4,331 research outputs found
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
Theoretical studies of photoexcitation and ionization in H_2O
Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H_2O employing FranckâCondon and staticâexchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the groundâstate equilibrium geometry, and previously devised momentâtheory techniques are employed in constructing the continuum oscillatorâstrength densities from the calculated spectra. Detailed comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impactâexcitation cross sections, and dipole (e,â2e)/(e,âe+ion) and synchrotronâradiation studies of partialâchannel photoionization cross sections. The various calculated excitation series in the outerâvalence (1b(^â1)_1, 3a(^â1)_1, 1b(^â1)_2) region are found to include contributions from valenceâlike 2b_2â(Ï*) and 4a_1(Îł*) virtual orbitals, as well as appropriate nsa_1, npa_1, nda_1, npb_1, npb_2, ndb_1, ndb_2, and nda_2 Rydberg states. Transition energies and intensities in the âŒ7 to 19 eV interval obtained from the present studies are seen to be in excellent agreement with the measured photoabsorption cross section, and to provide a basis for detailed spectral assignments. The calculated (1b(^â1)_1)X(^â2)B_1, (3a_1(^â1))^2A_1, and (1b_2(^â1))(^2)B_2 partialâchannel cross sections are found to be largely atomicâlike and dominated by 2pâkd components, although the 2b_2(Ï*) orbital gives rise to resonanceâlike contributions just above threshold in the 3a_1âkb_2 and 1b_2âkb_2 channels. It is suggested that the latter transition couples with the underlying 1b_1âkb_1 channel, accounting for a prominent feature in the recent highâresolution synchrotronâradiation measurements. When this feature is taken into account, the calculations of the three outerâvalence channels are in excellent accord with recent synchrotronâradiation and dipole (e,â2e) photoionization crossâsectional measurements. The calculated innerâvalence (2a_1(^â1)) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate finalâstate configurationâmixing effects that give rise to a modest failure of the Koopmans approximation, and to the observed broad PES band, in this case. Finally, the origins of the various spectral features present in the measured 1a_1 oxygen Kâedge electron energyâloss profile in H_2O are seen to be clarified fully by the present calculations
Resonant ion-pair formation in electron recombination with HF^+
The cross section for resonant ion-pair formation in the collision of
low-energy electrons with HF^+ is calculated by the solution of the
time-dependent Schrodinger equation with multiple coupled states using a wave
packet method. A diabatization procedure is proposed to obtain the electronic
couplings between quasidiabatic potentials of ^1Sigma^+ symmetry for HF. By
including these couplings between the neutral states, the cross section for
ion-pair formation increases with about two orders of magnitude compared with
the cross section for direct dissociation. Qualitative agreement with the
measured cross section is obtained. The oscillations in the calculated cross
section are analyzed. The cross section for ion-pair formation in electron
recombination with DF^+ is calculated to determine the effect of isotopic
substitution.Comment: 12 pages, 12 figure
Representation of a complex Green function on a real basis: I. General Theory
When the Hamiltonian of a system is represented by a finite matrix,
constructed from a discrete basis, the matrix representation of the resolvent
covers only one branch. We show how all branches can be specified by the phase
of a complex unit of time. This permits the Hamiltonian matrix to be
constructed on a real basis; the only duty of the basis is to span the
dynamical region of space, without regard for the particular asymptotic
boundary conditions that pertain to the problem of interest.Comment: about 40 pages with 5 eps-figure
Photoexcitation and ionization in molecular fluorine: StieltjesâTchebycheff calculations in the static-exchange approximation
Theoretical investigation of outer (1pig, 1piu, 3sigmag) and inner (2sigmau, 2sigmag) valence-shell electronic photoexcitation and ionization cross sections in molecular fluorine are reported employing separated-channel static-exchange calculations and StieltjesâTchebycheff (SâT) moment-theory techniques. The discrete vertical electronic 1pig excitation series are found to be in good agreement with recent spectral assignments and previously reported theoretical studies, and those for 1piu, 3sigmag, 2sigmau and 2sigmag excitations are in general accord with position and intensity estimates based on quantum-defect analysis. Certain of the partial-channel photoionization cross sections in F2 are seen to exhibit resonancelike features similar to those reported recently in related SâT studies of photoionization in N2, CO, and O2. The resonances can be attributed to valencelike and pre-Rydberg diabatic states that cross the outer limbs of appropriate Rydberg series and corresponding ionic-state potential curves as functions of internuclear coordinate, giving rise to large continuum transition intensities at the ground-state equilibrium internuclear separation. In contrast to the situation in N2, CO, and O2, however, there is no evidence of a resonance like sigma-->sigma* feature in the 3sigmag-->ksigmau photoionization channel in F2. Rather, this resonance in F2 appears as a strong N-->Vg transition below the 3sigmag ionization threshold, and the corresponding partial-channel photoionization cross section is seen to be structureless. Although experimental studies of partial-channel photoionization cross sections are apparently unavailable for comparison, the calculations reported here should provide reliable approximations to the dipole excitation/ionization spectra in F2, and are helpful in understanding and clarifying the dependences of photoionization spectra in light diatomic molecules on shell occupancy and equilibrium internuclear separation when compared with the results of previous studies of photoionization in N2, CO, and O2
Absence of polyphenol oxidase in cynomorium coccineum, a widespread holoparasitic plant
Polyphenol oxidase (PPO, E.C. 1.14.18.1) is a nearly ubiquitous enzyme that is widely distributed among organisms. Despite its widespread distribution, the role of PPO in plants has not been thoroughly elucidated. In this study, we report for the absence of PPO in Cynomorium coccineum, a holoparasitic plant adapted to withstand unfavorable climatic conditions, growing in Mediterranean countries and amply used in traditional medicine. The lack of PPO has been demonstrated by the absence of enzymatic activity with various substrates, by the lack of immunohistochemical detection of the enzyme, and by the absence of the PPO gene and, consequently, its expression. The results obtained in our work allow us to exclude the presence of the PPO activity (both latent and mature forms of the enzyme), as well as of one or more genes coding for PPO in C. coccineum. Finally, we discuss the possible significance of PPO deficiency in parasitic plants adapted to abiotic stress
Pseudospectral Calculation of the Wavefunction of Helium and the Negative Hydrogen Ion
We study the numerical solution of the non-relativistic Schr\"{o}dinger
equation for two-electron atoms in ground and excited S-states using
pseudospectral (PS) methods of calculation. The calculation achieves
convergence rates for the energy, Cauchy error in the wavefunction, and
variance in local energy that are exponentially fast for all practical
purposes. The method requires three separate subdomains to handle the
wavefunction's cusp-like behavior near the two-particle coalescences. The use
of three subdomains is essential to maintaining exponential convergence. A
comparison of several different treatments of the cusps and the semi-infinite
domain suggest that the simplest prescription is sufficient. For many purposes
it proves unnecessary to handle the logarithmic behavior near the
three-particle coalescence in a special way. The PS method has many virtues: no
explicit assumptions need be made about the asymptotic behavior of the
wavefunction near cusps or at large distances, the local energy is exactly
equal to the calculated global energy at all collocation points, local errors
go down everywhere with increasing resolution, the effective basis using
Chebyshev polynomials is complete and simple, and the method is easily
extensible to other bound states. This study serves as a proof-of-principle of
the method for more general two- and possibly three-electron applications.Comment: 23 pages, 20 figures, 2 tables, Final refereed version - Some
references added, some stylistic changes, added paragraph to matrix methods
section, added last sentence to abstract
Dissociative electron attachment to the H2O molecule. II. Nuclear dynamics on coupled electronic surfaces within the local complex potential model
We report the results of a first-principles study of dissociative electron
attachment to H2O. The cross sections are obtained from nuclear dynamics
calculations carried out in full dimensionality within the local complex
potential model by using the multi-configuration time-dependent Hartree method.
The calculations employ our previously obtained global, complex-valued,
potential-energy surfaces for the three (doublet B1, doublet A1, and doublet
B2) electronic Feshbach resonances involved in this process. These three
metastable states of H2O- undergo several degeneracies, and we incorporate both
the Renner-Teller coupling between the B1 and A1 states as well as the conical
intersection between the A1 and B2 states into our treatment. The nuclear
dynamics are inherently multidimensional and involve branching between
different final product arrangements as well as extensive excitation of the
diatomic fragment. Our results successfully mirror the qualitative features of
the major fragment channels observed, but are less successful in reproducing
the available results for some of the minor channels. We comment on the
applicability of the local complex potential model to such a complicated
resonant system.Comment: Corrected version of Phys Rev A 75, 012711 (2007
Two-particle interference of electron pairs on a molecular level
We investigate the photo-doubleionization of molecules with 400 eV
photons. We find that the emitted electrons do not show any sign of two-center
interference fringes in their angular emission distributions if considered
separately. In contrast, the quasi-particle consisting of both electrons (i.e.
the "dielectron") does. The work highlights the fact that non-local effects are
embedded everywhere in nature where many-particle processes are involved
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