Photoabsorption in formaldehyde: Intensities and assignments in the discrete and continuous spectral intervals

Theoretical investigations of total and partial‐channel photoabsorption cross sections in molecular formaldehyde are reported employing the Stieltjes–Tchebycheff (S–T) technique and separated‐channel static‐exchange (IVO) calculations. Vertical one‐electron dipole spectra for the 2b_2(n), 1b_1(π), 5a_1(σ), 1b_2, and 4a_1 canonical molecular orbitals are obtained using Hartree–Fock frozen‐core 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 zeroth‐order approximations to both valence and Rydberg transitions, and, in particular, the 2b_2(n) →nsa_1, npa_1, npb_2, and nda_2 IVO spectra are in excellent accord with recent experimental assignments and available intensity measurements. Convergent (S–T) photoionization cross sections in the static‐exchange (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 individual‐channel photoionization cross sections are attributed to contributions from valencelike a_1σ^∗ (CO), a_1σ^∗ (CH), and b_2σ^∗ (CH)/π_y^∗ (CO) molecular orbitals that appear in the photoionization continua, rather than in the corresponding one‐electron discrete spectral intervals. The vertical electronic cross sections for ^1A_1→^1B_1, ^1B_2, and ^1A_1 excitations are in generally good accord with previously reported CI (S–T) predictions of continuum orbital assignments and intensities, although some discrepancies due to basis‐set differences are present in the ^1B_1 and ^1B_2 components, and larger discrepancies apparently due to channel coupling are present in the ^1A_1→^1A_1 cross section. Partial‐channel vertical electronic cross sections for the production of the five lowest parent‐ion electronic states are found to be in general agreement with the results of very recent synchrotron‐radiation photoelectron branching‐ratio 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 partial‐ channel cross sections, providing support for the presence of a strong ka_1σ^∗ (CO) resonance in the (5a_1^(−1))^2A_1 channel. Finally, the total vertical electronic cross sections for absorption and ionization are in general accord with photoabsorption measurements, photoionization–mass–spectrometric studies, and the previously reported CI (S–T) 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 zeroth‐order account of discrete and continuum electronic dipole excitations in molecular formaldehyde

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

Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment.

Rate constants for the reactions of C+ + Cl-, Br-, and I- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10-8 cm3 s-1 were found for the reaction of C+ with Br- and I-, and a rate constant of 4.2 ± 1.1 × 10-9 cm3 s-1 was measured for the reaction with Cl-. The C+ + Cl- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 × 10-10 cm3 s-1, an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C+ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant

Photoexcitation and ionization in molecular fluorine: Stieltjes–Tchebycheff 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 Stieltjes–Tchebycheff (S–T) 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 S–T 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

IR AND X-RAY PHOTOELECTRON SPECTROSCOPY OF V 2 O 5 , TiO 2 AND V/Ti-OXIDE SOL-GEL DERIVED FILMS INFRARDE^A IN RENTGENSKA FOTOELEKTRONSKA SPEKTROSKOPIJA V 2 O 5 , TiO 2 IN V/Ti-OKSIDNIH SOL-GEL FILMOV

Prejem rokopisa -received: 1998-12-06; sprejem za objavo -accepted for publication: 1998-12-14 V/Ti-oxide films were prepared by dip-coating from sols made by mixing of V-oxoisopropoxide and Ti-propoxide in V:Ti molar ratio in precursors 3:1, 1:1 and 1:3. Amorphous films were obtained after annealing at 300°C (1 h). IR spectroscopic analysis revealed that V=O stretching modes appeared at 1020 cm -1 in the IR spectrum of powder (V:Ti=1:1). However, in the IR spectra of a film with the same molar ratio the bands at 1008 and 914 cm -1 appeared suggesting the presence of V 4+ -O bonds. The new band at 790 cm -1 signalled the V-O-Ti bridging bonds connecting V-O and Ti-O polyhedra. XPS measurements confirmed that initial-state films contained V 5+ , V 4+ and Ti 4+ species, but at the film surface only V 5+ species existed. Sequential depth analysis of films performed with Ar + sputtering showed that the vanadium is reduced to 3+ oxidation state with progressive exposure while titanium is not affected. Key words: electrochromic (EC) devices, oxide films, IR spectroscopy, XPS, oxidation states, sputtering, sol-gel V/Ti-oksidne filme smo pripravili iz koloidnih raztopin V-oksoizopropoksida in Ti-propoksida v 2-propanolu s tehniko potapljanja. Molska razmerja V:Ti v prekurzorjih so zna{ala 3:1, 1:1 in 1:3. Amorfni filmi so nastali po segrevanju na 300°C (1h). V IR spektru pra{kastega vzorca (V:Ti = 1:1) so se valen~na nihanja V=O pojavila pri 1020 cm -1 . V IR spektru filma z istim molskim razmerjem pa se pojavita trakova pri 1008 c

Ab initio study of vibrational excitation of HF by low-energy electrons

Fixed-nuclei, static-exchange calculations have been performed to study the vibrational-rotational excitation of HF by electrons from threshold to 5 eV. Our calculated, R-dependent K-matrix elements in 2Σ+, 2Π, and 2Δ symmetry are combined with exact point-dipole and laboratory-frame first-Born-approximation results for higher symmetries to obtain converged, integrated cross sections. The calculations show strong threshold peaks which are related to the dependence of the 2Σ+ K-matrix elements on internuclear distance at small scattering energies

Повышение надежности вторичного источника питания постоянного тока резервированием сигналов обратной связи

The paper deals with the problem of improving the reliability of DC-DC power supplies with pulse-width modulation. The topicality of the work is related to the importance of power supply issues in modern electronics, since the quality of operation of consumer electrical appliances, including critical ones, directly depends on the serviceability of sources. The object of the study is feedback circuits aimed at stabilization of the parameters of power supply of consumers. Failures of the mentioned feedback circuits most often occurs due to the electronic components degradation under harsh operating conditions as well as under severe mechanical overloads. Such failures are dangerous for uncontrolled increase of power supply output voltage and output current. To avoid this, a new method of reserving voltage feedback signals is presented in the paper which is implemented on the basis of flyback supply topology. Feedback signals are formed from the optocoupler located on the load side and from the auxiliary winding of the power transformer, together forming two independent output voltage control circuits. Only one circuit performs stabilization at any given moment of time. If one of these circuits fails, the second one can simply replace it in its operation. The proposed method does not require any digital signal processing algorithms or microprocessor control modules and can be implemented on the basis of cheap, widely available analog chips that perform pulse-width control of the output voltage. As a result, the problem of sudden feedback loop failure is solved and the reliability of electrical equipment is increased. The validity of the proposed method is confirmed by the results of computer simulation with the use of MatLab-Simulink environment. The obtained results can be used in design of fault-tolerant secondary power supplies that operate in harsh operating conditions.В статье рассматривается проблема повышения надежности импульсных источников вторичного электропитания постоянного тока. Актуальность работы связана со значимостью электропитания в современной электронной технике, поскольку от исправности источников напрямую зависит качество работы электроприборов-потребителей, в том числе критически важных. Объектом исследования являются цепи обратной связи, осуществляющие стабилизацию параметров электропитания потребителей. Выход из строя указанных цепей чаще всего происходит из-за деградации электронных компонентов схемы в жестких условиях эксплуатации и при сильных механических перегрузках и опасен бесконтрольным увеличением выходного напряжения и тока источника. Чтобы этого избежать, авторами предложен способ резервирования сигналов обратной связи по напряжению, реализованный на базе источника, выполненного по обратноходовой топологии. Сигналы обратной связи формируются с оптрона, расположенного на стороне нагрузки, и с дополнительной обмотки силового трансформатора, вместе образуя два независимых контура регулирования выходного напряжения. В каждый момент времени стабилизацию осуществляет только один контур. При выходе из строя первого контура второй способен заменить его работу. Представленный способ не требует цифровой обработки сигналов и микропроцессорного управления и может быть реализован на базе доступных аналоговых микросхем, осуществляющих широтно-импульсное регулирование выходного напряжения источника. В результате решается проблема аварийного отказа цепей обратной связи, повышается надежность работы электрооборудования. Правильность предлагаемого подхода подтверждена результатами имитационного компьютерного моделирования в программе MatLab-Simulink. Полученные результаты могут использоваться при проектировании отказоустойчивых вторичных источников электропитания для работы в жестких условиях эксплуатации