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Showing papers by "Instituto Politécnico Nacional published in 1988"



Journal ArticleDOI
TL;DR: In this paper, the spontaneous emergence of transverse patterns in lasers was investigated by using both the standard two-level model and the so-called cubic approximation, which is generally valid in the threshold regions.
Abstract: We investigate the spontaneous emergence of transverse patterns in lasers by using both the standard two-level model and the so-called cubic approximation, which is generally valid in the threshold regions. The stationary intensity configurations fall into two distinct classes. The first includes solutions of the single-mode type with the frequency and spatial structure of one of the transverse resonances. The solutions of the second group involve the simultaneous oscillation of several cavity modes, operating in such a way as to produce a stationary intensity profile. The stationary character of these multimode configurations emerges from the fact that the transverse modes of the resonator lock onto a common frequency during the nonlinear transient. We call this phenomenon cooperative frequency locking.

136 citations


Journal ArticleDOI
TL;DR: In this article, a general procedure for treating solid-state laser unstable resonators with tapered reflectivity mirrors within the framework of geometrical optics is presented, where a suitable choice of the analytical form of the reflectivity profile makes possible the derivation of simple design equations for the resonator parameters.
Abstract: A general procedure for treating solid-state laser unstable resonators with tapered reflectivity mirrors within the framework of geometrical optics is presented. A suitable choice of the analytical form of the reflectivity profile, which can simulate many experimental devices, makes possible the derivation of simple design equations for the resonator parameters. Experiments performed with a pulsed Nd:YAG laser containing a variable-reflectivity output coupler, implemented with vacuum evaporation techniques, are discussed. >

124 citations


Journal ArticleDOI
TL;DR: In this paper, a globally stable finite spectrum assignment adaptive controller for systems with delay is proposed, assuming that the process is minimum phase, though not necessarily stable, and the delay is known.
Abstract: We propose a globally stable finite spectrum assignment adaptive controller for systems with delay. It is assumed that the process is minimum phase, though not necessarily stable, and the delay is known. It is shown that the plant output asymptotically tracks a reference model output with all signals remaining bounded.

116 citations


Journal ArticleDOI
TL;DR: In this article, the Rietveld whole-fitting method was used to analyze the crystal structure of polythiophene and obtain a bidimensional characterization of the structure in the plane orthogonal to the chain axis with pgg symmetry.
Abstract: The crystal structure of polythiophene is analyzed by the Rietveld whole-fitting method on the X-ray diffraction powder profile. Experimental data allow only a bidimensional characterization of the structure in the plane orthogonal to the chain axis with a pgg symmetry (a = 7,79; b = 5,53 A). Best fitting is obtained by assuming rigorous coplanarity of thiophene rings along the chain. The presence of few and weak layer-peaks is interpreted in terms of only partial ordering of chains along their axes. The final disagreement factor, in terms of ∑|I0 − Ic|/∑ Inet, is 0,125.

103 citations


Journal ArticleDOI
TL;DR: The present findings support the existence of tonic and phasic depolarization of the intraspinal terminals of GS group Ia and Ib primary afferents during spontaneous fictive locomotion.
Abstract: The present study examines the modulation of gastrocnemius-soleus (GS) monosynaptic reflexes as well as the intraspinal threshold changes of GS group I primary afferent terminals ending in the intermediate and motor nuclei during fictive locomotion in high decerebrate cats. The amplitude of the monosynaptic reflexes (MSR's) evoked in the medial gastrocnemius by stimulation of the lateral gastrocnemius nerve was increased during the extensor (E) phase, decreased during the flexion (F) phase of the step cycle and remainded transiently increased after spontaneous episodes of fictive stepping. The intraspinal threshold of populations and of single group Ia GS afferent fibers ending in the motor pool, as well as of single Ia and Ib fibers ending in the intermediate nucleus, showed a sustained reduction during the episodes of fictive locomotion with superimposed cyclic changes in phase with the step cycle. During fictive walking and trotting the reduction of the intraspinal threshold of both Ia and Ib fiber terminals was maximal during the middle or late portion of the F-phase. During fictive gallop elicited by stimulation of the superficial peroneus nerve, the decrease in the intraspinal threshold of the Ia afferent fibers occurred however in phase with the activity of the GS motoneurons. During episodes of fictive locomotion slow, sustained negative DC potential shifts lasting tents of seconds, reflecting an increase in the extracellular potassium concentration were recorded at the base of the dorsal horn and in the intermediate nucleus. The present findings support the existence of tonic and phasic depolarization of the intraspinal terminals of GS group Ia and Ib primary afferents during spontaneous fictive locomotion. It is suggested that accumulation of potassium ions in the extracellular space contributes mainly to the sustained depolarization of group I fibers. The phasic depolarization would be mostly due to the activation of specific sets of interneurons and may, in the case of Ia fibers, contribute to the cyclic modulation of the MRS elicited during fictive locomotion.

93 citations


Journal ArticleDOI
TL;DR: The long control region of human genital papillomavirus type 18 harbors transcription regulatory elements, such as the E6 promoter and a cell type-specific enhancer independent of the E2 protein, which could explain its repression by bovine papilloma virus type 1 E2.
Abstract: The long control region of human genital papillomavirus type 18 harbors transcription regulatory elements, such as the E6 promoter and a cell type-specific enhancer independent of the E2 protein. By performing DNase I footprint experiments in vitro with protein extracts from different cell lines and tissues we searched for cellular factors interacting with the totality of the control region. We detected a total of eight different protected sites; most of them were found with all the extracts. Two of these sites, one in the enhancer and the other in the sequences proximal to the E6 cap site, interact with a member of the activator protein 1 family. However, in the absence of fine mutational analysis, we cannot readily discern an exact functional role for the different binding sites. We also characterized two regions, which are protected only in the presence of E2, corresponding to the perfect palindromes ACCGN4CGGT and present either 500 nucleotides or tandemly repeated about 70 nucleotides upstream of the E6 cap site. This last protected area covers a large part of the putative TATA box of the E6 promoter and could explain its repression by bovine papilloma virus type 1 E2, which could interfere with binding of the TFII D ubiquitous transcription factor to the TATA sequence of the promoter.

93 citations


Journal ArticleDOI
TL;DR: A super-Gaussian mirror, built by a special evaporation technique, has been tested in an unstable resonator of a pulsed Nd:YAG laser, demonstrating its effectiveness in generating diffraction-limited beams.
Abstract: A new class of tapered reflectivity mirrors with a super-Gaussian profile R ∝ exp(−krn) is introduced, and a geometrical-optics approach for analysis and design of unstable resonators made with these mirrors is presented. A super-Gaussian mirror, built by a special evaporation technique, has been tested in an unstable resonator of a pulsed Nd:YAG laser, demonstrating its effectiveness in generating diffraction-limited beams.

90 citations



Journal ArticleDOI
TL;DR: The results of these studies strongly suggest that division of E. histolytica involved a pleuromitotic process which is carried out in about 120 min.

61 citations


Journal ArticleDOI
TL;DR: This review on the use of indigenous solid-state fermentations (SSF) describes the microbiological transformation of plant raw materials into highly nutritious foods and flavor-enhancing ingredients.
Abstract: This review on the use of indigenous solid-state fermentations (SSF) describes the microbiological transformation of plant raw materials into highly nutritious foods and flavor-enhancing ingredients. Traditional fermented foods from most countries of the world may be classified into the following categories: fungal fermentation followed by brining, SSF principally using bacteria, lactic acid fermentation followed by fungal fermentation, production of fermented doughs, alcoholic fermentation, and fermented food ingredients. Scientific studies of the principles behind SSF, identification of the essential microorganisms, development of suitable and versatile equipment, control of the process, and quality control of the substrate and final product can very well lead to a substantial increase at a world level in the availability and consumption of these fermented foods. The application of modern biotechnology in some countries is already changing the ancient methods of making fermented foods.

Journal ArticleDOI
TL;DR: A renormalized polarizability is obtained for spheres embedded in a homogeneous matrix within the dipolar long-wavelength approximation with a relationship similar to the Clausius-Mossotti relation and it obeys a second-order algebraic equation.
Abstract: We develop a simple theory for the macroscopic dielectric function of a system of identical spheres embedded in a homogeneous matrix within the dipolar long-wavelength approximation. We obtained a relationship similar to the Clausius-Mossotti relation, but with a renormalized polarizability for the spheres instead of the bare polarizability. This renormalized polarizability obeys a second-order algebraic equation and it is given in terms of the bare polarizability, the volume fraction, and a functional of the two-particle correlation function of the spheres. We calculate the optical properties of metallic spheres within an insulating matrix and we compare our results with previous theories and with experiment.

Journal ArticleDOI
TL;DR: In this paper, the degradation of polypropylene film was studied up to 2424 h of exposure and a continuous reduction of the mechanical properties began after about 700 h. The degradation mechanism was based mainly on the β-scission reaction.

Journal Article
TL;DR: It is concluded that the AV node and upper bundle of His cells derive from the low interatrial septum, which has similar characteristics to those of the adult heart.
Abstract: The development of the atrioventricular node and bundle of His of embryonic chick hearts was studied by electrophysiological and morphological techniques. The dorsal wall of the AV canal and the interatrial septum were explored to determine if they contribute to the formation of the AV node and bundle of His. The resting membrane and action potentials of the interatrial septum cells were systematically analyzed and found to undergo progressive differentiation with development. The earliest identification of the AV node and upper bundle of His group of cells was achieved at 5 1/2-6 days of development by the electrical recording of their corresponding characteristic action potentials, from a circumscribed area located in the lowest and dorsal segment of the interatrial septum. The morphological and anatomical characterization of the cells was made following electrical recording and labelling with charcoal particles. The earlier AV node and bundle of His responses had similar characteristics to those of the adult heart. It is concluded that the AV node and upper bundle of His cells derive from the low interatrial septum. The possibility that AV canal cells contribute to this event was discarded. The functional relationship of the Av node and bundle of His with other cardiac tissues during the early development of the heart is discussed.

Journal ArticleDOI
TL;DR: The results suggest that the protective effect of colchicine may be mediated through its action on an early toxic event, because treatment of the animals with col chicine produced a significant decrease in CCl4-induced lipid peroxidation.

Journal ArticleDOI
TL;DR: In this article, it was shown that the atomic charges derived from electrostatic potentials give the same picture of the charge distribution within molecules as that obtained from the corrected Mulliken charges.

Journal ArticleDOI
TL;DR: Penicillinacylase from Ecoli, immoblized on Eupergit C beads catalyzes the hydrolysis in water/CH 3 CN 10:1, at pH 75 and 23° C, of a set of O-phenylacetate esters of primary carbinols as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, the authors analyzed the distortion of active feedback amplifier systems using a simple method, starting from the amplitude characteristic of both amplifiers, the main and the auxiliary, it is possible to calculate the intermodulation distortion of the complete system.
Abstract: Intermodulations distortion of active feedback amplifier systems has been analyzed using a simple method. Starting from the amplitude characteristic of both amplifiers-the main and the auxiliary-it is possible to calculate the intermodulation distortion of the complete system. Analytical results show that third-order intermodulation products can be considerably reduced when active feedback with correct amplitude and phase is used. Experimental measurements that confirm the theoretical analysis have been made on a one-stage bipolar transistor amplifier. >

Journal ArticleDOI
TL;DR: In this paper, the preparation, activation, and catalytic behavior of a Zn/sub 1/Mn/Sub 1.06/ oxide catalyst reported in the early literature for the synthesis of higher alcohols has been investigated by means of chemical analysis, XRD, IR, DTA-TG, SEM, surface area measurements, and flow reactor experiments.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the spectrum of squeezing in the light transmitted by a system of two-level atoms in an optical cavity driven by a coherent field, as recently observed by Kimble and collaborators.
Abstract: We study the spectrum of squeezing in the light transmitted by a system of two-level atoms in an optical cavity driven by a coherent field, as recently observed by Kimble and collaborators. Our analysis does not introduce any restriction on the detuning parameters or on the relative order of magnitude of the atomic and cavity relaxation rates. Using the dressed-mode formalism, we derive explicit analytic expressions for the spectrum of squeezing and for the second-order intensity correlation function. We describe how the spectrum depends on the various parameters of the system; this study provides general indications on how to optimize the conditions to observe squeezing in optical bistability. A special attention is devoted to the changes of the spectrum under variation of the detuning parameters or of the ratio of cavity to atomic-damping rate. In all cases, the changes are continuous and smooth, with quantitative variations in the level of squeezing. This indicates the existence of a single avenue to squeezing in this system. The bad-cavity direction is identified as especially conventient to obtain a significant amount of squeezing.

Journal ArticleDOI
TL;DR: The structures of the two major components, B3 and B4, were elucidated from their physico-chemical properties, 1H and 13C NMR spectra and fast atom bombardment mass spectra data in comparison with lipiarmycins A3 and A4.
Abstract: Actinoplanes deccanensis ATCC 21983, the producer of antibiotics lipiarmycin A3 and A4, furnished also a related antibiotic designated lipiarmycin B, active against Gram-positive bacteria, including anaerobes, and against Neisseria. The structures of the two major components, B3 and B4, were elucidated from their physico-chemical properties, 1H and 13C NMR spectra and fast atom bombardment mass spectra data in comparison with lipiarmycins A3 and A4.

Journal ArticleDOI
TL;DR: The results of Auger electron spectroscopy showed that there was no reduction of the oxide film when an a-Si : H film was deposited by the plasma decomposition of silane at substrate temperatures in the range 150-300°C as mentioned in this paper.
Abstract: Indium-doped zinc oxide films have been prepared by the spray pyrolysis technique using air as a carrier gas at atmospheric pressure. These films show the excellent optical and electrical characteristics of a transparent conductive coating, with a sheet resistance of 15 Ω/square and an average optical transmission of about 88%. All the studied films show the wurtzite hexagonal crystalline structure. The results of Auger electron spectroscopy show that there was no reduction of the oxide film when an a-Si : H film was deposited by the plasma decomposition of silane at substrate temperatures in the range 150–300°C. A steep interface between the transparent conductive film and the hydrogenated amorphous silicon film was observed. Also, it was found that the depth distribution of the constituent atoms zinc, oxygen and indium in the ZnO : In film was homogeneous.

Journal ArticleDOI
TL;DR: In this paper, the effects of changing a geometrical ratio and of including damping are shown over a range of load magnitudes, and the influence of damping is remarkable.
Abstract: Anomalous results, including counterintuitive permanent displacements and scatter of numerical solutions, have been found for certain fixed-pin supported beams deformed plastically by a transverse pressure pulse. Similar difficulties may be anticipated in other pulse loaded beams and plates with constraints that prevent in-plane displacements. The present studies show some of the special circumstances in which these occur, with a simple elastic-plastic model of Shanley type. In particular, effects of changing a geometrical ratio and of including damping are shown over a range of load magnitudes. The influence of damping is remarkable. Unpredictability of final displacement is evident in special circumstances, such that the final displacement is hypersensitive to the parameters of the loading, structure and solution technique.

Journal ArticleDOI
TL;DR: The structure of cladochrome A, a perylenequinone pigment isolated from etiolated cucumber seedlings infected with fungal spores of Cladosporium cucumerinum, has been revised and established as that of a diester of 3-hydroxybutyric acid with ent-isophleichrome.

Journal Article
TL;DR: Description de la technique d'isolement de proteines (albumines, glutenines, prolamines albumine) et de leur analyse a partir de farine entiere ou delipidee de graines d'Amaranthus hypochondriacus.
Abstract: Description de la technique d'isolement de proteines (albumines, glutenines, prolamines albumine) et de leur analyse a partir de farine entiere ou delipidee de graines d'Amaranthus hypochondriacus

Journal ArticleDOI
01 Aug 1988-Polymer
TL;DR: In this article, a study of five neodymium bromide complexes of general formula NdBr3·nL was conducted, and diffraction studies of I and II showed that both molecules are monomeric (coordination number 7).

Journal ArticleDOI
TL;DR: In this article, the quantum yield for random chain scission during environmental photo-degradation of a stabilised low density polyethylene film was calculated on the basis of the reduction of Mw during exposure.

Journal ArticleDOI
TL;DR: In this paper, the reciprocal role of silver and copper in disordered materials has been examined with the hope of investigating the reciprocal roles of the two metals in disorder materials. And the observed behaviour is discussed in connection with the structural and chemical features of the material, which shows a structure associated with fast ionic transport.

Journal ArticleDOI
TL;DR: Amorphous hydrogenated carbon films prepared by glow discharge decomposition of CH4 at a power density of 0.4 W cm−2 were bombarded with hydrogen and nitrogen ions of energies between 100 and 600 keV, and structural changes due to various ion doses were analyzed using Raman, photoluminescence, and infrared spectroscopy as mentioned in this paper.
Abstract: Amorphous hydrogenated carbon films prepared by glow discharge decomposition of CH4 at a power density of 0.4 W cm−2 were bombarded with hydrogen and nitrogen ions of energies between 100 and 600 keV. The structural changes due to various ion doses were analyzed using Raman, photoluminescence, and infrared spectroscopy. Nitrogen bombardment produces essentially a hydrogen depletion in the whole dose interval as shown by the IR spectra; as a consequence there is a drop of several orders of magnitude in the integrated photoluminescence intensity, and carbon–carbon double bonds, characteristic of a graphiticlike structure, appear in the Raman spectra. The same general effects are produced under proton bombardment, but their intensities change more gradually with the ion dose, in agreement with a vacancy production mechanism.

Journal ArticleDOI
TL;DR: In this paper, the 13C NMR spectra of all possible indoles substituted with one, two or three methyl groups on the pyrrole ring were assigned from heteronuclear two-dimensional 13C/1H contour plots and homonuclear hydrogen-hydrogen decoupling experiments at a specific concentration of 0.83 M in CDCl3 and in DMSO-d6.
Abstract: The 13C NMR spectra of all possible indoles substituted with one, two or three methyl groups on the pyrrole ring were assigned from heteronuclear two-dimensional 13C/1H contour plots and homonuclear hydrogen-hydrogen decoupling experiments at a specific concentration of 0.83 M in CDCl3 and in DMSO-d6. The results confirm controversial assignments for some compounds, and provide the assignments for those compounds not previously studied by this technique. In addition, deuterium/hydrogen NMR isotope effect on 13C chemical shifts (DHIECS) for indoles, wherein the labile NH was partially exchanged to ND, are reported. In this way DHIECS values over two, three, four and five bonds were measured. The results in each solvent are discussed in terms of the number of bonds between the exchanged hydrogen and the observed carbon, and also according to their geometric dependence. This provides some evidence of the vibrational origin of the DHIECS.