Observation and interpretation of a time-delayed mechanism in the hydrogen exchange reaction

Abstract

Extensive theoretical and experimental studies have shown the hydrogen exchange reaction H+H2 --> H2+H to occur predominantly through a 'direct recoil' mechanism: the H--H bonds break and form concertedly while the system passes straight over a collinear transition state, with recoil from the collision causing the H2 product molecules to scatter backward. Theoretical predictions agree well with experimental observations of this scattering process. Indirect exchange mechanisms involving H3 intermediates have been suggested to occur as well, but these are difficult to test because bimolecular reactions cannot be studied by the femtosecond spectroscopies used to monitor unimolecular reactions. Moreover, full quantum simulations of the time evolution of bimolecular reactions have not been performed. For the isotopic variant of the hydrogen exchange reaction, H+D2 --> HD+D, forward scattering features observed in the product angular distribution have been attributed to possible scattering resonances associated with a quasibound collision complex. Here we extend these measurements to a wide range of collision energies and interpret the results using a full time-dependent quantum simulation of the reaction, thus showing that two different reaction mechanisms modulate the measured product angular distribution features. One of the mechanisms is direct and leads to backward scattering, the other is indirect and leads to forward scattering after a delay of about 25 femtoseconds.

Quantum-classical transition for an analog of the double-slit experiment in complex collisions: Dynamical decoherence in quantum many-body systems

“…The physical picture of rotational wave packets and their interference, originally revealed for heavy-ion collisions [6,7,8], has been strongly supported by numerical calculations for, e.g. the H+D 2 [9], F+HD [10] and He+H + 2 [11] stateto-state chemical reactions. The many-body aspect is of primary importance for the QCT since the macroscopic world consists of complex systems.…”

“…In this reaction, the wave packets likely originate from interference of the overlapping resonances of the IC [18]. From the numerical calculations [9] we deduce Φ = 135 • andhω = 0.045 eV. It was found that the main contribution to the time-delayed reaction mechanism comes from the total angular momenta J = 15−20.…”

“…(6) to describe rotational wave packets and their interference revealed [9] for the chemical reaction H+D 2 (v i = 0, j i = 0) →HD(v f = 3, j f = 0)+D, where v i , j i and v f , j f are vibrational and rotational quantum numbers for the initial and final states, respectively. In this reaction, the wave packets likely originate from interference of the overlapping resonances of the IC [18].…”

Chemistry at surfaces: from ab initio structures to quantum dynamics

“…For gas-phase systems, for example, quantization of the internal modes at the transition state (quantized transition states or quantized dynamical bottlenecks) or Feshbach resonances on purely repulsive potential energy surfaces are typical quantum effects that have been predicted for several collision systems, but that have been observed only for a few of them. The H + H 2 isotopic family of reactions is the best known example for quantized dynamical bottlenecks which lead to an observable time-delayed mechanism [147,148], and interference features [149,150]. The F + HD→HF + D reaction is the first reaction for which conclusive evidence has been found for a Feshbach resonance in a full collisional dynamics [151,152] and only recently experimental evidence has been found for some polyatomic reactions [153][154][155].…”

Coriolis coupling effects in the calculation of state-to-state integral and differential cross sections for the H+D2 reaction

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Most of the previous state-to-state investigations of the H +D 2 reaction have been aimed at elucidating unsolved issues, such as the fact that the calculated rotational state distribution for HD͑vЈ =3, jЈ͒ was colder than that measured experimentally especially at higher collision energies, 6,10,11,13 the influence on state-to-state integral and differential cross sections of the geometric phase and nonadiabatic coupling, [16][17][18] etc. In contrast to this, the effect of Coriolis coupling on state-to-state reaction dynamics has received less attention, though several quantum studies have examined the reliability of the centrifugal sudden ͑CS͒ approximation, in which the Coriolis coupling is set to zero, for the calculation of more averaged quantities such as reaction probabilities and integral cross sections.…”