ABSTRACT
Reactive collisions between O(-) and H2 have been studied experimentally at temperatures ranging from 10 K to 300 K using a cryogenic radiofrequency 22-pole ion trap. The rate coefficients for associative detachment, leading to H2O + e(-), increase with decreasing temperature and reach a flat maximum of 1.8 × 10(-9) cm(3) s(-1) at temperatures between 20 K and 80 K. There, the overall reaction probability is in good agreement with a capture model indicating efficient non-adiabatic couplings between the entrance potential energy surfaces. Classical trajectory calculations on newly calculated potential energy surfaces as well as the topology of the conical intersection seam leading to the neutral surface corroborate this. The formation of OH(-) + H via hydrogen transfer, although occurring with a probability of a few percent only (about 5 × 10(-11) cm(3) s(-1) at temperatures 10-300 K), indicates that there are reaction paths, where electron detachment is avoided.
ABSTRACT
Stabilization of H3(+) collision complexes has been studied at nominal temperatures between 11 and 33 K using a 22-pole radio frequency (rf) ion trap. Apparent binary rate coefficients, k(*) = kr + k3[H2], have been measured for para- and normal-hydrogen at number densities between some 10(11) and 10(14) cm(-3). The state specific rate coefficients extracted for radiative stabilization, kr(T;j), are all below 2 × 10(-16) cm(3) s(-1). There is a slight tendency to decrease with increasing temperature. In contrast to simple expectations, kr(11 K;j) is for j = 0 a factor of 2 smaller than for j = 1. The ternary rate coefficients for p-H2 show a rather steep T-dependence; however, they are increasing with temperature. The state specific ternary rate coefficients, k3(T;j), measured for j = 0 and derived for j = 1 from measurements with n-H2, differ by an order of magnitude. Most of these surprising observations are in disagreement with predictions from standard association models, which are based on statistical assumptions and the separation of complex formation and competition between stabilization and decay. Most probably, the unexpected collision dynamics are due to the fact that, at the low translational energies of the present experiment, only a small number of partial waves participate. This should make exact quantum mechanical calculations of kr feasible. More complex is three-body stabilization, because it occurs on the H5(+) potential energy surface.
ABSTRACT
In this paper, we present a numerical and experimental study of W3-4 photonic crystal (PhC) waveguide lasers fabricated on InP substrate. In such a PhC waveguide, the dispersion curve of the fundamental mode folds in the two-dimensional gap of the triangular lattice. Folding occurs at the Brillouin zone edge as in the case of genuine distributed feedback (DFB) lasers. Single-mode emission is presently observed in both electrical and optical pumping configurations. This behavior is attributed to the different levels of out-of-plane losses experienced by the two DFB mode components. Three-dimensional finite-difference-time-domain calculations are used to finely quantify the quality factors of the waveguide modes. The modal discrimination is shown to be reinforced when lasing occurs far from the conduction band edge. This trend is also predicted for other canonical waveguides in triangular PhCs as for instance W2-3 waveguides.
ABSTRACT
We report new experimental results, and their theoretical analysis, on the mechanisms that control light transfer between two integrated waveguides connected by two-dimensional matrices of dielectric pillars. The optical properties of the system are analysed from the well-established formalism of classical field susceptibilities (Green dyadic functions). We apply this scheme to investigate the optical properties of two-dimensional arrays connected to two integrated waveguides. Comparisons with current experimental work based on near-field optical probing are provided together with a spectral analysis of the phenomenon.
