Kilohertz linewidth from frequency-stabilized mid-infrared quantum cascade lasers

Frequency stabilization of mid-IR quantum cascade (QC) lasers to the kilohertz level has been accomplished by use of electronic servo techniques. With this active feedback, an 8.5-microm QC distributed-feedback laser is locked to the side of a rovibrational resonance of nitrous oxide (N(2) O) at 1176.61cm (-1) . A stabilized frequency-noise spectral density of 42Hz/ radicalHz has been measured at 100 kHz; the calculated laser linewidth is 12 kHz.     

Efficient nonlinear frequency conversion in an all-resonant double- lambda system

We demonstrate efficient, pulsed, gas-phase, nonlinear frequency conversion in a quadruply resonant, double- Lambda system and, simultaneously, verify theoretical predictions of Rabi-frequency matching unique to absorbing nonlinear media. This system is used to up-convert ultraviolet light at 233 nm to the vacuum ultraviolet at 186 nm in atomic Pb vapor with small-signal conversion efficiencies exceeding 30% and with modest atomic density-length (NL) products (scale 10(14) cm(-2)) and optical power densities (10-100 kW/cm(2)).     

Specimen strain measurement in the split-Hopkinson-pressure-bar experiment

The validity of the split-Hopkinson-pressurebar technique for measuring dynamic strain and stress is examined by comparing the average strain computed by use of that technique with the strain at the midpoint of the sample. The midpoint strain is measured by use of the inter-ferometric-strain-gage method. It is found that the midpoint surface strain and the average strain agree quite well after a strain of approximately 1 percent. It is concluded that the pressure-bar technique is valid for large strains.     

Small attachable interferometric strain gages

Laser-based interferometry from two tiny reflective indentations can be used to measure in-plane strain/displacement over a very short gage length (on the order of 100 μm). If the specimen material is not reflective, then some other means of generating the interference patterns must be found. This paper describes two kinds of attachable gages: plated acetate replicas of indentations and reflective foils that are indented after application. In either case, the gage is applied with the techniques used for foil-resistance gages and the gage itself is very small. The manufacturing procedures are described. The results of experiments comparing the strain to that measured with foil-resistance gages are presented. Finally, the small interferometric gage is used to measure strain on one of the metal strips in a foil-resistance gage.     

COD measurements at various positions along a crack

Load versus crack-opening displacement (COD) was measured at various positions along the border of a fatigue crack as it grew from a small surface crack on the edge of an aluminum specimen into a through-the-thickness crack. Displacements were measured with a laser-based interferometric system with a gage length of 70 m and a resolution of 0.01 m. These load-COD curves can be used to determine opening loads and thereby investigate the effect of closure on the growth of cracks. In general, the opening loads decrease as the crack grows. The compliances after the crack is fully open can also be measured, and show generally good agreement with the predictions of linear-elastic fracture mechanics.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Storage of an excess proton in the hydrogen-bonded network of the d-pathway of cytochrome C oxidase: identification of a protonated water cluster

The mechanism of proton transport in the D-pathway of cytochrome c oxidase (CcO) is further elucidated through examining a protonated water/hydroxyl cluster inside the channel. The second generation multi-state empirical valence bond (MS-EVB2) model was employed in a molecular dynamics study based on a high-resolution X-ray structure to simulate the interaction of the excess proton with the channel environment. Our results indicate that a hydrogen-bonded network consisting of about 5 water molecules surrounded by three side chains and two backbone groups (S197, S200, S201, F108) is involved in storage and translocation of an excess proton to the extracellular side of CcO.     

High-resolution infrared spectroscopy in the 1,200-1,300 cm(-1) region and accurate theoretical estimates for the structure and ring-puckering barrier of perfluorocyclobutane

We present experimental infrared spectra and theoretical electronic structure results for the geometry, anharmonic vibrational frequencies, and accurate estimates of the magnitude and the origin of the ring-puckering barrier in C4F8. High-resolution (0.0015 cm-1) spectra of the nu12 and nu13 parallel bands of perfluorocyclobutane (c-C4F8) were recorded for the first time by expanding a 10% c-C4F8 in helium mixture in a supersonic jet. Both bands are observed to be rotationally resolved in a jet with a rotational temperature of 15 K. The nu12 mode has b2 symmetry under D2d that correlates to a2u symmetry under D4h and consequently has +/- <-- +/- ring-puckering selection rules. A rigid rotor fit of the nu12 band yields the origin at 1292.56031(2) cm-1 with B' = 0.0354137(3) cm-1 and B' ' = 0.0354363(3) cm-1. The nu13 mode is of b2 symmetry under D2d that correlates to b2g under D4h, and in this case, the ring-puckering selection rules are +/- <-- -/+ . Rotational transitions from the ground and first excited torsional states will be separated by the torsional splitting in the ground and excited vibrational states, and indeed, we observe a splitting of each transition into strong and weak intensity components with a separation of approximately 0.0018 cm-1. The strong and weak sets of transitions were fit separately again using a rigid rotor model to give nu13(strong) = 1240.34858(4) cm-1, B' = 0.0354192(7) cm-1, and B' ' = 0.0354355(7) cm-1 and nu13(weak) = 1240.34674(5) cm-1, B' = 0.0354188(9) cm-1, and B' ' = 0.0354360(7) cm-1. High-level electronic structure calculations at the MP2 and CCSD(T) levels of theory with the family of correlation consistent basis sets of quadruple-zeta quality, developed by Dunning and co-workers, yield best estimates for the vibrationally averaged structural parameters r(C-C) = 1.568 A, r(C-F)alpha = 1.340 A, r(C-F)beta = 1.329 A, alpha(F-C-F) = 110.3 degrees , thetaz(C-C-C) = 89.1 degrees , and delta(C-C-C-C) = 14.6 degrees and rotational constants of A = B = 0.03543 cm-1 and C = 0.02898 cm-1, the latter within 0.00002 cm-1 from the experimentally determined values. Anharmonic vibrational frequencies computed using higher energy derivatives at the MP2 level of theory are all within <27 cm-1 (in most cases <5 cm-1) from the experimentally measured fundamentals. Our best estimate for the ring-puckering barrier at the CCSD(T)/CBS (complete basis set) limit is 132 cm-1. Analysis of the C4F8 electron density suggests that the puckering barrier arises principally from the sigmaCC-->sigmaCF hyperconjugative interactions that are more strongly stabilizing in the puckered than in the planar form. These interactions are, however, somewhat weaker in C4F8 than in C4H8, a fact that is consistent with the smaller barrier in the former (132 cm-1) with respect to the latter (498 cm-1).     

Least median of squares estimation by optimization heuristics with an application to the CAPM and a multi-factor model

For estimating the parameters of models for financial market data, the use of robust techniques is of particular interest. Conditional forecasts, based on the capital asset pricing model, and a factor model are considered. It is proposed to consider least median of squares estimators as one possible alternative to ordinary least squares. Given the complexity of the objective function for the least median of squares estimator, the estimates are obtained by means of optimization heuristics. The performance of two heuristics is compared, namely differential evolution and threshold accepting. It is shown that these methods are well suited to obtain least median of squares estimators for real world problems. Furthermore, it is analyzed to what extent parameter estimates and conditional forecasts differ between the two estimators. The empirical analysis considers daily and monthly data on some stocks from the Dow Jones Industrial Average Index.