Engineering topics

Smith, Peter L.

Publications and source records attributed to Smith, Peter L..

17 records

NASA NTRS2006

Accurate VUV Laboratory Measurements of Fe III Transitions for Astrophysical Applications

We report preliminary measurements of Fe III spectra in the 1150 to 2500 A wavelength interval. Spectra have been recorded with an iron-neon Penning discharge lamp (PDL) between 1600 and 2500 A at Imperial College (IC) using high resolution Fourier (FT) transform spectroscopy. These FT spectrometer measurements were extended beyond 1600 A to 1150 A using high-resolution grating spectroscopy at the National Institute of Standards and Technology (NIST). These recorded spectra represent the first radiometrically calibrated measurements of a doubly-ionized iron-group element spectrum combining the techniques of vacuum ultraviolet FT and grating spectroscopy. The spectral range of the new laboratory measurements corresponds to recent HST/STIS observations of sharp-lined B stars and of Eta Carinae. The new improved atomic data can be applied to abundance studies and diagnostics of astrophysical plasmas.

Blackwell-Whitehead, R. J.↗

NASA NTRS2006

New Measurements of Doubly Ionized Iron Group Spectra by High Resolution Fourier Transform and Grating Spectroscopy

We report new measurements of doubly ionized iron group element spectra, important in the analysis of B-type (hot) stars whose spectra they dominate. These measurements include Co III and Cr III taken with the Imperial College VUV Fourier transform (FT) spectrometer and measurements of Co III taken with the normal incidence vacuum spectrograph at NIST, below 135 nm. We report new Fe III grating spectra measurements to complement our FT spectra. Work towards transition wavelengths, energy levels and branching ratios (which, combined with lifetimes, produce oscillator strengths) for these ions is underway.

Smillie, D. G.↗

NASA NTRS2002

New Atomic Data for Doubly Ionized Iron Group Atoms by High Resolution UV Fourier Transform Spectroscopy

Currently available laboratory spectroscopic data of doubly ionized iron-group element were obtained about 50 years ago using spectrographs of modest dispersion, photographic plates, and eye estimates of intensities. The accuracy of the older wavelength data is about 10 mAngstroms at best, whereas wavelengths are now needed to an accuracy of 1 part in 10(exp 6) to 10(exp 7) (0.2 to 2 mAngstroms at 2000 Angstroms). The Fourier transform (FT) spectroscopy group at Imperial College, London, and collaborators at the Harvard College Observatory have used a unique VUV FT spectrometer in a program focussed on improving knowledge of spectra of many neutral and singly and doubly ionized, astrophysically important, iron group elements. Spectra of Fe II and Fe III have been recorded at UV and VUV wavelengths with signal-to-noise ratios of several hundred for the stronger lines. Wavelengths and energy levels for Fe III are an order of magnitude more accurate than previous work; analysis is close to completion. f-values for Fe II have been published.

Smith, Peter L.↗

NASA NTRS2002

UV photoabsorption cross sections of CO, N2, and SO2 for studies of the ISM and planetary atmospheres

We report high-resolution laboratory measurements of photoabsorption cross sections of CO, N2, and SO2 in the wavelength range 80 to 320 nm. The motivation is to provide the quantitative data that are needed to analyze observations of absorption by, and to model photochemical processes in, the interstellar medium and a number of planetary atmospheres. Because of the high resolution of the spectrometers used, we can minimize distortion of the spectrum that occurs when instrument widths are greater than the widths of spectral features being measured. In many cases, we can determine oscillator strengths of individual rotational lines - a unique feature of our work.

Smith, Peter L.↗

NASA NTRS2002

[Activities of Harvard College Observatory]

With support from this grant, we have: 1) Developed techniques for improving wavelengths and f-values for singly and doubly charged ions of the iron group and have improved the accuracy of Fe III wavelengths by an order of magnitude. New Fe II f-values have also resulted from this work. 2) Measured line oscillator strengths and photoabsorption cross sections for UV molecular spectral feature that have been, or could be, used for searches for and detection of molecules in diffuse and translucent interstellar clouds and for determination of molecular column densities there. In addition, we have determined other molecular parameters -- line assignments, wavelengths, and line widths -- that are essential for theoretical descriptions of the abundance, fractionation, and excitation of interstellar molecules and for comparison of predictions with observations. 3) Measured A-values for spin-changing and other weak lines in low-Z ions. When A-values are available, these spectral features are useful for astrophysical plasma density and temperature diagnostics. Such lines are also used in interstellar abundance determinations in cases where the stronger allowed lines are saturated in astronomical spectra. 4) Taken an activist approach to ensuring that, (i), astronomers have ready access to our data, and, (ii), avenues of communication between data users and producers are strengthened.

Dalgarno, A.↗

NASA NTRS1997

Spectroscopic Data for an Astronomy Data Base

When we began this work, very few of the atomic and molecular data used by astronomers in the analysis of astronomical spectra were available in on-line searchable databases. Our principal goal was to: make the most useful of the atomic data files of R.L. Kurucuz (1995a,b) available on the WWW; and also to make the atomic data of R.L. Kelly for ultraviolet lines (i.e., essentially the same as the data in Kelly (1979) and Kelly (1987)) similarly available. In addition, we proposed to improve access to parameters for simple molecules of interest to astronomers.

Parkinson, W. H.↗

NASA NTRS1995

Spectroscopic data for an astronomy database

Very few of the atomic and molecular data used in analyses of astronomical spectra are currently available in World Wide Web (WWW) databases that are searchable with hypertext browsers. We have begun to rectify this situation by making extensive atomic data files available with simple search procedures. We have also established links to other on-line atomic and molecular databases. All can be accessed from our database homepage with URL: http:// cfa-www.harvard.edu/ amp/ data/ amdata.html.

Parkinson, W. H.↗

NASA NTRS1994

High-resolution oscillator strength measurements for the A(v') - X(0) bands of carbon monoxide with 11 less than or equal to v' less than or equal to 14

Band oscillator strengths (f-values) for four bands of the Fourth Positive system (A (1)Pi - Chi(sup 1) Sigma(+)) of CO have been determined from high-resolution (lambda/Delta lambda approximately equal to 150,000) absorption spectra. The bands, (14, 0) through (11, 0), are at wavelengths between 121 and 127 nm. Our f-values for the (11, 0) and (12, 0) bands support those of Chan, Cooper, & Brion (1993), which have been shown to be consistent with observations of CO in the clouds in the line of sight to zeta Oph. Our f-values for the (13, 0) and (14, 0) bands are the first directly measured values for these bands.

Smith, Peter L.↗

NASA NTRS1993

Transition probabilities for the 3s2 3p(2P0)-3s3p2(4P) intersystem lines of Si II

Intensity ratios of lines of the spin-changing 'intersystem' multiplet of S II (4P yields 2P0) at 234 nm have been used to determine electron densities and temperatures in a variety of astrophysical environments. However, the accuracy of these diagnostic calculations have been limited by uncertainties associated with the available atomic data. We report the first laboratory measurement, using an ion-trapping technique, of the radiative lifetimes of the three metastable levels of the 3s3p2 4P term of Si II. Our results are 104 +/- 16, 406 +/- 33, and 811 +/- 77 micro-s for lifetimes of the J = 1/2, 5/2, and 3/2 levels, respectively. A-values were derived from our lifetimes by use of measured branching fractions. Our A-values, which differ from calculated values by 30 percent or more, should give better agreement between modeled and observed Si II line ratios.

Calamai, Anthony G.↗

NASA NTRS1993

Measurement of the transition probability of the C III 190.9 nanometer intersystem line

A radio-frequency ion trap has been used to store C(2+) ions created by electron bombardment of CO. The transition probability for the 2s2p 3Po1-2s2 1S0 intersystem line of C m has been measured by recording the radiative decay at 190.9 nm. The measured A-value is 121 +/- 7/s and agrees, within mutual uncertainty limits, with that of Laughlin et al. (1978), but is 20 percent larger than that of Nussbaumer and Storey (1978). The effective collision mixing rate coefficient among the fine structure levels of 3Po and the combined quenching and charge transfer rate coefficients out of the 3Po1 level with the CO source gas have also been measured.

Kwong, Victor H. S.↗

NASA NTRS1992

Absorption band oscillator strengths of N2 transitions between 95.8 and 99.4 nm

Molecular nitrogen plays a central role in the energetics of the earth's upper atmosphere and is the major constituent of the atmospheres of the planetary satellites Titan and Triton. This paper reports a new set of absorption oscillator strengths measured at higher resolution for seven bands in the 95.8-99.4 nm region. The results are compared with earlier, lower resolution absorption measurements, electron scattering measurements, and calculations based on a deperturbation analysis of the excited states.

Stark, G.↗

NASA NTRS1992

Atomic and molecular data for space astronomy - Needs, analysis, and availability; 21st IAU General Assembly, Buenos Aires, Argentina, July 23-Aug. 1, 1991, Selected Papers

The present volume on atomic and molecular spectroscopic data for space astrophysics discusses scientific problems and laboratory data needs associated with the Hubble Space Telescope, atomic data needed for far ultraviolet astronomy with HUT and FUSE and for analysis of EUV and X-ray spectra, and data for observations of interstellar medium with the Hubble Space Telescope. Attention is also given to atomic and molecular data for analysis of IR spectra from ISO and SIRTF, atomic data from the opacity project, sources of atomic spectroscopic data for astrophysics, and summary of current molecular data bases.

Smith, Peter L.↗

NASA NTRS1991

High-resolution, VUV (147-201 nm) photoabsorption cross sections for C2H2 at 195 and 295 K

Results of measurements of photoabsorption cross sections of acetylene at 195 and 295 K in the wavelength range of 147-201 nm are reported. Short-wavelength data are obtained at 0.002 nm intervals, but no structure was observed on that scale. Emission and absorption lines from contaminant species in xenon and hydrogen discharges are used to determine the correct wavelength scale for the data. The uncertainty in the relative wavelengths is estimated to be about 0.004 nm, whereas the absolute wavelength values are accurate to + or - 0.043 nm. No significant photodestruction of C2H2 was found during the measurements. Cross-section values determined at the beginning portions of the measurements are indistinguishable from the values determined at the ends, thus demonstrating that there was no loss of absorbers.

Smith, Peter L.↗

NASA NTRS1991

High-resolution absorption cross sections of carbon monoxide bands at 295 K between 91.7 and 100.4 nanometers

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.

Stark, G.↗

NASA NTRS1988

Absolute, Extreme-Ultraviolet, Solar Spectral Irradiance Monitor (AESSIM)

AESSIM, the Absolute, Extreme-Ultraviolet, Solar Spectral Irradiance Monitor, is designed to measure the absolute solar spectral irradiance at extreme-ultraviolet (EUV) wavelengths. The data are required for studies of the processes that occur in the earth's upper atmosphere and for predictions of atmospheric drag on space vehicles. AESSIM is comprised of sun-pointed spectrometers and newly-developed, secondary standards of spectral irradiance for the EUV. Use of the in-orbit standard sources will eliminate the uncertainties caused by changes in spectrometer efficiency that have plagued all previous measurements of the solar spectral EUV flux.

Huber, Martin C. E.↗

NASA NTRS1987

Absolute oscillator strengths for 108 lines of Si I between 163 and 410 nanometers

Measurements of neutral silicon oscillator strengths (f-values) obtained by absorption and emission techniques have been combined using the numerical procedure of Cardon et al. (1979) to produce 108 f-values for the Si I lines between 163 and 410 nm. Beam-foil-lifetime measurements were employed to determine the absolute scale. The present measurements have uncertainties of about 0.07 dex (+ or - 16 percent) at the 1-sigma level of confidence. Good agreement is obtained between the results and previous data. The data also provide upper limits for the f-values of 22 other lines and information on the lifetimes for 36 levels in Si I.

Smith, Peter L.↗

NASA NTRS1987

Oscillator strengths for visible and ultraviolet observations of interstellar molecules

Optical observations of molecules in diffuse interstellar clouds are discussed. The role of oscillator strengths in determining cloud properties such as densities, temperatures, and particle and photon fluxes is examined. The use of an emission technique, molecular and atomic lifetime measurements, and absorption measurements to estimate oscillator strengths is described. It is noted that the lifetime and absorption measurements are more advantageous than the emission technique to estimate oscillator strengths.

Smith, Peter L.↗