The steady-state approximation: Fact or fiction?

The kinetic equations for an 81-reaction model of a photochemical smog chamber have been solved using a complete numerical integration as well as a quasi-steady-state approximation (QSSA) procedure. The two sets of results differ markedly in their prediction of experimentally significant factors such as the hydrocarbon depletion rate and the ozone and (NO)_x peaking times. The sources of the discrepancy are traced to the fact that the assumed steady-state conditions were not satisfied, leading to errors in the concentrations of intermediate radicals which in turn affect critical rates in the reaction model. The occurrence of such discrepancies in various types of reaction models, and with different QSSA strategies, is discussed, and it is concluded that the extent of such errors in QSSA calculations cannot be reliably predicted. Their impact on conclusions regarding reaction mechanisms and rate constants can surpass uncertainties in experimental data; conversely the credibility of predictions derived through QSSA calculations becomes highly suspect. Since recently devloped methods for complete numerical integration of systems of kinetic equations are now available, it is recommended that these be adopted in future work, and that the use of QSSA be abandoned.     

Molecular orientation of a model liquid crystal alignment layer

An analytical methodology, involving the use of a combination of second harmonic generation (SHG) and linear dichroism, was utilized to probe the molecular orientation and angular distribution of a model liquid crystal (LC) alignment layer. In order to determine which film structure would be best suited for use as an alignment layer, the azo dye o-methyl red (MR) was covalently bound to a glass substrate using both monofunctional and trifunctional silane chemistry. The influence of solvent on the orientation and angular distribution of both thin films was also investigated. For the monofunctional silane film under water, the mean orientation angle of the MR molecular long axis was 67±4° and the width of an assumed Gaussian distribution was 32±2°. Under hexanes, the mean orientation angle was the same within error (63±1°) but the distribution width narrowed considerably to 22±1°. Molecular orientation within the trifunctional silane film exhibited little dependence on solvent. Under water, the mean orientation angle and angular distribution width were 76±3° and 30±1°, respectively. With hexanes as the solvent, the mean orientation angle and angular distribution width were 79±1° and 30±1°, respectively. Orientation insensitive SHG measurements indicated that surface coverage in the tri-functional silane film was twice that in the mono-functional silane film. The observed orientational differences were attributed to differences in the forces that dictate molecular orientation for the two systems. Based on the higher orientation angle, higher surface coverage and the lack of solvent dependence, MR-tri exhibits more desirable characteristics for use as an LC alignment layer.     

Optical properties of Yb-doped fibers and fiber lasers at high temperature

Recent advances in power scaling of Yb^+ 3-doped fiber lasers to the kilowatt level suggest a need to examine the performance of Yb^+ 3-doped silica at temperatures well above ambient. We report experimental results for the absorption coefficient, emission cross-section, fluorescence lifetime, and slope efficiency of a Yb³⁺-doped large mode area (LMA) silica fiber for temperatures spanning 23 °C-977 °C. To the best of our knowledge these are the highest temperatures to date for which these optical properties have been measured. We find a sharp reduction in the energy storing capability and lasing performance of Yb^+ 3:SiO₂ above 500 °C that coincides with the onset of non-radiative transitions in the excited state manifold (thermal quenching). As the temperature increases from room temperature to 977 °C, absorption in the 1020-1120 nm operating band increases monotonically, concurrent with a reduction in absorption at the 920-nm and 977-nm pumping bands. Conversely, the spectral weight of the emission cross-section shifts from transitions above 1010 nm to those below, with the exception of the 977-nm emission band.     

Polarized pump-probe measurements of electronic motion via a conical intersection

Polarized femtosecond pump-probe spectroscopy is used to observe electronic wavepacket motion for vibrational wavepackets centered on a conical intersection. After excitation of a doubly degenerate electronic state in a square symmetric silicon naphthalocyanine molecule, electronic motions cause a approximately 100 fs drop in the polarization anisotropy that can be quantitatively predicted from vibrational quantum beat modulations of the pump-probe signal. Vibrational symmetries are determined from the polarization anisotropy of the vibrational quantum beats. The polarization anisotropy of the totally symmetric vibrational quantum beats shows that the electronic wavepackets equilibrate via the conical intersection within approximately 200 fs. The relationship used to predict the initial electronic polarization anisotropy decay from the asymmetric vibrational quantum beat amplitudes indicates that the initial width of the vibrational wavepacket determines the initial speed of electronic wavepacket motion. For chemically reactive conical intersections, which can have 1000 times greater stabilization energies than the one observed here, the same theory predicts electronic equilibration within 2 fs. Such electronic movements would be the fastest known chemical processes.     

Peak-power limits on fiber amplifiers imposed by self-focusing

We have numerically investigated the behavior of the fundamental mode of a step-index, multimode (MM) fiber as the optical power approaches the self-focusing limit (P(crit)). The analysis includes the effects of gain and bending (applicable to coiled fiber amplifiers). We find power-dependent, stationary solutions that propagate essentially without change at beam powers approaching P(crit) in straight and bent fibers. We show that in a MM fiber amplifier seeded with its fundamental eigenmode at powers 相似文献