Purely rotational coherence effect and time-resolved sub-Doppler spectroscopy of large molecules. I. Theoretical

In this and the accompanying paper we present a theoretical treatment and experimental study, respectively, of the phenomenon termed purely rotational coherence. This phenomenon has been demonstrated to be useful as a time domain means by which to obtain high resolution spectroscopic information on excited state rotational levels of large molecules [Felker et al., J. Phys. Chem. 90, 724 (1986); Baskin et al., J. Chem. Phys. 84, 4708 (1986)]. Here, the manifestations in temporally resolved, polarization-analyzed fluorescence of coherently prepared rotational levels in samples of isolated symmetric and asymmetric top molecules are considered. These manifestations, for reasonably large molecules at rotational temperatures characteristic of jet-cooled samples, take the form of polarization-dependent transients and recurrences with temporal widths of the order of tens of picoseconds or less. The transients, which arise from the thermal averaging of many single molecule coherences, are examined with respect to their dependences on molecular parameters (rotational constants, transition dipole directions) and experimental parameters (polarization directions and temperature). A physical picture of rotational coherence as a reflection of the time-dependent orientation of molecules in the sample is developed. And, the influence of rotational coherence in experiments designed to probe intramolecular energy flow is discussed. In the accompanying paper, we present experimental results for jet-cooled t-stilbene and anthracene. For t-stilbene we determine rotational constants for vibrational levels in the S1 electronic state (from the recurrences) and we monitor the trends in rotational coherence vs vibrational coherence as the total energy in the molecule increases

Calculation of Mono-Cyclic Molecular Structures using MINDO/3

Molecular geometries of selected, small cyclic compounds were calculated using the semi-epirical, S.C.F. molecular orbital method MINDO/3. Calculated geometries are compared with experiemental values to determine MINDO/3\u27s effectiveness. It is found that the method has several systematic deficiencies. In Particular, carbon-oxygen single bond distances are calculated to be too short

Dynamics of intramolecular vibrational-energy redistribution (IVR). III. Role of molecular rotations

Experimental results on jet-cooled anthracene pertaining to the role of rotations in IVR processes are presented. For theoretical comparison, we consider the effects of molecular rotational level structure on the beat-modulated decays that arise as manifestations of IVR. It is shown theoretically for anharmonic coupling that small differences in rotational constants between coupled vibrational states give rise to decays, the beat envelopes of which decay faster than the unmodulated portions of the decays. These envelope decay rates are shown to be rotational temperature dependent. The experimental results reveal behavior entirely consistent with the theoretical expectations. The results also show that although rotational effects are present in experimental decays, they are not so marked as to wash out the manifestations of vibrational coherence

Dynamics of intramolecular vibrational-energy redistribution (IVR). II. Excess energy dependence

The results of picosecond-resolved measurements of intramolecular vibrational-energy redistribution (IVR) in jet-cooled anthracene at different excess energies are presented. From these results, the nature of IVR as a function of vibrational energy, the relevant time scales for the process, and the details of pertinent vibrational couplings are determined

Dynamics of intramolecular vibrational-energy redistribution (IVR). I. Coherence effects

In this series of papers, theoretical and experimental results concerning the dynamical manifestations of intramolecular vibrational-energy redistribution (IVR) in temporally resolved fluorescence are presented. In this paper (I) we present a general treatment of IVR and coherence effects in multilevel vibrational systems. Specifically, the concern is with the derivation of the characteristics of the beat-modulated fluorescence decays which arise from vibrational coupling among N levels within a molecule. Relations connecting quantum beat frequencies, phases, and modulation depths to coupling parameters are presented. Likely sources of deviation of experimental results from theoretical predictions are considered. And, finally, the direct link between IVR and time-resolved fluorescence experiments is discussed with emphasis placed on the physical interpretation of vibrational quantum beats and the nature of IVR as a function of vibrational energy in a molecule

Leguminosas arbóreas para áreas semi-áridas e áridas

A review of the research and development activities on nitrogen fixing trees for semi-arid regions is presented. In the period from 1980 through 1983 much new information on pod productivity, on natural stand nitrogen fixation and on greenhouse mineral nutrition has become available. Early results from small plot plantation work indicate a biomass productivity of 10-13 dry metric tons per hectare per year. Clones have been made of promising individual trees and research on rooting of cuttings is rapidly approaching commercialization. Tissue culture propagation of semi-arid nitrogen fixing trees has only just begun. Procedures have been developed employing subsoiling and herbicides that yield more than 90% survival on sites receiving 500 mm annual rainfall. A tabular review is presented of the major research organizations and donors involved in semi-arid nitrogen fixing trees. Major technical constraints are coping with semi-arid soils that are generally low in available phosphorus, iron, zinc and manganese, range in soil texture from cracking clays to high bulk density sands or rocky desert pavement, and that vary in pH from 6.5 to 9.5. Pest management issues (insects and diseases) have not yet been studied in semi-arid nitrogen fixing trees and should be addressed before large scale plantations are undertaken. The excellent progress made from 1980 through 1983 suggests a very bright future for semi-arid nitrogen fixing trees.Foi feita uma revisão da pesquisa sobre leguminosas arbóreas para áreas semi-áridas. No período compreendido entre 1980 e 1983, muita informação sobre produção de vagens, fixação de N2 em condições naturais e nutrição mineral tornou-se disponível. Os resultados iniciais obtidos em pequenas parcelas indicam uma produção de biomassa de Prosopis entre 10-13 toneladas de peso seco por hectare/ano. Propagação vegetativa por cultura de tecidos é uma área de pesquisa recém-iniciada para as árvores fixadores de N2 de regiões semi-áridas. Por outro lado, clonagens de árvores individuais mais promissoras foram feitas, e a pesquisa sobre o enraizamento de estacas está rapidamente chegando ao nível comercial. Técnicas de manejo já foram desenvolvidas, empregando subsolagem e herbicidas, as quais permitem mais que 90% de sobrevivência de mudas em localidades com cerca de 500 mm anuais de chuvas. As principais instituições de pesquisa e financiadoras envolvendo árvores fixadoras de N2 para o semi-árido são listadas. Os maiores problemas técnicos a enfrentar são os solos do semi-árido que, geralmente, são deficientes em fósforo disponível, ferro, zinco e manganês, além de apresentarem ampla variação de textura, desde os argilosos aos arenosos e rochosos, e variação de pH de 6,5 a 9,5. O manejo das pragas e doenças não tem sido ainda objeto de estudo, e deve ser investigado antes que plantações em grande escala sejam estabelecidas. Entretanto, o excelente progresso obtido no período de 1980/83 sugere um futuro brilhante para as árvores fixadoras de nitrogênio, adaptadas às regiões semi-áridas

Opuntia ellisiana Griffiths as livestock feed in areas similar to USDA cold hardiness zones 6-7

The present review compiles the studies carried out so far on Opuntia ellisiana Griffiths. This species, of unknown origin, was first described at the beginning of the 20th century in southern Texas, USA, and introduced to Argentina in 1998. This species, like other Opuntia sps., can be cultivated in a wide range of environments and its lower transpiration per unit of carbon gained in relation to C3 and C4, has lead to in an important increase in water-use efficiency. While O. ellisiana has a lower growth and productivity than O. ficus-indica (L.) Mill. it stands out for its resistance to sub-zero temperatures. Fortunately, the intraspecies variation within O. ellisiana, shortens the time for its use after establishment. There is a wide variation in the nutrient content between the different forage species and clones of Opuntia. Due to the inherently low N availability in arid ecosystems, O. ellisiana, like the other species, has low protein content in natural unfertilized conditions. Some efforts, as the use of N-fertilizer, have been carried out to improve its protein level. About 15% protein levels have been obtained with other Opuntias. Other research has been directed to provide a favorable abiotic environment for a cactus to achieve higher biomass productivity and improved protein levels by interacting with nurse plants, such as Prosopis sps. The last alternative resulted in a significant increase in protein content and cladode quantity per plant of O. ellisiana.The present review compiles the studies carried out so far on Opuntia ellisiana Griffiths. This species, of unknown origin, was first described at the beginning of the 20th century in southern Texas, USA, and introduced to Argentina in 1998. This species, like other Opuntia sps., can be cultivated in a wide range of environments and its lower transpiration per unit of carbon gained in relation to C3 and C4, has lead to in an important increase in water-use efficiency. While O. ellisiana has a lower growth and productivity than O. ficus-indica (L.) Mill. it stands out for its resistance to sub-zero temperatures. Fortunately, the intraspecies variation within O. ellisiana, shortens the time for its use after establishment. There is a wide variation in the nutrient content between the different forage species and clones of Opuntia. Due to the inherently low N availability in arid ecosystems, O. ellisiana, like the other species, has low protein content in natural unfertilized conditions. Some efforts, as the use of N-fertilizer, have been carried out to improve its protein level. About 15% protein levels have been obtained with other Opuntias. Other research has been directed to provide a favorable abiotic environment for a cactus to achieve higher biomass productivity and improved protein levels by interacting with nurse plants, such as Prosopis sps. The last alternative resulted in a significant increase in protein content and cladode quantity per plant of O. ellisiana

Direct Observation of Nonchaotic Multilevel Vibrational Energy Flow in Isolated Polyatomic Molecules

With picosecond spectroscopy and molecular beams it is shown that nonchaotic multilevel vibrational energy flow is present in large polyatomic molecules. This Letter reports on this novel observation and its probing of the fundamental process of energy redistribution in molecules

Photodissociation of partially solvated molecules in beams by the picosecond-jet technique: Hydrogen bond breakage

In this communication we wish to report the measurement of photodissociation rates of jet-cooled solute-solvent complexes in various stages of solvation. The observation of fluorescence decays as a function of excess vibrational energy for isoquinoline (IQ), IQ-(methanol)n, and IQ-(water)n complexes reveals threshold behavior for the decays rates of 1:1 solute-solvent complexes. The thresholds at ~3 kcal/mol provide new information on the breakage of excited state hydrogen bonds

Sobre la ocurrencia del cretáceo superior marino en Coihaique , Provincia de Aisén

Using the CCSD­(T) model, we evaluated the intermolecular potential energy surfaces of the He–, Ne–, and Ar–phosgene complexes. We considered a representative number of intermolecular geometries for which we calculated the corresponding interaction energies with the augmented (He complex) and double augmented (Ne and Ar complexes) correlation-consistent polarized valence triple-ζ basis sets extended with a set of 3s3p2d1f1g midbond functions. These basis sets were selected after systematic basis set studies carried out at geometries close to those of the surface minima. The He–, Ne–, and Ar–phosgene surfaces were found to have absolute minima of −72.1, −140.4, and −326.6 cm^–1 at distances between the rare-gas atom and the phosgene center of mass of 3.184, 3.254, and 3.516 Å, respectively. The potentials were further used in the evaluation of rovibrational states and the rotational constants of the complexes, providing valuable results for future experimental investigations. Comparing our results to those previously available for other phosgene complexes, we suggest that the results for Cl₂–phosgene should be revised