Electron Nuclear Dynamics of H+ + C2H2 at ELab = 30, 200, and 450 eV.

We present a complete simplest-level electron nuclear dynamics (SLEND) investigation of H+ + C2H2 at collision energies ELab = 30, 200, and 450 eV. This reaction is relevant in astrophysics and provides a computationally feasible prototype for proton cancer therapy reactions. SLEND is a time-dependent, variational, direct, and nonadiabatic method that adopts a classical-mechanics description for the nuclei and a Thouless single-determinantal wave function for the electrons. We perform this study with our code PACE, which incorporates the One Electron Direct/Electron Repulsion Direct (OED/ERD) atomic integrals package developed by the Bartlett group. Current SLEND simulations with the 6-31G** basis set involves 2,646 trajectory calculations from 9 nonredundant, symmetry-inequivalent projectile-target orientations. For H+ + C2H2 at ELab = 30 eV, SLEND/6-31G** simulations predict one simple scattering process, and three reactive ones: C2H2 hydrogen substitution, C2H2 fragmentation into two CH moieties, and C2H2 fragmentation into CHC and H moieties, respectively. We reveal and analyze the mechanisms of these processes through computer animations; this valuable chemical information is inaccessible by experiments. The SLEND/6-31G** scattering angle functions exhibit primary and secondary rainbow scattering features that vary with the projectile-target orientations and collision energies. SLEND/6-31G** predicts 1-electron-transfer (1-ET) integral cross sections at ELab = 30, 200, and 450 eV in good agreement with their experimental counterparts. SLEND/6-31-G** predicts 1-ET differential cross sections (DCSs) at ELab = 30 eV that agree well with their experimental counterparts over all the measured scattering angles. In addition, SLEND/6-31G** predicts 0-ET DCSs at ELab = 30 eV that agree well with their experimental counterparts at low scattering angles, but less satisfactorily at higher ones. Remarkably, both the 0- and 1-ET DCSs from SLEND/6-31G** exhibit distinct primary rainbow scattering signatures in excellent agreement with their experimentally inferred counterparts. Furthermore, both SLEND/6-31G** and the experiment indicate that the primary rainbow scattering angles from the 0- and 1-ET DCSs are identical (an unusual fact in proton-molecule collisions). Through these rainbow scattering predictions, SLEND has also validated a procedure to extract primary rainbow angles from structureless DCSs. We analyze the obtained theoretical results in comparison with available experimental data and discuss forthcoming developments in the SLEND method.

Integrated polarization-free Bragg filters with subwavelength gratings for photonic sensing.

We present polarization-free Bragg filters having subwavelength gratings (SWGs) in the lateral cladding region. This Bragg design expands modal fields toward upper cladding, resulting in enhanced light interaction with sensing analytes. Two device configurations are proposed and examined, one with index-matched coupling between transverse electric (TE) and transverse magnetic (TM) modes and the other one with hybrid-mode (HM) coupling. Both configurations introduce a strong coupling between two orthogonal modes (either TE-TM or HM1-HM2) and rotate the polarization of the input wave through Bragg reflection. The arrangements of SWGs help to achieve two configurations with different orthogonal modes, while expanding modal profiles toward the upper cladding region. Our proposed SWG-assisted Bragg gratings with polarization independency eliminate the need for a polarization controller and effectively tailor the modal properties, enhancing the potential of integrated photonic sensing applications.

Polarization-independent photonic Bragg grating filter with cladding asymmetry.

A photonic Bragg grating is a fundamental building block that reflects the direction of wave propagation through spatial phase modulation and can be implemented using sidewall corrugation. However, due to the asymmetric aspect ratio of a waveguide cross section, typical Bragg gratings exhibit a strong polarization sensitivity. Here, we show that photonic Bragg gratings with cladding asymmetry can enable polarization-independent notch filters by rotating input polarizations. Such Bragg gratings strongly couple transverse electric (TE) and transverse magnetic (TM) modes propagating in opposite directions, filtering the input signal and reflecting the rotated mode. We analyzed this polarization-rotating Bragg grating using the coupled-mode theory and experimentally demonstrated it on a silicon-on-insulator platform. Our device concept is simple to implement and compatible with other platforms, readily available as polarization transparent Bragg components.

Electron nuclear dynamics of time-dependent symmetry breaking in H+ + H2O at ELab = 28.5-200.0 eV: a prototype for ion cancer therapy reactions.

Following our preceding research [P. M. McLaurin, R. Merritt, J. C. Domínguez, E. S. Teixeira and J. A. Morales, Phys. Chem. Chem. Phys., 2019, 21, 5006], we present an electron nuclear dynamics (END) investigation of H+ + H2O at ELab = 28.5-200.0 eV in conjunction with a computational procedure to induce symmetry breaking during evolution. The investigated system is a computationally feasible prototype to simulate water radiolysis reactions in ion cancer therapy. END is a time-dependent, variational, non-adiabatic, and on-the-fly method, which utilizes classical mechanics for nuclei and a Thouless single-determinantal state for electrons. In this study, a procedure inherent to END introduces low degrees of symmetry breaking into the reactants' restricted Hartree-Fock (RHF) state to induce a higher symmetry breaking during evolution. Specifically, the Thouless exponential operator acting on the RHF reference generates an axial spin density wave (ASDW) state according to Fukutome's analysis of HF symmetry breaking; this state exhibits spatial and spin symmetry breaking. By varying a Thouless parameter, low degrees of symmetry breaking are introduced into ASDW states. After starting the dynamics from those states, higher degrees of symmetry breaking may subsequently emerge as dictated by the END equations without ad hoc interventions. Simulations starting from symmetry-conforming states preserve the symmetry features during dynamics, whereas simulations starting from symmetry-broken states display an upsurge of symmetry breaking once the reactants collide. Present simulations predict three types of reactions: (I) projectile scattering, (II) hydrogen substitution, and (III) water radiolysis into H + OH and 2H + O fragments. Remarkably, symmetry breaking considerably increases the extent of the target-to-projectile electron transfers (ETs) occurring during the above reactions. Then, with symmetry breaking, 1-ET differential and integral cross sections increase in value, whereas 0-ET differential cross sections and primary rainbow scattering angles decrease. More importantly, END properties calculated from symmetry-breaking simulations exhibit better agreement with the experimental data. Notably, END 1-ET integral cross sections with symmetry breaking compare better with their experimental counterparts than 1-ET integral cross sections from high-level close-coupling calculations; moreover, END validates an undetected rainbow scattering peak inferred from the experimental data. A discussion of our symmetry-breaking procedure in the context of Fukutome's analysis of HF symmetry breaking is also presented.