Volume 121, Issue 11, 15 September 2004
Index of content:
121(2004); http://dx.doi.org/10.1063/1.1794652View Description Hide Description
We report a reversible liquid–solid transition upon heating of a simple solution composed of a-cyclodextrine (αCD), water, and 4-methylpyridine. These solutions are homogeneous and transparent at ambient temperature and solidify when heated to temperatures between 45° and 75°. Quasielastic and elastic neutron scattering show that molecular motions are slowed down in the solid and that crystalline order is established. The solution “freezes on heating.” This process is fully reversible, on cooling the solid melts. A rearrangement of hydrogen bonds is postulated to be responsible for the observed phenomenon.
Reaction-path switching induced by spatial-distribution change of reactants: CO oxidation on Pt(111)121(2004); http://dx.doi.org/10.1063/1.1796235View Description Hide Description
We studied the mechanism of CO oxidation on O-covered Pt(111) surfaces during CO exposure by means of time-resolvednear edge x-ray absorption fine structurespectroscopy. Two distinct reaction processes were found to occur sequentially; isolated O atoms and island-periphery O atoms contribute to each process. Combination of in situ monitoring of the reaction kinetics and Monte Carlo simulations revealed that CO coadsorption plays a role of inducing the dynamic change in spatial distribution of O atoms, which switches over the two reaction paths.
Stimulated emission three-pulse photo-echo peakshift: A mixed pump–probe and photon-echo technique for studying excited-state dynamics121(2004); http://dx.doi.org/10.1063/1.1794694View Description Hide Description
A novel four-pulse photon-echo technique for exploring condensed phase dynamics at different parts of the excited-statepotential energy surface is presented. In contrast to traditional three-pulse photon-echo signals, the introduction of a fourth pump pulse allows the use of photon-echo techniques to probe excited-state phenomena. Here, a “proof of principle” experiment is presented where the excited-statesolventdynamics of the coumarin 153 chromophore dissolved in methanol is explored. The fluctuations of the stimulated emission transition is probed, in contrast to the ground-state absorption transition explored in traditional echo measurements. Distinctly different excited-statedynamics, in contrast to ground-state signals, is observed and discussed.