The UV photochemistry of C2N2

The absorption, emission, and photodissociation yield spectra of C2N2 were measured in the 220 and 210 nm region near the 4(0)1 and 1(0)1 4(0)1 bands of the A 1 sigma + from the X 1 sigma + system. The emission spectrum showed very few lines which appeared in the absorption spectrum. Moreover, the emission had 660 ns lifetime and, at 210 nm a very large electronic emission quenching rate. Laser induced fluorescence was used to measure the relative yield of CN radicals as a function of photolysis wavelength. This spectrum seemed to follow the absorption spectrum below the dissociation threshold. Energy in the CN fragments appeared to be statistically distributed

The heat of formation of CN radicals and radiative lifetimes of the A 1 Sigma - state of C2N2

Radiative lifetimes have been measured for the stable vibrational levels of the A 1 sigma - electronic state of C2N2. They range from 1.3 microsec for the 4(sub 0)(sup 1) level to 0.66 microsec for the 1(sub 0)(sup 1) 4(sub 0)(sup 1) level and in general decrease with increasing vibrational excitation. Self-quenching rate constants range from gas kinetic to ten times that. Foreign gas quenching rates are slower. Observation of emission from the 1(sub 0)(sup 1) 4(sub 0)(sup 1) level sets a lower limit for the heat of formation of CN of 439.11 kJ/mol

Feasibility study of a synthesis procedure for array feeds to improve radiation performance of large distorted reflector antennas

The topics covered include the following: (1) performance analysis of the Gregorian tri-reflector; (2) design and performance of the type 6 reflector antenna; (3) a new spherical main reflector system design; (4) optimization of reflector configurations using physical optics; (5) radiometric array design; and (7) beam efficiency studies

Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility

We present a systematic study of the effect of crack blunting on subsequent crack propagation and dislocation emission. We show that the stress intensity factor required to propagate the crack is increased as the crack is blunted by up to thirteen atomic layers, but only by a relatively modest amount for a crack with a sharp 60$^\circ$ corner. The effect of the blunting is far less than would be expected from a smoothly blunted crack; the sharp corners preserve the stress concentration, reducing the effect of the blunting. However, for some material parameters blunting changes the preferred deformation mode from brittle cleavage to dislocation emission. In such materials, the absorption of preexisting dislocations by the crack tip can cause the crack tip to be locally arrested, causing a significant increase in the microscopic toughness of the crack tip. Continuum plasticity models have shown that even a moderate increase in the microscopic toughness can lead to an increase in the macroscopic fracture toughness of the material by several orders of magnitude. We thus propose an atomic-scale mechanism at the crack tip, that ultimately may lead to a high fracture toughness in some materials where a sharp crack would seem to be able to propagate in a brittle manner. Results for blunt cracks loaded in mode II are also presented.Comment: 12 pages, REVTeX using epsfig.sty. 13 PostScript figures. Final version to appear in Phys. Rev. B. Main changes: Discussion slightly shortened, one figure remove

EMISSION, ABSORPTION AND QUANTUM YIELDS IN THE C2N2A1Σ−←1Σ+C_{2}N_{2} A^{1}\Sigma^{-} \leftarrow {^{1}}\Sigma^{+} TRANSITION

Author Institution: Department of Chemistry, Howard University WashingtonWe have measured the absorption, emission end photodissociation spectra of cyanogen in the 220 and 210 nm regions near the $4_{O}{^{1}}$ and $1_{O}{^{1}}4_{O}{^{1}}$ bands of the $A^{1}\Sigma^{-} \leftarrow x^{1}\Sigma^{+}$ system. The spectra were excited with a frequency doubled Nd-YAG pumped dye laser. The doubled light was Reman shifted in high pressures of hydrogen gas. Emission spectra were measured by an unfiltered VUV photomultiplier. The emission spectra did not show all lines that appeared in the absorption spectra and almost all of the intensity was in the $1_{O}{^{n}}4_{O}{^{1}}$ progression. The radiative lifetime of the emission was 660 ns. We used laser induced fluorescence to measure the relative yield of CN radicals as a function of photolysis wavelength. Above the dissociation threshold the yield generally followed the absorption spectrum. A few nascent quantum state distributions of CN were measured. Energy in the CN fragments appeared to be statistically distributed. These results can be understood if some of the excited $C_{2}N_{2}$ is crossing over into a non-dissociating triplet manifold. We are attempting to establish quantum yields for dissociation and emission. If possible fluorescence spectra will also be measured. Dispersed fluorescence spectra will identify the emitting state

THE FLUORESCENCE SPECTRA OF C2N2C_{2}N_{2}

Author Institution: Department of Chemistry, Howard University; Department of Chemistry, Ballistic Research Laboratory; Department of Chemistry, Howard UniversityDispersed fluorescence spectra have been measured following excitation of all of the non-dissociative levels of the $A^{1}\Sigma^{-}_u$ levels of ethanedinitrile $C_{2}N_{2}$. The spectra are characterized by long progressions in the $v_{1}$ and $v_{4}$ modes. The longest wavelength fluorescence terminates on ground state vibrational levels more than $14,000 cm^{-1}$ above the origin. In addition the radiative lifetimes have an inverse dependence on the amount of excitation in the $v_{4}$ mode, which may indicate a crossing to high lying levels of the $x^{1}\Sigma^{+}_g$ electronic state