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Small concentrations of selected gases measured automatically. Proposed multiple-laser-diode spectrometer part of system for measuring automatically concentrations of selected gases at part-per-billion level. Array of laser/photodetector pairs measure infrared absorption spectrum of atmosphere along probing laser beams. Adaptable to terrestrial uses as monitoring pollution or control of industrial processes.
Molecular Beam Epitaxy (MBE) computer program developed to aid in design of single- and double-junction cascade cells made of silicon. Cascade cell has efficiency 1 or 2 percent higher than single cell, with twice the open-circuit voltage. Input parameters include doping density, diffusion lengths, thicknesses of regions, solar spectrum, absorption coefficients of silicon (data included for 101 wavelengths), and surface recombination velocities. Results include maximum power, short-circuit current, and open-circuit voltage. Program written in FORTRAN IV.
Radio absorptivity data for planetary atmospheres obtained from spacecraft radio occultation experiments and earth-based radio astronomical observations can be used to infer abundances of microwave absorbing atmospheric constituents in those atmospheres, as long as reliable information regarding the microwave absorbing properties of potential constituents is available. The key activity for this grant year has continued to be laboratory measurements of the microwave and millimeter-wave properties of the simulated atmospheres of the outer planets and their satellites. A Fabry-Perot spectrometer system capable of operation from 32 to 41 GHz was developed. Initially this spectrometer was used to complete laboratory measurements of the 7.5 to 9.3 mm absorption spectrum of ammonia. Laboratory measurements were begun at wavelengths near 3.2 mm, where a large number of observations of the emission from the outer planets were made. A description of this system is presented.
The high-dispersion absorption spectrum of Fe I is reported in the 1550-3215-A region. Included are wavelengths of about 3000 observed spectral features, improved spectral assignments, 248 new energy levels, and a value for the ionization potential of 63 737/cm obtained from extrapolation of Rydberg series. Improved wavelengths for several hundred V I and Ti I spectral lines determined on the same spectrograms as the iron data also are presented.
The results of a number of experiments are reported in which SiH4-Fe(CO)5-H2O ice mixtures were irradiated by 1 MeV protons at 15 K and then warmed to room temperature in approximately 50 K steps. The properties of a sample were monitored by observing its infrared absorption spectrum as a function of its processing history. In each experiment in which ice samples containing SiH4 were irradiated, a refractory amorphous silicate residue remained after warming to room temperature. In addition to the usual broad silicate features at 10 and 20 microns, features due to water or hydroxyl at 3 and 6.2 microns, and a feature due to CO at 5 microns, a surprisingly stable infrared feature at 4.6 microns, attributed to the SiH stretching fundamental, was observed.
Extended x ray absorption fine structure (EXAFS) is observed as a modulation on the high energy side of an x ray absorption edge. It occurs when the photo-ejected electron wave is scattered by neighboring atoms in a solid, and interference occurs between the outgoing and scattered waves. The result is that the absorption spectrum carries a signature that is characteristic of the identity and disposition of scattering atoms around the absorbing atom. Therefore, it may be shown that the Fourier transform of the normalized EXAFS can provide detailed information about the immediate environment of specific atoms in a solid and is ideally suited to the study of cosmic dusts. A study of cosmic dust was initiated using EXAFS and other techniques. The simplest type of cosmic material, namely iron meteorites, was investigated.
Many carbon stars in the IRS low resolution spectra (LRS) catalog were found which display emission spectra that compare favorable with the absorption spectrum of alpha-C:H. These stars have largely been classified as 4X in the LRS which has led to their interpretation by others in terms of displaying a mixture of the UIRF's 8.6 micron band and SiC at 11.5 microns. It was also found that many of these stars have a spectral upturn at 20+ microns which resembles the MgS band seen in carbon stars and planetary nebulae. It was concluded that this group of carbon stars will evolve into planetary nebulae like NGC 7027 and IC 418. In the presence of hard ultraviolet radiation the UIRF's will light up and be displayed as narrow emission bands on top of the broad alpha-C:H emission bands.
A flashlamp-pumped 2.1 micron Ho:Tm:Cr:YAG laser was studied for both normal mode and Q-switched operations under a wide variety of experimental conditions in order to optimize performance. Laser output energy, slope efficiency, threshold and pulselength were determined as a function of operating temperature, output mirror reflectivity, input electrical energy and Q-switch opening time. The measured normal-mode laser thresholds of a Ho(3+) (0.45 atomic percent):Tm(3+) (2.5 atomic percent):Cr(3+) (0.8 atomic percent):YAG crystal ranged form 26 to 50 J between 120 and 200 K with slope efficiencies up to 0.36 percent with a 60 percent reflective output mirror. Under Q-switched operation the slope efficiency was 90 percent of the normal-mode result. Development of solid state lasers with Ho(3+), Tm(3+) and/or Er(3+) doped crystals has been pursued by NASA for eye-dafe mid-infrared LIDAR (light detection and ranging) application. As a part of the project, the authors have been working on evaluating Ho(3+):Tm(3+):Cr(3+):YAG crystals for normal-mode and Q-switched 2.1 micron laser operations in order to determine an optimum Tm(3+) concentration under flashlamp pumping conditions. Lasing properties of the Ho(3+) in the mid-infrared region have been studied by many research groups since the early 1960's. However, the technology of those lasers is still premature for lidar application. In order to overcome the inefficiency related to narrow absorption bands of the Ho(3+), Tm(3+) and Er(3+), the erbium has been replaced by chromium. The improvement in flashlamp-pumped Ho(3+) laser efficiency has been demonstrated recently by several research groups by utilizing the broad absorption spectrum of Cr(3+) which covers the flashlamp's emission spectrum. Efficient energy transfer to the Tm(3+) and then the Ho(3+) occurs subsequently. It is known that high Tm(3+) concentration and low Ho(3+) concentration are preferred to achieve a quantum efficiency approaching two and to avoid large reabsorption losses. However, determination of the optimum Tm(3+) concentration required to ensure efficient energy transfer from Cr(3+) to Tm(3+) and from Tm(3+) to Ho(3+) has not been made in the Ho:Tm:CR:YAG crystal. The results obtained so far are given.
Re2(CO)6(dmpm)2 shows photophysical behavior in a rigid medium that differs dramatically from that observed in fluid solution. In a hydrocarbon glass at 77 K, metal-metal bond homolysis is suppressed and an intense phosphorescence is observed. The transient absorption spectrum, which shows only weak transitions to the red of the ground state 1(sigma-sigma asterisk) transition, permits assignment of the emitting state to a 3(sigma-sigma asterisk) transition. The crystal structure of Re2(CO)6(dmpm)2 is also reported. The ground-state electronic structure is discussed relative to the structural data.
Theoretical descriptions of the abundance and excitation of carbon monoxide in interstellar clouds require accurate data on the vacuum-ultraviolet absorption spectrum of the molecule. The 6.65 m spectrometer at the Photon Factory synchrotron light source was used to measure photoabsorption cross sections of CO features between 91.2 and 100.4 nm. These data were recorded at a resolving power of 170,000, more than 20 times greater than that used in previous work.
High-level ab initio calculations are reported for the low-lying states of the FeH(+) molecule. The ground state is determined to be X 5Delta with the low-lying 5Pi state lying about 600/cm higher. The excited states of FeH(+) are repulsive in the Franck-Condon region of the ground state. Thus it is likely that FeH(+) is photodissociated in stellar atmospheres. The strongest transition should be the (3) 5Delta - X 5Delta transition near 30,000/cm, which should give rise to a structureless feature in the absorption spectrum due to the repulsive nature of the upper state. All of the perpendicular transitions are computed to be very weak. The D0 value of FeH(+) is computed to be 50.2 kcal/mole, which should be the most accurate theoretical value to date.
Thirteen spectra of Mars, in the 7.5 to 12.8 micron wavelength were obtained on 7 Dec. 1990 from the Infrared Telescope Facility (IRTF). For these observations, a grating with an ultimate resolving power of 120 to 250 was used and wavelengths were calibrated for each grating setting by comparison with the absorption spectrum of polystyrene measured prior to each set of observations. By sampling the Nyquist limit at the shortest wavelengths, an effective resolving power of about 120 over the entire wavelength range was achieved. A total of four grating settings were required to cover the entire wavelength region. A typical observing sequence consisted of: (1) positioning the grating in one of the intervals; (2) calibrating the wavelength of positions; and (3) obtaining spectra for a number of spots on Mars. Several observations of the nearby stellar standard star, alpha Tauri, were also acquired throughout the night. Each Mars spectrum represents an average of 4 to 6 measurements of the individual Mars spots. As a result of this observing sequence, the viewing geometry for a given location or spot on Mars does not change, but the actual location of the spot on Mars's surface varies somewhat between the different grating settings. Other aspects of the study are presented.
The global distribution of ozone in the atmosphere of Mars was determined from Doppler-limited infrared heterodyne spectroscopy measurements at the NASA Infrared Telescope Facility (IRTF) facility during June 3-7, 1988. Mars spectra near two O3 lines arising from the v sub 3 band near 1031.45 cm (-1) were used. The lines were Doppler shifted out of the strong terrestrial ozone absorption spectrum and its effect was removed. Ozone measurements were obtained at eight beam positions over a range of latitudes and local solar zenith angles. The beam size of the planet was 1.4 arcsec. A Martian CO2 line appeared in the spectra and was inverted to retrieve local temperature profiles. Using these temperature profiles, the total ozone column abundance at each position was retrieved by fitting the measured line with synthetic spectra generated by a radiative transfer program. The only previous measurement of ozone at this season was made above the South polar cap by Mariner 7 and revealed an abundance of 10 micron-atm. However, the retrieved O3 column abundances from this investigation are less than 2.2 micron-atm at all positions sampled. These results are consistent with mid-spring abundances predicted by photochemical models of Liu and Donahue, and Shimazaki and Shimizu.
This study presents high-resolution measurements of the pure rotational absorption spectrum of CO in its ground state for the range J arcsec - 5-37. A least-squares fit to this data set, augmented by previous microwave measurements of the J arcsec = 0-4 rotational transitions in the literature, determined accurate values for the molecular constants. A table of calculated CO rotational frequencies is provided for the range J arcsec = 0-45.
The diffuse reflectance spectra of Hawaiian palagonite mixtures with an Fe-rich montmorillonite have prompted their present use as spectral analogs of the Martian surface. Like the Mars spectrum and unlike clays, the 2.2-micron reflectance spectrum absorption band is not present in the palagonite sample; neither is the 2.2-micron Al-OH clay lattice band seen in palagonite-montmorillonite mixtures, where the latter component remains below 15 wt pct. Fe-rich montmorillonite clay may therefore be present in Mars, in combination with palagonite, while remaining undetected in remotely sensed spectra.
We have investigated the impact of high resolution, temperature-dependent CO2 cross-section measurements, reported by Lewis and Carver (1983), on calculations of photodissociation rate coefficients in the Martian atmosphere. We find that the adoption of 50 A intervals for the purpose of computational efficiency results in errors in the calculated values for photodissociation of CO2, H2O, and O2 which are generally not above 10 percent, but as large as 20 percent in some instances. These are acceptably small errors, especially considering the uncertainties introduced by the large temperature dependence of the CO2 cross section. The inclusion of temperature-dependent CO2 cross sections is shown to lead to a decrease in the diurnally averaged rate of CO2 photodissociation as large as 33 percent at some altitudes, and increases of as much as 950 percent and 80 percent in the photodissociation rate coefficients of H2O and O2, respectively. The actual magnitude of the changes depends on the assumptions used to model the CO2 absorption spectrum at temperatures lower than the available measurements, and at wavelengths longward of 1970 A.
The project was to measure the optical constants of CO2 ice in the spectral regions of weak absorption. Many previous measurements had been made by others in the strong absorption bands, but the weak regions had been poorly measured or not measured at all. In these regions the emissivity of CO2 frost and CO2 clouds is quite sensitive to particle sizes and to the value of the absorption coefficient, so the new measurements will have applications to energy budget and remote sensing of the Martian surface and atmosphere. During the time period covered by this grant, a method was developed for growing clear crystals of CO2, 4 cm in length. An apparatus was constructed for accurate measurement of spectral transmission through these crystals, for wavelengths 0.18-3.8 micrometers, covering the near-ultraviolet, visible, and near-infrared regions, bounded by the strong ultraviolet absorption and the 4.3-micrometer band. Our new best estimate of the spectral absorption coefficient (expressed as imaginary refractive index) is shown. It is a composite result of many measurements on dozens of crystals. Now that we have a good dataset for the shortwave absorption spectrum, the focus of the research has moved to the thermal infrared, for which a Fourier Transform interferometer has been adapted for use with C02-ice crystals.
The current research project (VVNRS) involves VV Cephei, which is a binary system in which a 20 solar mass main sequence star orbits within the extended atmosphere of the 40 solar mass M-type supergiant primary. Although the period of the system is 20.3 years, the components are close enough for a stream of matter from the primary to form an accretion disk around the hot secondary. We had previously completed an ADP-sponsored study of twelve years of archival spectra of this system and were surprised to find intermittent changes in the line profiles of the chromosphere-like absorption spectrum. This we think may be related to the accretion process. The previously existing spectra were not closely spaced enough in time to clearly delineate the nature of these changes. Therefore, we proposed and were granted eleven half-shifts to obtain closely spaced IUE spectra during IUE's 14th year of operations (1991/1992).