5,007 research outputs found
Cross sections for short pulse single and double ionization of helium
In a previous publication, procedures were proposed for unambiguously
extracting amplitudes for single and double ionization from a time-dependent
wavepacket by effectively propagating for an infinite time following a
radiation pulse. Here we demonstrate the accuracy and utility of those methods
for describing two-photon single and one-photon double ionization of helium. In
particular it is shown how narrow features corresponding to autoionizing states
are easily resolved with these methods.Comment: 9 pages, 9 figure
Decoding sequential vs non-sequential two-photon double ionization of helium using nuclear recoil
Above 54.4 eV, two-photon double ionization of helium is dominated by a
sequential absorption process, producing characteristic behavior in the single
and triple differential cross sections. We show that the signature of this
process is visible in the nuclear recoil cross section, integrated over all
energy sharings of the ejected electrons, even below the threshold for the
sequential process. Since nuclear recoil momentum imaging does not require
coincident photoelectron measurement, the predicted images present a viable
target for future experiments with new short-pulse VUV and soft X-ray sources.Comment: 4 pages, 3 figure
Photoabsorption in formaldehyde: Intensities and assignments in the discrete and continuous spectral intervals
Theoretical investigations of total and partialâchannel photoabsorption cross sections in molecular formaldehyde are reported employing the StieltjesâTchebycheff (SâT) technique and separatedâchannel staticâexchange (IVO) calculations. Vertical oneâelectron dipole spectra for the 2b_2(n), 1b_1(Ï), 5a_1(Ï), 1b_2, and 4a_1 canonical molecular orbitals are obtained using HartreeâFock frozenâcore functions and large basis sets of compact and diffuse normalizable Gaussians to describe the photoexcited and ejected electrons. The calculated discrete excitation spectra provide reliable zerothâorder approximations to both valence and Rydberg transitions, and, in particular, the 2b_2(n) ânsa_1, npa_1, npb_2, and nda_2 IVO spectra are in excellent accord with recent experimental assignments and available intensity measurements. Convergent (SâT) photoionization cross sections in the staticâexchange (IVO) approximation are obtained for the 15 individual partial channels associated with ionization of the five occupied molecular orbitals considered. Resonance features in many of the individualâchannel photoionization cross sections are attributed to contributions from valencelike a_1Ï^â (CO), a_1Ï^â (CH), and b_2Ï^â (CH)/Ï_y^â (CO) molecular orbitals that appear in the photoionization continua, rather than in the corresponding oneâelectron discrete spectral intervals. The vertical electronic cross sections for ^1A_1â^1B_1, ^1B_2, and ^1A_1 excitations are in generally good accord with previously reported CI (SâT) predictions of continuum orbital assignments and intensities, although some discrepancies due to basisâset differences are present in the ^1B_1 and ^1B_2 components, and larger discrepancies apparently due to channel coupling are present in the ^1A_1â^1A_1 cross section. Partialâchannel vertical electronic cross sections for the production of the five lowest parentâion electronic states are found to be in general agreement with the results of very recent synchrotronâradiation photoelectron branchingâratio measurements in the 20 to 30 eV excitation energy interval. Most important in this connection is the tentative verification of the predicted orderings in intensities of the partialâ channel cross sections, providing support for the presence of a strong ka_1Ï^â (CO) resonance in the (5a_1^(â1))^2A_1 channel. Finally, the total vertical electronic cross sections for absorption and ionization are in general accord with photoabsorption measurements, photoionizationâmassâspectrometric studies, and the previously reported CI (SâT) calculations. Although further refined calculations including vibrational degrees of freedom and autoionization line shapes are required for a more precise quantitative comparison between theory and experiment, the present study should provide a reliable zerothâorder account of discrete and continuum electronic dipole excitations in molecular formaldehyde
Selective bond-breaking in formic acid by dissociative electron attachment.
We report the results of a joint experimental and theoretical study of dissociative electron attachment to formic acid (HCOOH) in the 6-9 eV region, where H- fragment ions are a dominant product. Breaking of the C-H and O-H bonds is distinguished experimentally by deuteration of either site. We show that in this region H- ions can be produced by formation of two or possibly three Feshbach resonance (doubly-excited anion) states, one of which leads to either C-H or O-H bond scission, while the other can only produce formyloxyl radicals by O-H bond scission. Comparison of experimental and theoretical angular distributions of the anion fragment allows the elucidation of state specific pathways to dissociation
An FPT Algorithm for Spanning Trees with Few Branch Vertices Parameterized by Modular-Width
The minimum branch vertices spanning tree problem consists in finding a spanning tree T of an input graph G having the minimum number of branch vertices, that is, vertices of degree at least three in T. This NP-hard problem has been widely studied in the literature and has many important applications in network design and optimization. Algorithmic and combinatorial aspects of the problem have been extensively studied and its fixed parameter tractability has been recently considered. In this paper we focus on modular-width and show that the problem of finding a spanning tree with the minimum number of branch vertices is FPT with respect to this parameter
Speeding up Networks Mining via Neighborhood Diversity
Parameterized complexity was classically used to efficiently solve NP-hard problems for small values of a fixed parameter. Then it has also been used as a tool to speed up algorithms for tractable problems. Following this line of research, we design algorithms parameterized by neighborhood diversity (nd) for several graph theoretic problems in P (e.g., Maximum Matching, Triangle counting and listing, Girth and Global minimum vertex cut). Such problems are known to admit algorithms parameterized by modular-width (mw) and consequently - being the nd a "special case" of mw - by nd. However, the proposed novel algorithms allow to improve the computational complexity from a time O(f(mw)? n +m) - where n and m denote, respectively, the number of vertices and edges in the input graph - which is multiplicative in n to a time O(g(nd)+n +m) which is additive only in the size of the input
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