Without the need to construct complicated potential energy surfaces, a multicenter impulsive model is developed to characterize the dynamical feature of conventional transition state (TS) and roaming pathways in the photodissociation of formaldehyde, H2CO → CO + H2. The photofragment energy distributions (PED) resulting from the roaming mechanism are found to closely correlate to a particular configuration that lies close to the edge of the plateau-like intrinsic reaction coordinate, whereas such a PED is associated with the configuration at the saddle point when the conventional TS pathway is followed. The evaluated PED results are consistent with those by experimental findings and quasi-classical trajectory calculations. Following impulsive analysis, the roaming pathway can be viewed as a consequence of energy transfer events between several vibrational modes. For H2CO, the available energy initially accumulated at the C-H bond is transferred to other transitional mode(s) via stretching-bending coupling, and finally to the HH stretching. (Chemical Presented). © 2014 American Chemical Society.

We report a ratiometric fluorescent metal ion sensor based on the mechanism of fluorescence resonance energy transfer (FRET) between synthesized 15-crown-5-ether capped CdSe/ZnS quantum dots (QDCE) and 15-crown-5-ether attached rhodamine B (RBCE) in pH 8.3 buffer solution. Fluorescence titration with different metal ions in pH 8.3 buffer solution of the QDCE-RBCE conjugate showed a decrease and an increase in the fluorescence intensity for QDCE and RBCE moieties respectively due to FRET from QDCE to RBCE. This sensor system shows excellent selectivity towards K+ ions resulting in increasing efficiency of FRET. Energy transfer efficiency depends on the affinity between metal ions and crown ether functionalized with QDCE/RBCE. The detailed analysis of FRET was explored. This water soluble ratiometric sensor system can act as a good FRET probe for sensing applications especially in biological systems. © The Royal Society of Chemistry 2015.

The photodissociation channels of methyl formate have been extensively investigated by two different advanced experimental techniques, ion imaging and Fourier-Transform-Infrared emission spectroscopy, combined with quantum chemical calculations and molecular dynamics simulations. Our aim is to characterize the role of alternative routes to the conventional transition-state mediated pathway: the roaming and the triple fragmentation processes. The photolysis experiments, carried out at a range of laser wavelengths in the vicinity of the triple fragmentation threshold, beside the simulation of large bunches of classical trajectories with different initial conditions, have shown that both mechanisms share a common path that involves a conical intersection during the relaxation process from the electronic excited state S1 to the ground state S0. © 2015 AIP Publishing LLC.

Photodissociation of isobutyraldehyde (C3H7CHO) at 248 nm is investigated using time-resolved Fourier-transform infrared emission spectroscopy to demonstrate the growing importance of the roaming pathway with increasing molecular size of aliphatic aldehydes. Each acquired CO rotational distribution from v = 1 to 4 is well characterized by a single Boltzmann rotational temperature from 637 to 750 K, corresponding to an average rotational energy of 5.9 ± 0.6 kJ mol-1. The roaming signature that shows a small fraction of CO rotational energy disposal accompanied by a vibrationally hot C3H8 co-fragment is supported by theoretical prediction. The energy difference between the tight transition state (TS) and the roaming saddle point (SP) is found to be -27, 4, 15, 22, and 30 kJ mol-1 for formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, and 2,2-dimethyl propanal, respectively. The roaming SP is stabilized by a larger alkyl moiety. It is suggested that the roaming photodissociation rate of aldehydes increasingly exceeds those via the tight TS, resulting in the dominance of the CO + alkane products, as the size of aldehydes becomes larger. Along with formaldehyde, acetaldehyde, and propionaldehyde, in this work isobutyraldehyde is further demonstrated that this aldehyde family with special functional group is the first case in the organic compound to follow predominantly a roaming dissociation pathway, as the molecular size becomes larger. © the Owner Societies 2015.

This paper aims at discussing new facets on stereodynamical behaviors in chemical reactions, i.e. the effects of molecular orientation and alignment on reactive processes. Further topics on macroscopic processes involving deviations from Arrhenius behavior in the temperature dependence of chemical reactions and chirality effects in collisions are also discussed. © 2015 AIP Publishing LLC.

We present an experimental study of the coherence properties of amplified slow light by four-wave mixing (FWM) in a three-level electromagnetically induced transparency (EIT) system driven by one additional pump field. High energy gain (up to 19) is obtained with a weak pump field (a few mW∕cm2) using optically dense cold atomic gases. A large fraction of the amplified light is found to be phase incoherent to the input signal field. The dependence of the incoherent fraction on pump field intensity and detuning and the control field intensity is systematically studied. With the classical input pulses, our results support a recent theoretical study by Lauk et al. [Phys. Rev. A 88, 013823 (2013)], showing that the noise resulting from the atomic dipole fluctuations associated with spontaneous decay is significant in the high gain regime. This effect has to be taken into consideration in EIT-based applications in the presence of FWM.

We present an experimental study to achieve ultrahigh optical depths for cold atomic media with a two dimensional magneto-optical trap (MOT) of cesium. By combining large atom number, a temporally dark and compressed MOT, and Zeeman-state optical pumping, we achieve an optical depth of up to 1306 for the open transition of the cesium D1 line. Our work demonstrates that it is feasible to push the optical depth up to the 1000 level with a convenient MOT setup. This development paves the way to many important proposals in quantum optics and many-body physics.