A fully state- and angle-resolved study of the H+HD→D+H2 reaction: Comparison of a molecular beam experiment to ab initio quantum reaction dynamics

Abstract

We present the results of a joint experimental and theoretical investigation of the reaction dynamics of the HϩHD→DϩH 2 chemical reaction. The experiment was performed using a crossed molecular beam apparatus that employed the Rydberg-atom time-of-flight detection scheme for the product D atom. The photolysis of a HI precursor molecule produced a beam source of hot H atoms, which, when crossed with a cold HD beam, yielded two well-defined center-of-mass collision energies, E C ϭ0.498 and 1.200 eV. The resolution of the experiment was sufficient to allow the measurement of the rovibrationally state-resolved differential cross section from the ground state of the HD reagent. The reaction was modeled theoretically using a converged coupled channel scattering calculation employing the BKMP2 potential energy surface: The S matrix was computed on a grid of 56 energies in the range E C ϭ0.245-1.551 eV. It is found that the experimental and theoretical state-to-state differential cross sections are in quantitative agreement at the two experimental energies. The geometric phase, which was not included in the calculation, is apparently not required at the energies considered. The spin statistics for the two identical protons is observed to have a dramatic effect on the rotational distribution of H 2 products, giving rise to a saw-toothed distribution with odd-jЈϾeven-jЈ. The differential cross section for several of the product states exhibited a dramatic forward peak that may be the signature of trapped quantum states near the saddle point. A detailed analysis of the reaction attributes is presented based on the energy dependence of the computed S matrix.

The observation of quantum bottleneck states

“…A simple picture to interpret the energy dependence of the state-to-state reaction probabilities, based on the adiabatic model, has recently been advanced by Chao et al [41] and Harich et al [35]. As illustrated by the schematic diagram in figure 12, the adiabatic potential curves correlate the initial and final rovibrational states along the reaction coordinate.…”

“…n 0 ;E, J ) have been observed in a number of reactions studied by using accurate quantum scattering calculations. For example, Chao et al [41] have analysed the oscillation in P R (n ! n 0 ;E, J ) vs. E for the H þ HD reaction and established the close connection to the QBS energies.…”

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

“…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]. Finally, Kliner et al [20] have studied the H + para-H 2 → ortho-H 2 (ν , j ) + H reaction.…”

Probing Quantum Dynamics of Elementary Chemical Reactions via Accurate Potential Energy Surfaces

“…In a later report, Althorpe and coworkers analyzed the forward scattering peak using full quantum dynamics calculations based on the BKMP2 PES [45], which was claimed to have an accuracy of about 5 cm −1 , and concluded that the forward scattering peaks observed in the DCS for the H + D 2 reaction were associated with the time-delay in the transition state region [46]. [47,48]. Theoretical state-resolved differential cross section using the highly accurate BKMP2 PES are in quantitative agreement with the experimental results (see also Fig.…”