Spectrum analysis

One of the limiting factors restricting aircraft landings at maJor airports is the
minimum spacing requirements due to vortex wake avoidance. If it can be shown that the
separation requirements are too conservative, then it may be possible to increase the rate
of landings on a given runway. During August/September 2003, NASA and the (United
States Department of Transportation) USDOT sponsored a wake acoustics test at the
Denver International Airport. The central instrument of the test was a large microphone
phased array. Different types of aircrafts were recorded during landing and the acoustic
data obtained was stored. From acoustic data the spectrograms were generated using the
technique of AutoRegressive (AR) spectral estimation from multitaper autocorrelation
estimates.
Several sources of sound that are recorded in the audio files can be observed in the
spectrograms. Some these signals, such as the noise generated from the aircraft engine can be identified easily because of their strength and the Doppler shift they undergo. In
contrast to this, the wake vortex signal is weaker and does not exhibit a Doppler shift
because it's stationary in space. Therefore it may not be identified easily because of the
existence of stronger signals. The motive in our research is to develop methods to
determine these strong signals that appear as spectral lines in the spectrogram. In the
future, the results obtained in this work can be used to eliminate these strong signals from
the spectrogram thus allowing us to see and identify wake vortex signal which is more
important to us.

G-quadruplexes (G4s) are nucleic acid structures formed from π-stacked planar sets of four Hoogsteen hydrogen bonded guanine bases. G4s emerged as potential therapeutic targets based on their ability to modulate gene expression and inhibit the ability of telomerase to elongate chromosomal telomeres. Raman spectroscopy, polarized Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and other optical spectroscopic techniques were used to characterize the G4s formed by four different DNA sequences: human telomeric (HT), thrombin-binding aptamer (TBA), nuclease hypersensitive element III1 region of the c- Myc gene promoter (Myc), and a single loop-isomer of Myc (MycL1).

This research evaluated the applicability of electrochemical impedance spectroscopy (EIS) as a non-destructive technique to predict and characterize the degradation of carbon fiber reinforced polymer (CFRP) composites exposed to aqueous environments at ambient and 6.2 $\pm$ 0.3 MPa. Changes in EIS data were related to water uptake into the composite material as a function of exposure time. Electrochemically induced damage (both anodic and cathodic) were also evaluated using impedance measurements. Three point flexure tests with concurrent EIS measurements were employed to study the effect of stresses on water uptake and mechanical degradation. Visual observation of the extent of damage (i.e., fiber-matrix debonding) was made using scanning electron microscopy (SEM) and correlated with EIS observation.

The optical properties of the diluted magnetic semiconductor Zn1-x MnxS have been studied by using photoluminescence emission, lifetime and excitation spectroscopies. In photoluminescence spectroscopy three bands were found peaking at 585 nm (yellow band), 650 nm (orange band) and 750 nm (red band). For the yellow and red bands the peak position changes little with Mn concentration. For the orange band the peak energy decreases with increasing Mn concentration. The lifetimes of the yellow, orange and red bands were found to be about 70, 50 and 7 mus. The results of the lifetime measurements support the assumption that there is energy transfer from the yellow to the red band through radiationless transitions. In the excitation spectroscopy of the yellow band four peaks were found peaking near 526, 487, 461 and 426 nm. These four bands have been ascribed to transitions from the four lowest excited states of Mn ++ to the ground state. (Abstract shortened with permission of author.)

Raman spectroscopy was used to investigate how phase structure and phase stability of zirconia mixtures depend on temperature, type and concentration of added rare earth oxides, and fabrication process. All 8wt% yttria samples exhibited the tetragonal structure between 300 and 10 K. The TZP sample, prepared under pressure and fully hardened, exhibited the most completely stabilized tetragonal structure. The plasma coating process did not improve the stability of the tetragonal structure. Adding ceria to zirconia did not completely stabilize the tetragonal structure, while adding alumina actually destabilized it. Mode Gruneisen parameters, obtained from changes in frequency for yttria samples with different concentration, agreed with published results under pressure, except for one Eg mode. The negative value obtained for this mode was ascribed to changes in cation-anion interactions. The temperature dependence of the linewidth for this mode was quantitatively explained by cubic anharmonic interactions.

Spectroscopic observations were made of the resonance absorption of
sunlight by the atmospheric hydroxyl radical (OH). The abundance of
0 terrestrial OH was measured from the Fritz Peak Observatory (39.92 N,
105.50°W) in Colorado during September of 1978. These measurements
were made on the absorption of the P1 (1) line of the A^2sigma^+ - x^2pi(O , O)
rotational band of OH at 3081.7 A. The instrument used was the
PEPSIOS (Poly Etalon Pressure Scanned Interferometric Optical Spectrometer)
because of its high luminosity - resolution product . The
computer methods ·~sed in the analysis procedure are described . These
measurements indicated an average daytime vertical column abundance
of (5.40 ± 0.67) x 10^13 OH molecules/cm^2.

Polarized electronic spectra of trans-diacidobis (ethylenediamine )-
chromium(III) complexes were measured at liquid nitrogen temperature,
the diacido groups being aquochloro, aquobromo, bromochloro, bis(N, N-dimethylformamide), chloro(N, N-dimethyl£ormamide) and bromo(N, N-dimethylformamide). The quadrate components of the first two cubic
bands were polarized and assigned on the basis of predicted polarization
pattern for D4h vibronic mechanism for d^3 configuration. The higher energy intraconfigurational doublets were uncovered in most of the systems. The bis(N, N-dimethylformamide) complex showed two high-energy spin-forbidden bands at ~32,000 cm^-1 not previously observed in
the spectra of chromium(III) complexes. The ligand field and electron
correlation parameters were evaluated by fitting the observed band maxima with d^3 quadrate energy equations including full configuration interaction but neglecting spin-orbit perturbation. The parameters of these complexes
and of the other trans-diacidobis (ethylenediamine) systems obtained by
others were analysed in terms of the bonding nature of ligands.