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The òE" ← X2A'2 spectrum of NO3 radical from 7550 cm-1 to 9750 cm-1 has been recorded and analyzed. Our spectrum differs from previously recorded spectra of this transition due to jet-cooling, which narrows the rotational contours and eliminates spectral interference from hot bands. Assignments of numerous vibronic features can be made based on both band contour and position including the previously unassigned 3¹0 band and several associated combination bands. We have analyzed our spectrum first with an independent anharmonic oscillator model and then by a quadratic Jahn-Teller vibronic coupling model. The fit achieved with the quadratic Jahn- Teller model is excellent, but the potential energy surface obtained with the fitted parameters is in only qualitative agreement with one obtained from ab initio calculations. [ABSTRACT FROM AUTHOR]

New experimental data have been obtained for the methoxy radical by observing at high-resolution laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) transitions between the X 2E and lowest excited à 2A1 state. The SEP transitions were from the à state (pumped from the X 2E3/2 spin-orbit component) to the X 2E1/2 spin-orbit component. These data for the first time directly connect with high precision the spin-orbit components of the X 2E ground state of CH3O. Surprisingly these new SEP observations are inconsistent with predictions of the X state structure based on long-standing analyses primarily based on the microwave spectra of ground state CH3O. It is found that all the experimental data can be understood consistently when the previously accepted value of the spin-orbit coupling constant is adjusted and the reflection parity assignments in the X state are reversed. The latter action changes the sign of a number of reflection-parity-dependent parameters in the X state. The ramifications of the changes and the physical interpretation of the resulting parameters are discussed in some detail. [ABSTRACT FROM AUTHOR]

The dynamics of photodissociation of glyoxal (HOC–COH) near the dissociation threshold on the triplet manifold are studied through measurement of distributions of nascent fragment HCO in various internal states. Three rotational levels 1[sub 01][sup *], 4[sub 13][sup *], and 3[sub 21][sup *]+3[sub 22][sup *] of vibrational state U (excitation wavelength ∼394.4 nm, origin at 25 331.865 cm-1) of glyoxal in state à [sup 1]A[sub u] and two other vibrational states at excitation wavelengths 390.33 and 382.65 nm are selected to produce fragment HCO. By means of fluorescence in the transition B [sup 2]A[sup ′]–X [sup 2]A[sup ′] of HCO, we determined the relative populations of internal states of that fragment. Rotational states of product HCO up to N=26 and K=2 are populated, and bimodal distributions of these rotational states are observed for the photolysis wavelengths used in this work. The high rotational part of the distribution with average energy near values calculated on the basis of the statistical model—phase-space theory is assigned to arise from glyoxal on its S[sub 0] surface, and the low rotational part from the T[sub 1] surface with an exit barrier. After photolysis near the threshold region on the triplet surface, HCO arising from the T[sub 1] state appears to be a major component of products because these rotational levels 1[sub 01][sup *], 4[sub 13][sup *], and 3[sub 2][sup *] of U state selected are gateway states with an enhanced rate of intersystem crossing. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Fluorescence excitation spectra for transition à [sup 1]A[sub u]–X [sup 1]A[sub g] of trans-glyoxal in a supersonic jet were recorded with laser excitation in a wavelength region near 395 nm. With resolution 0.04 cm[sup -1], most lines in these spectra are resolved and are assigned rotationally; of six bands assigned, three have c-type rotational structure, and another three have types a/b hybrid, a and b. Fluorescence decays with quantum beats, resulting from coherent excitation of S[sub 1] and T[sub 1] states, are observed for most rotational levels in this region. In spectra transformed to the frequency domain for these decays, the widths are larger than those obtained at low excitation energy. The widths from nonzero frequency lines, reflecting mostly the triplet character, indicate that the triplet state is dissociating. Fluorescence spectra of fragment HCO confirm that the dissociation channel correlated to the triplet surface of glyoxal involves formation of radical products. From an abrupt decrease of lifetime of the triplet state and onset of disappearance of fluorescence of glyoxal, the threshold for formation of fragment HCO from trans-glyoxal is determined to be 394.4 nm. With data for heat of formation this threshold yields an exit barrier 2.5±0.5 kcal/mol. Signal of HCO for wavelengths greater than the threshold position up to 400.5 nm is observed and is assigned to arise from the S[sub 0] surface. Rotationally state-resolved appearance rates of HCO for a vibrational state near the dissociation threshold are measured. Relative to lifetimes obtained from eigenstates in the quantum-beat data, the appearance time of product HCO reflecting the overall depletion of glyoxal is on average longer. Some observed gateway states with enhanced yields of HCO are considered to result from strong singlet–triplet interaction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Hot spots produced by long-wavelength infrared (LWIR) radiation in an energetic material, crystalline RDX (1,3,5-trinitroperhydro-1,3,5-triazine), were studied by thermal-imaging microscopy. The LWIR source was a CO2 laser operating in the 28-30THz range. Hot spot generation was studied using relatively low intensity (~100W cm-2), long-duration (450 ms) LWIR pulses. The hot spots could be produced repeatedly in individual RDX crystals, to investigate the fundamental mechanisms of hot spot generation by LWIR, since the peak hot-spot temperatures were kept to ~30K above ambient. Hot spots were generated preferentially beneath RDX crystal planes making oblique angles with the LWIR beam. Surprisingly, hot spots were more prominent when the LWIR wavelength was tuned to be weakly absorbed (absorption depth ~30 μm) than when the LWIR wavelength was strongly absorbed (absorption depth ~5 μm). This unexpected effect was explained using a model that accounts for LWIR refraction and RDX thermal conduction. The weakly absorbed LWIR is slightly focused underneath the oblique crystal planes, and it penetrates the RDX crystals more deeply, increasing the likelihood of irradiating RDX defect inclusions that are able to strongly absorb or internally focus the LWIR beam. [ABSTRACT FROM AUTHOR]

We have observed and characterized hot spot formation and hot-spot ignition of energetic materials (EM), where hot spots were created by ultrasonic or long-wavelength infrared (LWIR) exposure, and were detected by high-speed thermal microscopy. The microscope had 15-20 μm spatial resolution and 8.3 ms temporal resolution. LWIR was generated by a CO2 laser (tunable near 10.6 μm or 28.3 THz) and ultrasound by a 20 kHz acoustic horn. Both methods of energy input created spatially homogeneous energy fields, allowing hot spots to develop spontaneously due to the microstructure of the sample materials. We observed formation of hot spots which grew and caused the EM to ignite. The EM studied here consisted of composite solids with 1,3,5-trinitroperhydro-1,3,5-triazine crystals and polymer binders. EM simulants based on sucrose crystals in binders were also examined. The mechanisms of hot spot generation were different with LWIR and ultrasound. With LWIR, hot spots were most efficiently generated within the EM crystals at LWIR wavelengths having longer absorption depths of ~25 μm, suggesting that hot spot generation mechanisms involved localized absorbing defects within the crystals, LWIR focusing in the crystals or LWIR interference in the crystals. With ultrasound, hot spots were primarily generated in regions of the polymer binder immediately adjacent to crystal surfaces, rather than inside the EM crystals. [ABSTRACT FROM AUTHOR]

We report transmission electron tomography of nanoporous gold fabricated by chemically dealloying Au35Ag65 films. A number of algorithms were employed to quantitatively characterize the complex three-dimensional nanoporous structure. It was found that gold ligaments and nanopore channels are topologically and morphologically equivalent, i.e., they are inverses of each other in three-dimensional space. Statistical analysis reveals that this bicontinuous nanostructured material is actually quasiperiodic and has, on average, a near zero surface curvature. These quantitative measurements will help in understanding the structural stability of nanoporous gold and in modeling its physical and chemical performances. [ABSTRACT FROM AUTHOR]