William E. Blass

Thermal spectroscopy of Neptune: The stratospheric temperature, hydrocarbon abundances, and isotopic ratios☆

Abstract Portions of Neptunes disk-averaged spectrum were observed from the NASA/Infrared Telescope Facility in 1989 and 1990 with a cryogenic echelle array spectrometer, with resolution of ν/Δν ∼ 104. A lower-resolution spectrum (Orton et al. 1990, Icarus 85, 257–265) was used in conjunction with the analysis of these spectra to provide a more reliable absolute intensity calibration. Together, these data imply a stratospheric temperature of 168 K near 1–10 μbar pressure. Considering the infrared data, Voyager UVS experiment, and Earth-based stellar occultation results, we adopt an uncertainty of ±10 K. The temperature profile adopted in this study is consistent with the size and shape of the H2 J = 3−1 quadrupole feature which was detected in emission. The maximum mixing ratios of CH4, C2H2, and C2H6 which are consistent with this temperature and its uncertainty are 7.5−5.6+18.6 × 10−4, 5.1−4.3+2.0 × 10−8, and 1.0−0.8+0.2 × 10−6, respectively. These C2H2 and C2H6 abundances are completely consistent with previous measurements, after correcting for different temperature profile assumptions. The combined measurements imply a CH3D/CH4 ratio of 3.6 ± 0.5 × 10−4, from which we deduce D/H = 1.13 ± 0.16 × 10−4; this is enhanced with respect to solar values and consistent with near-infrared high-resolution spectroscopy (deBergh et al. 1990, Astrophys. J. 355, 661–666). High-resolution measurements of 13C12CH6 and 12C2H6 imply that 12C/13C = 78 ± 26, consistent with solar and telluric values.

Diode laser spectra of the torsional splittings in the ν9 band of ethane: Torsion‐vibration‐rotation interactions and the barrier to internal rotation

Diode laser spectra of most of the Q branches of the ν9 band of ethane from RQ8–PQ15 have been recorded. The Q branches RQ0–RQ4 were deconvoluted to yield an effective resolution of (0.5–1.0)×10−3 cm−1 FWHM. Torsional splittings were observed for most lines. In contrast to predictions based on first order theory, the splittings which range from (2–53)×10−3 cm−1, have a marked J and K dependence. A second order theory of torsion‐vibration‐rotation interaction between ν9 and 3ν4 is developed, which fits the splittings with an rms error of 0.0006 cm−1, using only three adjustable parameters: the barrier to internal rotation in ν9, the energy difference between ν9 and 3ν4, and an effective coupling constant. The barrier to internal rotation in ν9 is found to be 1123±10 cm−1.

Probing Titan's lower atmosphere with acousto-optic tuning

Abstract Narrow-band images of Titan were obtained in November 1999 with the NASA/GSFC- built acousto-optic imaging spectrometer (AImS) camera. This instrument utilizes a tunable filter element that was used within the 500- to 1050-nm range, coupled to a CCD camera system. The images were taken with the Mount Wilson 2.54-m (100 in.) Hooker telescope, which is equipped with a natural guide star adaptive optics system. We observed Titan at 830 and 890 nm and at a series of wavelengths across the 940-nm window in Titan’s atmosphere where the methane opacity is relatively low. We determined the absolute reflectivity (I/F) of Titan and fit the values at 940 nm to a Minnaert function at Titan’s equator and at −30° latitude (closer to the subsolar point) and obtained average values for the Minnaert limb-darkening slope, k, of 0.661 ± 0.007 and 0.775 ± 0.018, respectively. Comparison with models suggests that the equatorial value of k is consistent with rain removal of haze in the lower atmosphere. The higher value of k at −30° is consistent with the southern hemisphere being brighter than the equator. However, the fits are not unique. The data and models at 890 are consistent with no limb brightening or darkening at this wavelength either at the equator or at −30°.

An infrared study of the bending region of acetylene

Abstract Acetylene spectra observed with instrumental resolutions of 0.0025 and 0.005 cm−1 were obtained at The National Optical Astronomy Observatory, Tucson, Arizona using the FTS spectrometer at the National Solar Observatory McMath telescope facility. These resolutions are factors of 6 and 3 times, respectively, that of the best prior study in the literature. The higher resolution allowed the assignment of many more low-J Q-branch transitions. The range of optical densities available in this study also allowed the extension of observed and assigned transitions to significantly higher J-values than reported in prior studies. Availability of improved standard lines resulted in high quality transition wavenumbers which are accurate to approximately ±0.0001 cm−1. Using proven combination difference techniques and analysis software, this study produces the best available molecular parameters for the ν4 and ν5 states of acetylene. In addition, because acetylene has no permanent dipole moment, this study extends the precision of available ground state parameters B0 and D0 and produces for the first time an estimator of H0. In addition to H0 values for both isotopic species treated, we have obtained for the first time a value for H5 for the dominant isotopmer. The value of H5 for the 13C12CH2 was not determined and thus thought to be significantly smaller than H5 for 12C2H2. This study also obtains l-doubling parameters for both degenerate fundamental states in the lesser isotopmer for the first time as well as H4 for both isotopmers and B4 and D4 for the lesser isotopmer.

10 mu m ethylene: spectroscopy, intensities and a planetary modeler's atlas

FTS and TDL spectra of ethylene in the 10 mum region have been observed, measured, calibrated, assigned and intensities have been measured. The ultimate goal of this work is the production of a planetary modelers atlas. A spectrum taken in double-pass configuration at the McMath-Pierce FTS instrument at Kitt Peak National Observatory has been frequency calibrated using CO2 laser bands. Results of a previous analysis (Cauuet et al., J Mol Spectrosc 1990;139:191) have enabled the assignment of the FTS spectrum wherein we have measured over 500 line intensities in the 900-1000 cm(-1) region. These FTS intensities have been calibrated against 13 isolated transitions, taken as secondary intensity standards. These standard lines have been measured independently using TDL (tunable-diode-laser) spectrometers at University of Tennessee and Goddard Space Flight Center. A calculated spectrum, including mixing coefficients for v(4), v(7), v(10) and v(12), and calculated relative intensities, and the TDL-calibrated FTS line intensities were used as data in a non-linear regression analysis to determine the 296 K vibrational band intensities of S-0-7(v) = 321.69 +/- 0.36 cm(-1)/cm atm, S-0-10(v) = 1.16 +/- 0.47 cm(-1)/cm atm, and S-0-12(v) = 31.60 +/- 9.80 cm(-1)/cm atm. These vibrational band intensities combined with the theoretical spectral-line atlas make possible the generation of an ethylene spectrum at an arbitrary temperature. Such spectra prove useful to the planetary-atmosphere modeling-community. A web site is available where an individual can interact with the model and download a custom atlas. The URL is http://aurora.phys.utk.edu/(similar to)blass/ethyatl/

Self- and foreign-gas broadening of ethane lines determined from diode laser measurements at 12 μm

Abstract Self- and foreign-gas broadening of ethane lines have been measured in the v 9 band at 12 μm. A coefficient of 0.125 cm -1 -atm -1 was determined for self broadening. Foreign-gas broadening coefficients determined are (in cm -1 -atm -1 ) 0.090 for N 2 , 0.069 for He, 0.068 for Ar, 0.108 for H 2 , and 0.096 for CH 4 . Results are given for a sample temperature of 296 K.

High-resolution infrared spectrum and analysis of the ν9 band of ethane at 12.17 μm

Abstract The ν9 degenerate fundamental of C2H6 near 12.17 μm (822 cm−1) has been recorded at medium resolution (Δν = 0.125−0.500 cm−1) and high resolution (Δν = 0.04−0.05 cm−1) using both grating and interferometric spectrometers. The spectra have been assigned and analyzed by both standard and band contour techniques. The high-resolution spectrum showed a pronounced l-doubling in the K = 0 subband and the presence of a (2,2) type l-resonance. The assigned lines were corrected for these effects and analyzed using an energy expression through third order ( h 3 † ) with partial fourth-order ( h 4 † ) contributions. The standard deviation for the analysis of 549 of 569 assigned transitions was 6.7 × 10−3 cm−1. The ν9 band has been observed recently on Jupiter, Saturn, Titan, and Neptune by several workers. Spectra, frequency listings, and assignments have been included for use in future work on C2H6 in the atmospheres of the outer planets.

Adaptive noise filtering using an error-backpropagation neural network

A neural network of the feedforward-error backpropagation type proposed by D.E. Rumelhart et al. (1986) was applied to filter noise from spectral data commonly encountered in infrared absorption of molecular transitions. The purpose was to gain insight into the way a neural network can be trained to remove noise from a noise-corrupted signal with implications for signal processing in general. The neural network simulation was implemented in Fortran and run on a VAX 8800. Training of the neural network occurred on a set of spectral data with random transitions and line shape parameters. Preliminary results of the performance of the adopted neural network are reported and discussed along with observed limitations. Future improvements on noise filtering using a neural network are proposed. >

Hydrogen and nitrogen broadening of the lines of C2H2 at 14 μm

Abstract Collision broadening of 14 μm acetylene transitions was studied using a diode laser spectrometer. Fifteen P(J) and Q(J) transitions in the v5 fundamental band, broadened by hydrogen and nitrogen, were used in the study. The reduced Lorentz-widths per unit pressure obtained from the observed Voigt-widths were fitted to a model quadratic in J, i.e. γ0L = a + bJ + cJ2. The results for the nitrogen-broadened lines are 0.0853 cm-1-atm-1 for the collision broadening coefficient a at J = 0, b = -1.63 x 10-3, and c = 3.92 x 10-5 cm-1-atm-1. The results for the hydrogen broadened lines in cm-1-atm-1 are 0.0812, -1.16 x 10-3, and 2.93 x 10-5, respectively.

Carbon rod furnace infrared source

A carbon rod furnace infrared source has been built and has proven to be a reliable and trouble‐free source despite the high rod temperature of 2500 K. The furnace offers several advantages over previous furnaces. These include an increase in usable rod length to 6.35 cm, and extended rod life (up to 400 h), and a fully automated power supply for the furnace. Construction and operational details are discussed.