Quantum Calculations On The Vibrational Predissociation Of NeBr2: Evidence For Continuum Resonances

Quantum mechanical calculations on the vibrational predissociation dynamics of NeBr2 in the B electronic state have been performed and the results compared with both experimental data and other computational studies. For vibrational levels with v less than or equal to 20 we find that the vibrational state dependence of the predissociation lifetimes is in qualitative agreement with experimental measurements, as are the calculated Br-2 fragment rotational distributions. For higher vibrational levels, the B \u3c-- X excitation profiles are well represented by a sum of two Lorentzian line shapes. We attribute this result to the presence of long-lived resonances in the dissociative continuum that are reminiscent of long-lived dissociative trajectories in previous classical studies of NeBr2. (C) 2000 American Institute of Physics. [S0021-9606(00)00205-1]

Vibrational Branching Ratios From The Dissociation Of The NeIBr Van Der Waals Molecule

The degree of vibrational excitation in the IBr fragment from the vibrational predissociation of NeIBr (A (3)PI(1)) has been measured using two-color pump-probe laser-induced fluorescence spectroscopy. We find that for the lowest initial vibrational states examined, DELTA-upsilon = -1 dissociation pathways dominate the dynamics, while this channel is closed for upsilon greater-than-or-equal-to 17. From this result, the A state binding energy (D0) of the complex is determined to be 67 +/- 4 cm-1, while that in the X electronic state is found to be 73 +/- 4 cm-1. The X state binding energy is identical to that for NeI2 and NeBr2, suggesting that the potential energy surface for NeIBr can be constructed from a summation of atom-atom pair potentials; we present such a model potential energy surface. The variations in the vibrational branching ratios, when combined with the trends in the predissociation rates, point to the importance of fragment rotational excitation in the dynamics of the dissociation

Fragment Rotational Distributions From The Dissociation Of NeBr2: Experimental And Classical Trajectory Studies

The Br-2 fragment rotational distributions that result from the vibrational predissociation of NeBr2 in the B electronic state have been measured for several initial vibrational levels. In each case, the rotational distributions extend to the effective energetic Limit determined by the amount of energy available (E(av1)) for disposal into the fragment rotational and translational degrees of freedom. Analysis of the data allows refinement of the NeBr2 dissociation energy; we find that D-0=70.0 +/- 1.1 cm(-1) for the X electronic state, v = 0. Both Delta v = - 1 and -2 dissociation events have been examined. For dissociation pathways with approximately the same value of E(av1) the Delta v = -2 pathways are observed to have a higher fraction of the fragment energy in rotational excitation. The overall shape of the Delta v = -1 distributions are insensitive to the value of E(av1), suggesting that a Franck-Condon model for the dissociation may have some validity, though quantitative quantum mechanical calculations demonstrate that this model does not reproduce the large degree of fragment rotational excitation. Two classical models for the dissociation also fail to reproduce the extent of fragment rotational distribution. This result is discussed in light of previous experimental and theoretical investigations, focusing on the apparent agreement of classical models with the IBr fragment rotational distributions that result from the dissociation of NeIBr. (C) 1997 American Institute of Physics

Nonadiabatic Electronic Interactions In The Ion-Pair States Of NelCl

Nonadiabatic interactions in the NeIC1 van der Waals complex have been explored in the lowest energy triad of IC1 ion-pair states (approximately 39 000 cm-1). Dispersed fluorescence measurements reveal emission characteristic of multiple ion-pair electronic states, with the relative contributions from the E(O+ ), beta(1), and D\u27(2) states changing with the initial IC1 vibrational excitation (v(IC1)). Emission directly from NeIC1 (v(IC1) = O) complexes indicates that the initially prepared NeIC1 levels have mixed electronic character and that the IC1 electronic parentage changes with the initial van der Waals vibrational level selected. NeIC1 complexes prepared with 1-4 quanta of IC1 stretch undergo rapid vibrational predissociation with a strong propensity for DELTA-V(IC1) = - 1 relaxation. The electronic state(s) populated in the IC1 fragments differ from the mixed electronic character of the initially prepared level, demonstrating that vibrational predissociation is accompanied by nonadiabatic electronic state changing processes. The observed final state selectivity may be attributed to the relative strength of the nonadiabatic couplings between the initial NeIC1 bound state and the final IC1 states or a momentum gap rationale based on the overlap between the NeIC1 bound state wave function and the highly oscillatory continuum wave function of the separating fragments

Fragment Rotational State Distributions From The Dissociation Of NeIBr: Experimental And Theoretical Results

The IBr fragment rotational state distributions that result when the NeIBr van der Waals molecule undergoes vibrational predissociation have been measured in a pump-probe laser-induced fluorescence experiment. Independent of initial vibrational state and the number of quanta of vibrational energy lost from the I-Br coordinate, the rotational distributions extend over the full range of energetically accessible states. From the observation of energetic constraints on the rotational distribution, the dissociation energy (D0) is calculated to be 65.5 +/- 1.2 cm-1 for the A electronic state, v=16. For the X electronic state, v=0, D0=71.8 +/- 1.2 cm-1. Quantum mechanical bound state calculations carried out on a model A electronic state potential energy surface are in quantitative agreement with this result. The rotational distributions are broader than that predicted by either a Franck-Condon or classical impulsive model for the dissociation. The distributions are qualitatively in accord with classical trajectory calculations. Analysis of specific rotational distributions in the context of the energy gap law shows poor quantitative agreement. The average fragment rotational energy relative to analogous Ne-containing complexes is, however, qualitatively predicted by the energy gap relations

Influence of silicon concentration on the nanoindentation and nanoscratch behavior of pyrolytic carbon

The influences of silicon on the nanoindentation and nanoscratch behavior of pyrolytic carbon were studied using a Nanoindenter XP. The elastic behavior was compared to polycrystalline POCO graphite, single crystalline Ticonderoga graphite, and four actual heart valve components, two of which had high porosity and two which had low porosity. The elastic modulus and hardness were relatively constant below a critical silicon concentration of approximately 8 wt%. Above this critical concentration, the modulus and hardness increase with increasing silicon concentration. This behavior appears to be related to the formation of silicon carbide particles that are embedded in and surround the carbon growth features in the microstructure of an 8 wt% silicon sample. The scratch behavior in the “snow plow” mode also changes abruptly at about 8 wt% silicon. It was observed that pyrolytic carbon is scratch resistant at silicon concentrations less than about 8 wt%, while deep troughs are produced above about 8 wt%. The abrupt change in scratch behavior may be due to the lower fracture toughness of silicon carbide and the distribution of silicon carbide in the growth features. Indentation deformation with the Berkovich indenter in pyrolytic carbon is almost fully elastic and recoverable, due to the sliding of weakly bonded hexagonal planes of carbon atoms that are constrained by neighboring crystallites. Indentation of single crystals of Ticonderoga graphite, which have no constraint, resulted in only a small amount of elastic recovery during unloading. The mechanical behavior of the actual heart valve components can be explained based on their silicon content, with the exception of the hardness of the high porosity samples was about 15 % lower than expected

Task Persistence in Early Childhood Education

This study was designed to discover developmental trends in task persistence (TP). It was hoped that the results would suggest when and for what groups a task persistence (TP) curriculum is crucial. To study this development a sample of two, four, and six year old children were given a work task. The task required 120 subjects (Ss) to sort and fold 33 items of clothing and linen. After the task instructions and demonstration, the experimenter did not intervene with reinforcers or further directions. The dependent variables investigated were time spent on the task, time spent away from the task, and number of tasks completed. The independent variables were age, race (Black and White), sex, and social class (advantaged and disadvantaged). The data were analyzed with analysis of variance, Pearson\u27s Product-Moment Correlation, and chi square. The results indicated that older children spent significantly more time on task and significantly less time away from task than younger children. Black children spent significantly more time (than White children) on and away from the task at all ages. Advantaged children spent significantly more time (than disadvantaged children) on task at all ages and significantly less time away from task at ages two and four. Advantaged and disadvantaged subjects were not differentiated by the time spent away from the task at age six. These results imply that task persistence training programs could begin as early as age two. They also suggest that certain subgroups (i.e., disadvantaged White) may require more intensive task persistence training then other subgroups (i.e., advantaged Black). Completion of tasks (CT) significantly differentiated six year olds from two and four year olds, but failed to differentiate two and four year olds. Tasks were completed significantly more often at ages two and four by males than females and by advantaged than disadvantaged children. However, these differences disappeared by age six. Since the differences faded at age six, the use of completion of tasks training as a general curriculum goal may not be warranted. Some of the most surprising results were described by the significant correlation coefficients. The total time spent on the task was negatively related to completion of task. Completion of task was positively related to the time spent away from the task and number of times away from task. These findings suggest that brief periods of absence from a task may improve the chances of completing the task. Therefore, optimal work performance for early childhood education may be realized by programming frequent, brief breaks. A secondary purpose of the study was to investigate verbalizations expressed during the experimental sessions. To study this variable, a verbatim record was established and maintained for each subject. The verbalizations were then totaled and classified as task relevant or irrelevant. The results were analyzed with an analysis of variance (age x race x sex x social class) and Pearson\u27s Product Moment Correlation. Although the number of verbalizations increased with age, the differences between ages four and six were not significant. This finding may reflect a shift to covert verbal mediation during work performance on or about age four. The developmental patterns of verbalizations were different for boys and girls. The girls used more overt verbalizations (than boys) at ages two and four; however, at age six the girls\u27 number of verbalizations decreased noticeably. The verbalizations of boys increased with age. At age six, they used more verbalizations than the girls. The writer concluded that this finding reflected advanced verbalization skills in the girls. If the conclusion is valid, the results may mean that overt verbalizations are more necessary for boys to organize their experiences. White children used significantly more task relevant verbalizations than Black children. This suggests that early childhood educational programs for predominately Black populations may need to stress task relevant verbalization training more than programs which serve predominately White populations. Disadvantaged children used more task irrelevant verbalizations at ages two and six than advantaged children. Apparently, they need extra emphasis on task oriented verbalization training. When considered with the previous paragraph, the disadvantaged Black population has the greatest need for task relevant verbalization training

How Agriculture Could Uplift a Nation and its People: Providing Analytical Data to Farmers in Belize

Agriculture is the bedrock of the Belize economy, but is plagued by routine issues. These problems hurt not only the economy of Belize, but also the health and well being of its citizens. By working with the Ministry of Agriculture my team surveyed the Stann Creek district of Belize to address these issues and provide much needed information to local farmers

Collision-Induced Electronic Energy Transfer From v=0 Of The E(0+g) Ion-Pair State In I2: Collisions With He And Ar

The electronic energy transfer pathways that occur following collisions between I-2 in the E ion-pair electronic state (v = 0, J = 55) and He and Ar atoms have been determined. The nearby D, D\u27, and beta ion-pair states are populated, but with relative branching ratios that vary with the rare gas collision partner. In He/I-2 collisions, the D state is preferentially populated, while Ar/I-2 collisions preferentially populate the beta electronic state. Bimolecular rate constants and effective hard sphere collision cross sections have been determined for each channel; the cross sections range from 7.0 +/- 1.0 Angstrom(2) for populating the beta state with Ar collisions to 0.9 +/- 0.2 Angstrom(2) for populating the D\u27 state with He collisions. For both rare gas collision partners, and all three final electronic states, low vibrational levels are populated, in rough accord with the relevant Franck-Condon factors. There is little propensity observed for population of vibrational levels that are in near resonance with the initially prepared level in the E state. (C) 2002 American Institute of Physics

C-Cl Bond Fission, Hcl Elimination, And Secondary Radical Decomposition In The 193 Nm Photodissociation Of Allyl Chloride

The primary photodissociation dynamics of allyl chloride upon excitation at 193 nm is investigated in a crossed laser-molecular beam scattering apparatus. Tunable vacuum ultraviolet (VUV) photoionization of the products provides a unique ability to learn about the secondary reaction products of the nascent photoproducts formed. The data show evidence for four significant primary reaction channels: a previously unidentified low kinetic energy C-Cl bond fission channel producing unstable allyl radicals, an excited state C-Cl bond fission channel producing Cl atoms with high translational energy, an HCl elimination pathway releasing significant energy to product translation to HCl and its momentum-matched mass 40 partner, and an HCl elimination channel producing low kinetic energy HCl products and predominantly unstable mass 40 products. The measured branching of these primary reaction channels of [all C-Cl] : [fast C-Cl] : [slow C-Cl] : [fast HCl] : [slow HCl] : [all HCl] is 1.00: 0.971: 0.029: 0.291: 0.167: 0.458 (where fast refers to the high recoil kinetic energy channels). The high internal energy allyl radicals formed in the slow C-Cl fission pathway of allyl chloride further dissociate/isomerize, as do the unstable mass 40 products formed in the HCl elimination pathways, and these products are investigated. Photoionization efficiency (PIE) curves of the HCl product suggest that a three-centered elimination mechanism contributes significantly to an observed HCl elimination reaction. (C) 2002 American Institute of Physics