4,180 research outputs found
Vibrational branching ratios and shape resonant photoionization dynamics in N_2O
Vibrational branching ratios and photoelectron asymmetry parameters for alternative vibrational modes in the photoionization of N_2O(7Ļ^(ā1)) have been studied using accurate photoelectron continuum orbitals. Earlier dispersed ionic fluorescence measurements [E. D. Poliakoff, M. H. Ho, M. G. White, and G. E. Leroi, Chem. Phys. Lett. 130, 91 (1986)] revealed strong nonāFranckāCondon vibrational ion distributions for both the symmetric and antisymmetric stretching modes at low photoelectron energies. Our results establish that these features arise from a Ļ shape resonance which, based on its dependence on internuclear geometry, must be associated with the molecular framework as a whole and not with either of its fragments, NāN or NāO. This behavior accounts for the more pronounced deviations of the vibrational branching ratios from FranckāCondon values observed in the symmetric than in the antisymmetric mode. The Ļ continuum also supports a second shape resonance at higher energy which does not influence the vibrational branching ratios but is quite evident in the photoelectron asymmetry parameters around a photon energy of 40 eV. These vibrationally resolved studies of the photoelectron spectra of this polyatomic system provide an interesting example of the rich shape resonant behavior that can be expected to arise in polyatomic molecules with their alternative vibrational modes
Shape resonance behavior in 1Ļ_g photoionization of O_2
We report calculations of vibrationally resolved cross sections and photoelectron angular distributions for photoionization of O_2 leading to the X^ā2 Ī _g (Ī½^+ =0ā4) states of O^+_2 using HartreeāFock continuum photoelectron orbitals. These studies were motivated by recent results which show that a Ļ_u shape resonance plays a dominant role in producing nonāFranckāCondon vibrational distributions in resonant multiphoton ionization of O_2 via the Cā^3Ī _g (1Ļ_g3sĻ_g) Rydberg state. In the present study, we investigate how this shape resonance influences photoionization dynamics in singleāphoton ionization. Below 21 eV photon energy, we find significant nonāFranckāCondon effects in the vibrational branching ratios as well as in the vibrationally resolved photoelectron angular distributions. Substantial autoionization hinders a direct comparison between theory and experiment
Multiplet-specific shape resonant features in vibrationally resolved 3Ļ_g photoionization of O_2
We report multipletāspecific vibrationally resolved photoionization cross sections and photoelectron angular distributions for the 3Ļ_g orbital of O_2 leading to the v^+=0ā3 levels of the b^4Ī£^ā_g and B^2Ī£^ā_g states of O^+_2. These studies were motivated by recent work which shows significant nonstatistical behavior in the vibrationally unresolved spectrum at low photoelectron energies arising from the sensitivity of the kĻ_u shape resonance to the multipletāspecific exchange potentials. In addition to the anticipated nonāFranckāCondon vibrational distributions arising from the kĻ_u shape resonance, we also find substantial nonstatistical effects in our vibrationally resolved cross sections and particularly in our photoelectron angular distributions over a broad energy range. Extensive electronic autoionization due to Rydberg levels leading to the c^ā4Ī£^ā_u (2Ļ^(ā1)_u) ion makes it difficult to assess these effects in the available experimental data
A rigorous analysis of the cavity equations for the minimum spanning tree
We analyze a new general representation for the Minimum Weight Steiner Tree
(MST) problem which translates the topological connectivity constraint into a
set of local conditions which can be analyzed by the so called cavity equations
techniques. For the limit case of the Spanning tree we prove that the fixed
point of the algorithm arising from the cavity equations leads to the global
optimum.Comment: 5 pages, 1 figur
Characterization of Collective Gaussian Attacks and Security of Coherent-State Quantum Cryptography
We provide a simple description of the most general collective Gaussian
attack in continuous-variable quantum cryptography. In the scenario of such
general attacks, we analyze the asymptotic secret-key rates which are
achievable with coherent states, joint measurements of the quadratures and
one-way classical communication.Comment: 4 pages, 1 figure + 1 Table, REVteX. More descriptive titl
Exponentially Enhanced Quantum Metrology
We show that when a suitable entanglement generating unitary operator
depending on a parameter is applied on N qubits in parallel, and an appropriate
observable is measured, a precision of order 2 raised to the power (-N) in
estimating the parameter may be achieved. This exponentially improves the
precision achievable in classical and in quantum non-entangling parallel
strategies. We propose a quantum-optics model of laser light interacting with
an N-qubit system, say a polyatomic molecule, via a generalized Jaynes-Cummings
interaction which, in principle, could achieve the exponentially enhanced
precision.Comment: 4 pages, 1 postscript figure ; typos correcte
On Strong Superadditivity of the Entanglement of Formation
We employ a basic formalism from convex analysis to show a simple relation
between the entanglement of formation and the conjugate function of
the entanglement function E(\rho)=S(\trace_A\rho). We then consider the
conjectured strong superadditivity of the entanglement of formation , where and are the
reductions of to the different Hilbert space copies, and prove that it
is equivalent with subadditivity of . As an application, we show that
strong superadditivity would follow from multiplicativity of the maximal
channel output purity for all non-trace-preserving quantum channels, when
purity is measured by Schatten -norms for tending to 1.Comment: 11 pages; refs added, explanatory improvement
Shape-resonance-induced non-FranckāCondon effects in (2+1) resonance enhanced multiphoton ionization of the C 3Ī g state of O2
We show that strong non-FranckāCondon effects observed in (2+1) resonance enhanced multiphoton ionization of the C 3Pig state of O2 are due to the ksigmau shape resonance previously observed in single-photon studies of diatomic molecules. Calculated vibrational branching ratios for the v=2,3 levels of the C 3Ī g state are in reasonable agreement with experiment. Certain discrepancies remain in comparing theoretical results with the measured spectra, and possible electron-correlation effects which underly this are discussed
Clustering by soft-constraint affinity propagation: Applications to gene-expression data
Motivation: Similarity-measure based clustering is a crucial problem
appearing throughout scientific data analysis. Recently, a powerful new
algorithm called Affinity Propagation (AP) based on message-passing techniques
was proposed by Frey and Dueck \cite{Frey07}. In AP, each cluster is identified
by a common exemplar all other data points of the same cluster refer to, and
exemplars have to refer to themselves. Albeit its proved power, AP in its
present form suffers from a number of drawbacks. The hard constraint of having
exactly one exemplar per cluster restricts AP to classes of regularly shaped
clusters, and leads to suboptimal performance, {\it e.g.}, in analyzing gene
expression data. Results: This limitation can be overcome by relaxing the AP
hard constraints. A new parameter controls the importance of the constraints
compared to the aim of maximizing the overall similarity, and allows to
interpolate between the simple case where each data point selects its closest
neighbor as an exemplar and the original AP. The resulting soft-constraint
affinity propagation (SCAP) becomes more informative, accurate and leads to
more stable clustering. Even though a new {\it a priori} free-parameter is
introduced, the overall dependence of the algorithm on external tuning is
reduced, as robustness is increased and an optimal strategy for parameter
selection emerges more naturally. SCAP is tested on biological benchmark data,
including in particular microarray data related to various cancer types. We
show that the algorithm efficiently unveils the hierarchical cluster structure
present in the data sets. Further on, it allows to extract sparse gene
expression signatures for each cluster.Comment: 11 pages, supplementary material:
http://isiosf.isi.it/~weigt/scap_supplement.pd
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