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Fluorescence excitation spectra for transition A 1Au–X 1Ag of trans-glyoxal in a supersonic jet were recorded with laser excitation in a wavelength region near 395 nm. With resolution 0.04 cm−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 S1 and T1 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 on...

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 28-30 THz range. Hot spot generation was studied using relatively low intensity (∼100 W 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 ∼30 K 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.

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.

The jet-cooled Ã-X̃ near IR origin band spectra of the G(1)G(2)G(3) conformer of four β-hydroxyethylperoxy isotopologues, β-HEP (HOCH(2)CH(2)OO), β-DHEP (DOCH(2)CH(2)OO), β-HEP-d(4) (HOCD(2)CD(2)OO), and β-DHEP-d(4) (DOCD(2)CD(2)OO), have been recorded by a cavity ringdown spectrometer with a laser source linewidth of ~70 MHz. The spectra of all four isotopologues have been analyzed and successfully simulated with an evolutionary algorithm, confirming the cyclic structure of the molecule responsible for the observed origin band. The analysis also provides experimental à and X̃ state rotational constants and the orientation of the transition dipole moment in the inertial axis system; these quantities are compared to results from electronic structure calculations. The observed, broad linewidth (Δν2 GHz) is attributed to a shortened lifetime of the à state associated with dynamics along the reaction path for hydrogen transfer from the OH to OO group.

Rotationally resolved laser induced fluorescence and stimulated emission pumping Ã(2)A(1)-X̃(2)E spectra, along with pure rotational spectra in the 153-263 GHz region within the E(3/2) component of the ground state in asymmetrically deuterated methoxy radicals CH(2)DO and CHD(2)O have been observed. The combined data set allows for the direct measurement with high precision of the energy separation between the E(1/2) and E(3/2) components of the ground state and the energy separation between the parity stacks in the E(3/2) component of the ground state. The experimentally observed frequencies in both isotopologues are fit to an effective rotational Hamiltonian accounting for rotational and spin-rotational effects arising in a near-prolate asymmetric top molecule with dynamic Jahn-Teller distortion. Isotopic dependencies for the molecular parameters have been successfully implemented to aid the analysis of these very complex spectra. The analysis of the first and second order contributions to the effective values of molecular parameters has been extended to elucidate the physical significance of resulting molecular parameters. Comparisons of measured parameters, e.g., spin-orbit coupling, rotational and spin-rotation constants, are made among the 5 methoxy isotopologues for which data is now available. Comparisons of experimental results, including the derived geometric structure at both the C(3v) conical intersection and at the Jahn-Teller distorted minima, are made with quantum chemistry calculations.

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 (2)E and lowest excited A (2)A(1) state. The SEP transitions were from the A state (pumped from the X (2)E(3/2) spin-orbit component) to the X (2)E(1/2) spin-orbit component. These data for the first time directly connect with high precision the spin-orbit components of the X (2)E ground state of CH(3)O. 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 CH(3)O. 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.