2,934 research outputs found
Floquet theory of radical pairs in radiofrequency magnetic fields.
This is the final version of the article. Available from AIP Publishing via the DOI in this record.We present a new method for calculating the product yield of a radical pair recombination reaction in the presence of a weak time-dependent magnetic field. This method successfully circumvents the computational difficulties presented by a direct solution of the Liouville-von Neumann equation for a long-lived radical pair containing many hyperfine-coupled nuclear spins. Using a modified formulation of Floquet theory, treating the time-dependent magnetic field as a perturbation, and exploiting the slow radical pair recombination, we show that one can obtain a good approximation to the product yield by considering only nearly degenerate sub-spaces of the Floquet space. Within a significant parameter range, the resulting method is found to give product yields in good agreement with exact quantum mechanical results for a variety of simple model radical pairs. Moreover it is considerably more efficient than the exact calculation, and it can be applied to radical pairs containing significantly more nuclear spins. This promises to open the door to realistic theoretical investigations of the effect of radiofrequency electromagnetic radiation on the photochemically induced radical pair recombination reactions in the avian retina which are believed to be responsible for the magnetic compass sense of migratory birds.We gratefully acknowledge funding from the European
Research Council under the European Union’s 7th Framework
Programme, FP7/2007-2013/ERC Grant Agreement
No. 340451, and from the US Air Force (USAF) Office of
Scientific Research under the Air Force Materiel Command,
USAF Award No. FA9550-14-1-0095
Using quantum rotational polarization moments to describe the stereodynamics of the H+D-2(v=0,j=0)-> HD(v ',j ')+D reaction
We present results of quantum calculations we have performed on the title reaction in order to study its stereodynamics at collision energies of 0.54 and 1.29 eV. Our theoretical model is based on a representation where directional properties are expressed in terms of real rotational polarization moments instead of magnetic quantum numbers. We analyze the physical meaning of rotational polarization moments and show that, when defined as in the present work, these quantities directly describe the reaction stereodynamics in terms of intuitive chemical concepts related to preferences in the reaction mechanism for particular planes and senses of molecular rotation. Using this interpretation, we identify two distinct regimes for the stereodynamics of the title reaction, observed when HD is formed with low or high rotational excitation. We also identify relevant characteristics of both regimes: (i) the existence and location of preferred planes and senses of molecular rotation, (ii) correlations between these preferences, the scattering angle and the reaction probability, and (iii) their dependence on the collision energy. (C) 1998 American Institute of Physics.10883142315
Dynamics of OH(2Pi)-He collisions in combined electric and magnetic fields
We use accurate quantum mechanical calculations to analyze the effects of
parallel electric and magnetic fields on collision dynamics of OH(2Pi)
molecules. It is demonstrated that spin relaxation in 3He-OH collisions at
temperatures below 0.01 K can be effectively suppressed by moderate electric
fields of order 10 kV/cm. We show that electric fields can be used to
manipulate Feshbach resonances in collisions of cold molecules. Our results can
be verified in experiments with OH molecules in Stark decelerated molecular
beams and electromagnetic traps.Comment: 20 pages, 5 figures, submitted to Faraday Discuss. 142: Cold and
Ultracold Molecule
Cold heteromolecular dipolar collisions
We present the first experimental observation of cold collisions between two
different species of neutral polar molecules, each prepared in a single
internal quantum state. Combining for the first time the techniques of Stark
deceleration, magnetic trapping, and cryogenic buffer gas cooling allows the
enhancement of molecular interaction time by 10. This has enabled an
absolute measurement of the total trap loss cross sections between OH and
ND at a mean collision energy of 3.6 cm (5 K). Due to the dipolar
interaction, the total cross section increases upon application of an external
polarizing electric field. Cross sections computed from \emph{ab initio}
potential energy surfaces are in excellent agreement with the measured value at
zero external electric field. The theory presented here represents the first
such analysis of collisions between a radical and a closed-shell
polyatomic molecule.Comment: 7 pages, 5 figure
Quantum Theory of Reactive Scattering in Phase Space
We review recent results on quantum reactive scattering from a phase space
perspective. The approach uses classical and quantum versions of normal form
theory and the perspective of dynamical systems theory. Over the past ten years
the classical normal form theory has provided a method for realizing the phase
space structures that are responsible for determining reactions in high
dimensional Hamiltonian systems. This has led to the understanding that a new
(to reaction dynamics) type of phase space structure, a {\em normally
hyperbolic invariant manifold} (or, NHIM) is the "anchor" on which the phase
space structures governing reaction dynamics are built. The quantum normal form
theory provides a method for quantizing these phase space structures through
the use of the Weyl quantization procedure. We show that this approach provides
a solution of the time-independent Schr\"odinger equation leading to a (local)
S-matrix in a neighborhood of the saddle point governing the reaction. It
follows easily that the quantization of the directional flux through the
dividing surface with the properties noted above is a flux operator that can be
expressed in a "closed form". Moreover, from the local S-matrix we easily
obtain an expression for the cumulative reactio probability (CRP).
Significantly, the expression for the CRP can be evaluated without the need to
compute classical trajectories. The quantization of the NHIM is shown to lead
to the activated complex, and the lifetimes of quantum states initialized on
the NHIM correspond to the Gamov-Siegert resonances. We apply these results to
the collinear nitrogen exchange reaction and a three degree-of-freedom system
corresponding to an Eckart barrier coupled to two Morse oscillators.Comment: 59 pages, 13 figure
An atomic and molecular database for analysis of submillimetre line observations
Atomic and molecular data for the transitions of a number of astrophysically
interesting species are summarized, including energy levels, statistical
weights, Einstein A-coefficients and collisional rate coefficients. Available
collisional data from quantum chemical calculations and experiments are
extrapolated to higher energies. These data, which are made publically
available through the WWW at http://www.strw.leidenuniv.nl/~moldata, are
essential input for non-LTE line radiative transfer programs. An online version
of a computer program for performing statistical equilibrium calculations is
also made available as part of the database. Comparisons of calculated emission
lines using different sets of collisional rate coefficients are presented. This
database should form an important tool in analyzing observations from current
and future (sub)millimetre and infrared telescopes.Comment: Accepted for publication in A&A, 14 pages, 5 figure
Activation of muscarinic cholinergic receptors enhances the volume-sensitive efflux of myo-inositol from SH-SY5Y neuroblastoma cells
A mechanism used by cells to regulate their volume under hypo-osmotic conditions is the release of organic osmolytes, one of which is myo-inositol. The possibility that activation of phospholipase-C-linked receptors can regulate this process has been examined for SH-SY5Y neuroblastoma cells. Incubation of cells with hypo-osmolar buffers (160–250 mOsm) led to a biphasic release of inositol which persisted for up to 4 h and could be inhibited by inclusion of anion channel blockers – results which indicate the involvement of a volume-sensitive organic anion channel. Inclusion of oxotremorine-M, a muscarinic cholinergic agonist, resulted in a marked increase (80–100%) in inositol efflux under hypo-osmotic, but not isotonic, conditions. This enhanced release, which was observed under all conditions of hypo-osmolarity tested, could be prevented by inclusion of atropine. Incubation of the cells with either the calcium ionophore, ionomycin, or the phorbol ester, phorbol 12-myristate 13-acetate, partially mimicked the stimulatory effect of muscarinic receptor activation when added singly, and fully when added together. The ability of oxotremorine-M to facilitate inositol release was inhibited by removal of extracellular calcium, depletion of intracellular calcium or down-regulation of protein kinase C. These results indicate that activation of muscarinic cholinergic receptors can regulate osmolyte release in this cell line.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65241/1/j.1471-4159.2003.02021.x.pd
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