Dynamics of the D+ + H2 and H+ + D2 reactions: a detailed comparison between theory and experiment

Abstract

ion-molecule reactions is compared with the results of quantum mechanical (QM), quasiclassical trajectory (QCT), and statistical quasiclassical trajectory (SQCT) calculations. The dynamical observables considered include specific rate coefficients as a function of the translational energy, E T , thermal rate coefficients in the 100-500 K temperature range. In addition, kinetic energy spectra (KES) of the D + ions reactively scattered in H + + D 2 collisions are also presented for translational energies between 0.4 eV and 2.0 eV. For the two reactions, the best global agreement between experiment and theory over the whole energy range corresponds to the QCT calculations using a Gaussian binning (GB) procedure, which gives more weight to trajectories whose product vibrational action is closer to the actual integer QM values. The QM calculations also perform well, although somewhat worse over the more limited range of translational energies where they are available (E T o 0.6 eV and E T o 0.2 eV for the H + + D 2 and D + + H 2 reactions, respectively). The worst agreement is obtained with the SQCT method, which is only adequate for low translational energies. The comparison between theory and experiment also suggests that the most reliable rate coefficient measurements are those obtained with the merged beams technique. It is worth noting that none of the theoretical approaches can account satisfactorily for the experimental specific rate coefficients of H + + D 2 for E T r 0.2 eV although there is a considerable scatter in the existing measurements. On the whole, the best agreement with the experimental laboratory KES is obtained with the simulations carried out using the state resolved differential cross sections (DCSs) calculated with the QCT-GB method, which seems to account for most of the observed features. In contrast, the simulations with the SQCT data predict kinetic energy spectra (KES) considerably cooler than those experimentally determined.

Dynamics of the D+ + H2 → HD + H+ reaction at the low energy regime by means of a statistical quantum method

“…In that work, Honvault and Scribano concluded that their results for the j = 0 case were about between two and four times smaller than the statistical predictions of McCarroll; 51 for the reaction initiated with H 2 (v = 0, j = 1) however the TIQM rates were found larger than those statistical results. In turn, the value at T = 100 K reported by Jambrina et al 47 As in the case of the H + +H 2 reaction, 4, 56 the statistical predictions are found to reproduce correctly the qualitative behaviour of the TIQM results as a function of the temperature. In addition, the comparison with the TIQM values shows that the quantitative agreement is also remarkable.…”

“…47, the D + +H 2 process has been experimentally studied at much more extent for higher values of T. Fehsenfeld et al 15 reported a value larger than 0.7 × 10 −9 cm 3 molec −1 s −1 at T = 80 K and almost independent value with respect to the temperature of about 1.0 × 10 −9 cm 3 molec −1 s −1 with error bars of +0.5/ − 0.25 × 10 −9 cm 3 molec −1 s −1 for the range 100 K < T < 287 K. Both experimental results remain below the output of both theoretical results obtained in the present study. The k SQM (T) results overestimate the measured rate constants, as observed beyond E c ∼ 0.04 eV in the comparison established by Jambrina et al 47 for rate constants as a function of the energy for the title reaction calculated by means of the SQCT approach and measurements from ion beam investigations.…”

“…46 The title reaction has also been investigated by means of the SQCT approach, in combination with a variety of different theoretical methods. 47 The rate coefficients in terms of the collisional energy obtained with this quasi-classical version of the SQM approach were found to diverge from purely quasi-classical trajectory (QCT) calculations rapidly beyond E c ∼ 3 × 10 −1 eV for processes initiated from different initial rotational states of H 2 . A straightforward comparison for the entire energy range there considered with the time independent quantum mechanical (TIQM) method employed (the ABC code 48 ) was, however, not reported, since the QM calculation was limited up to E c ∼ 0.4 eV and the SQCT results were only shown from E c ∼ 10 −3 eV for j = 0 and between ∼ 5 × 10 −3 and 0.5 × 10 −2 eV for j = 1-3.…”

“…19 up to E c ∼ 0.18 eV, and within the experimental uncertainties of some other ion beam techniques for larger energies up to ∼1 eV. 47 The comparison of thermal rate constants between 100 and 500 K revealed that the statistical predictions are larger than both QM and QCT results, but are on the contrary, perhaps accidentally, the closest theoretical rates to the measurements by Henchman et al 18 Recently, some of us have carried out TIQM calculations on the D + +H 2 → HD+H + reaction to obtain ICSs and thermal rate constants at low temperature. 49 The comparison with previous experimental work 19,50 revealed that the calculation reproduced rate constants beyond a collision energy of 0.006 eV.…”

Dynamically biased statistical model for the ortho/para conversion in the ${\rm H}_2 + {\rm H}_3^+$H2+H3+ → ${\rm H}_3^{+} +$H3++ H2 reaction

“…45,46 This procedure has been applied recently to the study of the triatomic D + + H 2 and H + + D 2 reactions providing a very good agreement with experiments. 47 However, in the present five-atom case the ZPE is very high. This implies a very wide energy range of internal energy of the two products.…”