Yuxin Tan

Photodissociation dynamics of deuterium sulfide cations (D2S+) via the A2A1 state were investigated using the time-sliced velocity map ion imaging technique. High-resolution images of S+(4S) from the S+-loss channel, D2(X1Σg+) + S+(4S), were acquired at five wavelengths near 320 nm. From the high-resolution images, we derived the total product kinetic energy releases, angular distributions, and rovibrational-state populations of the D2(X1Σg+) co-products. The product angular distributions are nearly isotropic across all photolysis wavelengths studied. While the available energy distributions in internal and translational energy show only weak variations, the internal state populations of products exhibit clear wavelength dependence. Based on MRCI+Q/aug-cc-pV5Z calculations, the energy correlation diagram of D2S+ was constructed. The excitation photon energies employed in this study lie near the dissociation threshold of the SD+(3Σ−) + D(2S) channel. The results reveal rich photofragmentation dynamics arising from complex non-adiabatic couplings among several electronic states. Multiple dissociation pathways, including possible roaming mechanisms, contribute to the formation of D2(X1Σg+) + S+(4S) products at the excitation energy near the dissociation threshold of the SD+(3Σ−) + D(2S) channel.

The H + CH4 reaction and its isotopic variant reactions serve as a benchmark reaction system to understand the dynamics of polyatomic reactions. In this paper, we report a vibrationally state-resolved crossed molecular beam study on the D + CD4 → D2 + CD3 reaction using the time-sliced velocity map ion imaging method. Pronounced collision energy dependence is found in product kinetic energy distributions and angular distributions. At 0.67 eV, CD3 products are dominantly distributed in the backward direction with D2 products in the vibrational ground state. As collision energy increases, the angular distributions of CD3 products shift from the backward direction to the sideways and forward directions, and vibrational excited D2 appears and intensifies. By comparing our result of the D + CD4 → D2 + CD3 reaction with previous studies on the D + CH4 → HD + CH3 and H + CD4 → HD + CD3 reactions, isotope effects have been discussed. The product kinetic energy and angular distributions vary significantly among the three isotopic variant reactions.

How the non-reacting moiety of a molecule influences a polyatomic reaction has been a topic of much research interest. Here we present a comprehensive investigation of the D + CH₄ → HD + CH₃ reaction, a benchmark polyatomic elementary reaction with CH₃ as the non-reacting moiety, employing a high-resolution crossed molecular beams apparatus and an accurate seven-dimensional wave packet method. An interesting angular distribution of the CH₃(v’ = 1) product umbrella bending vibrational state is observed to scatter more in the sideways direction than the CH₃(v’ = 0) one. By monitoring the wave functions on a dividing surface in the transition state region, the CH₃ umbrella bending mode is established as a reporter mode that faithfully reveals how the D atom dynamically approaches CH₄ at different total angular momenta or impact parameters. This discovery of the reporter mode provides an opportunity for the detailed study of polyatomic reaction dynamics.

The photodissociation dynamics of the hydrogen sulfide cation (H2S+) (X2B1) were investigated using the time-sliced velocity map ion imaging technique. S+ (4Su) product images were measured at four photolysis wavelengths around 393.70 nm, corresponding to the excitation of the H2S+ (X2B1) cation to the A2A1 (0, 8, 0) state. The raw images and the derived total kinetic energy releases (TKERs) spectra exhibited partial rotational resolution for the H2 products. A sensitive dependence on the photolysis wavelength was observed in the TKER spectra and anisotropy parameters. Within a narrow excitation energy range of ∼12 cm−1, the H2 products showed two distinct rotational excitations. Furthermore, clear differences in anisotropy parameters were observed. These phenomena indicate that the rotational excitation of the H2S+ ions plays a role in the non-adiabatic photodissociation dynamics.

Intriguing dynamics pattern in F + HD: Despite decades of studies, the role of relativistic spin-orbit interactions in the dynamics of chemical reactions remains an intriguing topic. Using a high-resolution velocity map imaging crossed beams technique, Chen et al. observed an interesting pattern in the differential cross sections in the F + HD → HF + D reaction near the partial wave resonances (see the Perspective by Rakitzis). Further theoretical analysis showed that this pattern originates from quantum interference between spin-orbit split partial wave resonances with different total parities. The effect of the fine structure of the partial waves observed for this long known yet not completely explored three-atom system represents one more remarkable demonstration of the truly quantum nature of chemical reaction dynamics. Science , this issue p. 936 ; see also p. 886