J. Klavins - Academia.edu (original) (raw)

Papers by J. Klavins

Resumen. -Nidificación del Tarefero (Sittasomus griseicapillus). -El tarefero (Sittasomus griseic... more Resumen. -Nidificación del Tarefero (Sittasomus griseicapillus). -El tarefero (Sittasomus griseicapillus) es una especie común que está ampliamente distribuida en bosques del Neotrópico, pero sorprendentemente se conoce muy poco acerca de sus hábitos de nidificación, y hay información conflictiva sobre si ambos padres o solo la hembra cuidan a los huevos y los pichones. Encontramos seis nidos en el Chaco y la selva Atlántica en el norte de Argentina, en cavidades de árboles formadas por degradación natural (no nidos abandonados de carpinteros), 4.5-16 m arriba del suelo, con entradas de 3-7 cm de diámetro, y 18-35 cm de profundidad. Los tareferos pusieron tres huevos sobre un lecho de cortezas y/u hojas. Aunque dos adultos rondaron el nido, anillar uno de los adultos en el nido que estudiamos con mayor esfuerzo (40 h durante 17 días) nos permitió confirmar que un solo individuo atendió a los pichones. El individuo pasó muy poco tiempo en la cavidad del nido, haciendo hasta 12 viajes en una hora alimentando a los pichones con artrópodos, especialmente con larvas de Lepidópteros y polillas adultas. La tasa de alimentación se incrementó durante el período que los pichones estaban en el nido, mientras disminuyó la tasa de remoción de los sacos fecales. Adultos, pichones y volantones fueron inconspícuos, vocalizando solo suavemente y llamando poco la atención hacia el nido. Los adultos no defendían el nido de otras aves, sino que volaban del árbol cada vez que eran asustados. Estos comportamientos, el pequeño tamaño de las cavidades de los nidos, y su origen por degradación natural en vez de excavación por pájaros carpinteros, podrían explicar por qué se han encontrado tan pocos nidos.

Russian Journal of Electrochemistry, 2007

The conditions of electrochemical formation of nickel hydride are investigated. The electrochemic... more The conditions of electrochemical formation of nickel hydride are investigated. The electrochemical properties of different nickel-based materials (bulk, porous, foamed tapes) are examined to state the hydrogen adsorption/absorption phenomena and possibility to use them as a negative electrode in hydrogen devices (electrolyzers, rechargeable batteries, fuel cells). Surface activation of nickel materials are made by electroplating and etching methods. Thin palladium coating is used to prove the formation of nickel hydride during cathodic charging. Volt-amperometric and kinetic measurements show that not only palladium, but also activated nickel plays important role in the surface activation of electrode materials and promotion of hydrogen absorption in nickel substrate materials.

Physical Review A, 2005

We report results from studies of the Autler-Townes ͑AT͒ effect observed in sodium molecules from... more We report results from studies of the Autler-Townes ͑AT͒ effect observed in sodium molecules from a molecular beam. A relatively weak laser field P couples an initially populated rovibronic level g in the electronic ground state ͑here X 1 ⌺ g + , vЉ =0, JЉ =7͒ to a selected excited rovibronic level e ͑here A 1 ⌺ u + , vЈ = 10, JЈ =8͒, which in turn is coupled by a relatively strong laser field S to a more highly excited level f ͑here 5 1 ⌺ g + , v = 10, J =9͒, a scheme we idealize as a three-state ladder. The AT effect is seen by scanning the frequency of the P field while recording fluorescence from both the e and f levels in separate detection channels. We present qualitative theoretical considerations showing that, when the P field is weak, the ratio of doublet component areas in the excitation spectrum from level f can be used to determine the lifetime of this level. We obtain a value of 17± 3 ns. When the P field is stronger, such that its Rabi frequency is larger than the decay rate of level e, the fraction of f-level population that decays to the intermediate electronic state A 1 ⌺ u + can be deduced from the AT spectrum. When supplemented with values of Franck-Condon and Hönl-London factors, our measurements give a value for the branching ratio ͑the fraction returning to level e͒ of r e = 0.145 with a statistical error of ±0.004. The use of a strong P field on the g-e transition and a weak S field as a probe on the e-f transition results in complex line shapes in the excitation spectrum of level f, not showing the familiar Autler-Townes doublet structure.

$Research paper thumbnail of Excitation transfer between the rubidium 5 ^{2}D fine-structure levels in collisions with ground-state rubidium atoms: Experiment and theory$

Physical Review A, 2000

We report a study of fine-structure mixing Rb(5 2 D 5/2 )→Rb(5 2 D 3/2 ) in collisions with groun... more We report a study of fine-structure mixing Rb(5 2 D 5/2 )→Rb(5 2 D 3/2 ) in collisions with ground-state Rb atoms. In the experiment, two-photon cw laser excitation was applied to the Rb vapor cell. The measured cross section for the process was (5.8Ϯ1.9)ϫ10 Ϫ14 cm 2 . Theoretical calculations using nonadiabatic collision theory gave a value of 3.4ϫ10 Ϫ14 cm 2 .

Physical Review A, 2008

The theoretical and experimental values of the 5f, 6f, 7f, and 8f radiative lifetimes of neutral ... more The theoretical and experimental values of the 5f, 6f, 7f, and 8f radiative lifetimes of neutral potassium are reported. The reduced matrix elements for all allowed electric-dipole nf 5/2 -nЈd 5/2 , nf 5/2 -nЈd 3/2 , and nf 7/2 -nЈd 5/2 transitions with n =5-8 in K are calculated using the relativistic linearized coupled-cluster method with single and double excitations of Dirac-Fock wave functions included to all orders in many-body perturbation theory. The resulting electric-dipole matrix elements are used to evaluate the lifetimes of the 5f, 6f, 7f, and 8f states in neutral K and their uncertainties. The contributions from the nf 5/2 -nЈg 7/2 , nf 7/2 -nЈg 7/2 , and nf 7/2 -nЈg 9/2 transitions to the lifetimes of the 6f, 7f, and 8f states are evaluated using the third-order manybody perturbation theory and are found to be very small. The theoretical results are 5f = 117͑4͒ ns, 6f = 195͑4͒ ns, 7f = 301͑6͒ ns, and 8f = 441͑9͒ ns. The experiment is performed in a cell using time-resolved fluorescence spectroscopy. The nf states are excited stepwise, 4s → 4p → nf, using two pulsed synchronous dye lasers for the dipole and quadrupole transitions, respectively. The measured values 5f = 117͑3͒ ns, 6f = 190͑6͒ ns, 7f = 309͑8͒ ns, and 8f = 428͑10͒ ns are in excellent agreement with the present theoretical calculations.

Physica Scripta, 1992

NaRb molecules in a thennal cell in a mixture of Na-Rb were excited to the D'n state by means of ... more NaRb molecules in a thennal cell in a mixture of Na-Rb were excited to the D'n state by means of a 514.5nm beam from an Ar+ laser. The natural lifetime for the NaRb Din, v' = 0, J' = 44 state, populated by the selective laser excitation, was found to be T = 22.6 ± 1.8ns. The kinetics of the molecular fluorescence lines as a function of the Rb atom concentration were also studied. The rate constant for collisional quenching of this molecular level by Rb atoms could be detennined to k q = 2.1 . 10-9 em' S-I. The experiments at steady state conditions demonstrate that energy transfer to the atomic 4 2 D levels of Rb dominates the collisional quenching with a rate constant of k ET = 1.2' 10-9 em' S-I. The results suggest that such molecule-atom transfer of electronic energy can be used to efficiently populate 2D levels of alkali metal atoms.

Physica Scripta, 1996

The cross section u = (8 ± 4) x 10-15 cm 2 was determined for the Rb(7S) + Rb(5S) --> Rb(5D) + Rb... more The cross section u = (8 ± 4) x 10-15 cm 2 was determined for the Rb(7S) + Rb(5S) --> Rb(5D) + Rb(5S) excitation energy transfer process, and the quenching cross section uq = (2 ± 1) x 1O-14 cm 2 for the Rb(5D)

Journal of Physics: Conference Series, 2007

During thermogravimetric research of metal hydrides we noticed mass growth of samples above 200°C... more During thermogravimetric research of metal hydrides we noticed mass growth of samples above 200°C even in an argon atmosphere. Further heating is leading to the growth of weight up to 2-7 weight% till 500°C. Second run of the same sample without taking out of DTA instrument gave only small mass changes, indicating that noticed mass increase during first run is permanent. Microscope and elemental analyses were made to determine the reason of mass growth. XRD inspection revealed the formation of new phase with bunsenite NiO structure with deformed cubic structure. The new phase is no more active to hydrogen sorption/desorption. Our results demonstrated that the usage of hydrogen storage alloys AB5 must be taken with care - it is important not to exceed some critical temperature were irreversible structural, compositional and morphological changes will occur.

Zeitschrift f�r Physik D Atoms, Molecules and Clusters, 1994

Cross sections for electron energy transfer from the initial rotational state 1 1 of the two lowe... more Cross sections for electron energy transfer from the initial rotational state 1 1 of the two lowest vibrational levels VI = 0 and VI = 1 of excited dimers NaiA) to potassium atoms as described by NaiNL'; ,

Chemical Physics Letters, 1994

Chemical Physics Letters, 1999

Associative ionizing NaiA II; , v') + Na(3P3/2) collisions have been studied in a single superson... more Associative ionizing NaiA II; , v') + Na(3P3/2) collisions have been studied in a single supersonic beam with respect to the influence of the initial vibrational excitation of the molecules on the reaction efficiency. An enhancement of trimer ion yield of nearly one order of magnitude has been observed as the vibrational excitation increases from v' = 0 to v' = 14.

Canadian Journal of Physics, 2001

Collisional energy transfer (ET) between the 2 S and 2 D states of alkali atoms is reviewed. Rece... more Collisional energy transfer (ET) between the 2 S and 2 D states of alkali atoms is reviewed. Recent experimental results on the ET in homonuclear collisions in Na, K, and Rb vapours upon cw (continuous wave) and pulsed laser excitation, as well as theoretical calculations of the respective cross sections are discussed. The mechanisms of these processes are considered in an attempt to clarify the role of the dipole-dipole interaction.

To enhance the volume of stored hydrogen in solid hydrogen carriers, the volume and the surface o... more To enhance the volume of stored hydrogen in solid hydrogen carriers, the volume and the surface of material must be effectively used. Physicochemical properties are strongly influenced by the composition of material and the dimensions of grains used in the composite. In our work the hydrogen absorption of materials with selected surface and bulk properties were tested. The gravimetric hydrogen uptake of AB 5 and AB 5 glass phase samples was measured at different pressure steps. AB 5 was used as a catalyst and bulk material, but silica based Pyrex glass was used for high surface composite formation.

A new nano structured and fully inorganic composite zirconium phosphate proton conducting materia... more A new nano structured and fully inorganic composite zirconium phosphate proton conducting material was synthesized by support impregnation with zirconium oxide suspensions or sols and subsequent phosphorization. The composite inorganic zirconium phosphate membranes prepared using the zirconium oxide suspension were found to give rise to high proton conductivity. The proton conductivity obtained was in the order of 10 -2 S/cm at room temperature (RH = 100 %), which is comparable to Nafion proton conductivity. An inorganic membrane based direct methanol fuel cell was tested and efficiency equivalent to more than 50 % of the Nafion efficiency was obtained when using standard platinum catalyst inks.

Latvian Journal of Physics and Technical Sciences, 2013