9]. The last source of uncertainty in the experimental measurement of HD(ν , j ) laboratory speed is the HD + recoil imparted by the departing electron.…”

“…A detailed description of an experimental set-up has been given elsewhere [8,9,29] and only the most pertinent details are described herein. Table 1 The required ultraviolet (UV) laser light was obtained by either frequency tripling the output of a dye laser (Lambda Physik, LPD 3000) pumped by the second harmonic of an Nd:YAG 3+ laser (Spectra Physics, GCR Series), or by frequency doubling the output of a dye laser (Lambda Physik, LPD 3000) pumped by the third harmonic of an Nd:YAG 3+ laser (Quanta-Ray, DCR-3).…”

“…According to this model, small-impactparameter collisions lead to rotationally cold, backward scattered products, while glancing, large-impact-parameter collisions produce rotationally excited, sideways/forward scattered products. A different reaction mechanism has been discovered for highly internally excited HD(ν , j ) products of the H + D 2 → HD(ν , j ) + D reaction, wherein rotationally excited molecules become more backward scattered than the rotationally cold ones [9,10]. In addition, highly internally excited HD(ν , j ) products, particularly the HD(ν = 4, j ) manifold, exhibit a more isotropic angular distribution [9][10][11][12].…”

“…A different reaction mechanism has been discovered for highly internally excited HD(ν , j ) products of the H + D 2 → HD(ν , j ) + D reaction, wherein rotationally excited molecules become more backward scattered than the rotationally cold ones [9,10]. In addition, highly internally excited HD(ν , j ) products, particularly the HD(ν = 4, j ) manifold, exhibit a more isotropic angular distribution [9][10][11][12]. The deuterium atom Rydberg tagging technique was employed in the study of an H + HD → H 2 (ν , j ) + D hydrogen exchange variant [13,14], where the forward peak has been attributed to the well-known time-delayed mechanism observed in the H + D 2 → HD(ν = 2, 3, j = 0) + D reaction [8,[15][16][17][18][19].…”