Justin Jankunas

Seemingly Anomalous Angular Distributions in H + D2 Reactive Scattering

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. An elementary chemical reaction manifests unexpectedly complex rotational dynamics. 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.

Is the simplest chemical reaction really so simple

Modern computational methods have become so powerful for predicting the outcome for the H + H2 → H2 + H bimolecular exchange reaction that it might seem further experiments are not needed. Nevertheless, experiments have led the way to cause theorists to look more deeply into this simplest of all chemical reactions. The findings are less simple.

Communication: Importance of rotationally inelastic processes in low-energy Penning ionization of CHF{sub 3}

Low energy reaction dynamics can strongly depend on the internal structure of the reactants. The role of rotationally inelastic processes in cold collisions involving polyatomic molecules has not been explored so far. Here we address this problem by performing a merged-beam study of the He((3)S1)+CHF3 Penning ionization reaction in a range of collision energies E/kB = 0.5-120 K. The experimental cross sections are compared with total reaction cross sections calculated within the framework of quantum defect theory. We find that the broad range of collision energies combined with the relatively small rotational constants of CHF3 makes rotationally inelastic collisions a crucial player in the total reaction dynamics. Quantitative agreement between theory and experiment is only obtained if the energy-dependent probability for rotational excitation is included in the calculations, in stark contrast to previous experiments where classical scaling laws were able to describe the results.

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

Abstract The H + HD → HD(ν′, j′) + H reaction has been studied experimentally and theoretically. Differential cross sections of HD(ν′, j′) products have been measured by means of a Photoloc technique and calculated using a time-independent quantum mechanical theory. Three product states: HD(ν′ = 1, j′ = 8) at a collision energy (Ecoll) of 1.97 eV; HD(ν′ = 2, j′ = 3) at Ecoll = 1.46 eV; and HD(ν′ = 2, j′ = 5) at Ecoll = 1.44 eV, show very good agreement between theory and experiment. The other two, highly rotationally excited states studied, HD(ν′ = 1, j′ = 12) and HD(ν′ = 1, j′ = 13) at Ecoll = 1.97 eV, exhibit a noticeable disagreement between experiment and theory. This is consistent with our most recent findings on the H + D2 → HD(ν′, j′) + D reaction, wherein the differential cross sections of HD(ν′ = 1, high j′) product states showed similar disagreement between the experiment and theory [J. Jankunas, M. Sneha, R. N. Zare, F. Bouakline, and S. C. Althorpe, J. Chem. Phys. 138 (2013) 094310]. In all five cases, however, we find overwhelming support that the experimental signal is a sum of reactive and inelastic scattering events. The interference term escapes detection, frustrating our attempt to observe geometric phase effects. Nevertheless, this work constitutes a first experimental example in which the indistinguishability of reactive and inelastic channels must be taken into account explicitly when constructing differential cross sections.

D + C(CH3)4 → HD (v ′, j ′) + C(CH3)3CH2: possible concerted flow of vibration energy into translation

The HD (v ′, j ′) product speed distributions from the D + C(CH3)4  → HD(v ′, j ′) + C(CH3)3CH2 reaction at a centre-of-mass collision energy of 1.20 eV (27.7 kcal/mol) were measured using a three-dimensional ion imaging apparatus. Anomalously fast HD (v ′, j ′) molecules are observed that move at speeds beyond the energetically allowed limit for reagents in their ground states plus the average thermal vibrational energy present in neopentane. These products are attributed to a reaction between D atoms and thermally excited C(CH3)4 molecules in which it is speculated that vibrational energy from more than one vibrational mode of neopentane is funneled into product translation in what seems to be an unprecedented coordinated process.