Our UV FT spectrometer was first applied to the spectrum of iron, the most
abundant of the astrophysically important transition elements. The spectra
of Fe I and Fe II were recorded here from the visible to 200 nm and (later)
into the VUV region, using as source an iron-neon hollow cathode lamp. These
spectra were combined with infrared spectra from the
National Solar Observatory to make a revised
multiplet table for Fe I . The analysis of the Fe II data is nearing
completion by S.Johansson at Lund University. The spectra were also used
to generate recommended iron wavelength standards in the visible region
("Wavelength calibration of Fourier transform emission spectra with applications
to Fe I", R.C.M.Learner and A.P.Thorne, J.Opt.Soc.Am., B5, 2045--2059, 1988).
This paper was followed by three others, giving
precision Fe I and Fe II wavelengths in the ultraviolet and
precision Fe I and Fe II wavelengths in the infrared,
and, finally, precision vuv wavelengths of Fe II.
I have recorded the spectra of platinum at high resolution from 300 nm to 140 nm,
using both a high current demountable hollow cathode lamp and a low current
quasi-commercial lamp of the type used to calibrate space-borne instruments.
Very many of the lines of both Pt I and Pt II show hyperfine structure and
isotope shift. These spectra have not yet been used for a full spectral
analysis, but the structure of particular lines has been used to reveal
isotopic anomalies in the spectrum of the chemically peculiar star Chi Lupi
obtained with the Hubble Space Telescope.
I also recorded the spectrum of nickel in the UV and VUV. The
analysis of the Ni I spectrum was carried out by
U.Litzen, but the Ni II spectrum, containing most of the
VUV lines, still awaits analysis.
At present I am working on the analysis of the spectra of vanadium, both V I and
V II, using data recorded in our laboratory by J.Semeniuk supplemented by
infrared spectra from the National Solar Observatory.
Accurate laboratory wavelength measurements have found a rather different
application in cosmology. It has been suggested that certain physical
constants may in fact have varied with time, and a test of this hypothesis
is to compare laboratory measurements of certain resonance lines with
telescope observations of the same lines formed in very distant objects with
large red shifts. We have collaborated in this with J.Webb and his colleagues
at the Australian National University by measuring absolute wavelengths with
an uncertainty of 0.002 cm-1, or about 0.008 pm,
for resonance lines of Mg I and II and
Cr, Zn and Ni.
Evidence for the variation at
present appears to be statistically significant but not conclusive.
In addition to wavelengths, energy levels and hyperfine structure, oscillator
strengths and transition probabilities are urgently needed by the astrophysical
community. Accurate intensity measurements, using calibrated radiometric
standards, can be used to find branching fractions for sets of lines with a
common upper level, and these, when combined with measurements of calculations
of the lifetime of the upper level, yield absolute transition probabilities.
We have recently applied this method to determine
transition probabilities for about 700 lines in the spectrum of Ti II.
In the field of molecular spectroscopy, I have been involved in the
measurements of absorption cross-sections in the two
UV band systems of SO2 mentioned
above (this publication refers to the lower wavelength system, 220-198 nm).
These measurements have since been extended to low temperatures, relevant to
the atmospheres of Venus
and Io, and are being continued by J.Rufus, formerly a postgraduate student
in the group and now a post-doctoral R.A. at CfA. We developed a new
technique for establishing a reliable baseline by using the second output
from the FT interferometer to monitor the background continuum source.
The work on the very weak, forbidden Herzberg bands of
O2 in the region 250-230 nm was carried out
with a 3-metre multi-pass absorption cell in front of the FT spectrometer to
achieve adequate absorption. Long integration times were required to
obtain a satisfactory signal-to-noise ratio. There are three overlapping
band systems, all of which have now been analysed at CfA. For the Herzberg I
bands, line wavelengths and
line intensities were published separately.
The publications for the weaker Herzberg II
and Herzberg III bands report both wavelengths
and intensities. The
band parameters are essential for reliable calculation of the regions of
weak continuous absorption adjoining the discrete bands; these continua
control the penetration of solar radiation into the atmosphere in this
spectral region.
The three months during which our VUV FTS was at Photon Factory yielded
extensive data on the many overlapping bands of NO
from 195 to 160 nm and on the Schumann-Runge bands of
O2 from 195 nm to 175 nm (where the
continuous absorption begins). The high resolution of FTS is necessary to avoid errors
in intensity arising from saturation of the absorption, which cannot be
detected when the instrumental width exceeds the true line width, as well as
to obtain accurate line positions. Analysis of all this data is proceeding
slowly but steadily in Japan and at CfA; to date the results for the
NO b(9,0) band, the
NO d(1,0) band, and
the NO e(1,0) band
have been published, and a fourth paper is ready for publication.
I have retained a strong interest in the instrumental aspects of UV FTS
since our own spectrometers became functional. For several years I was a
member of a group funded by ESTEC to investigate the feasibility of an
imaging VUV FT spectrometer for detailed solar
studies from space (part
of the SIMURIS project). I collaborated with R.C.M.Learner and J.W.Brault
in two papers on different aspects of FTS, one on phase
correction and the other on
ghosts and artefacts. I have been closely involved with the various
upgrades of our instruments that have enabled them to deliver their unique
spectra and with the assessment of their
performance and potential .
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