A High-Resolution Infrared Spectrum (2200-8800 cm-1) of a

Thorium-Argon-Neon Hollow-Cathode Lamp

A Th-Ar-Ne hollow cathode source was built into the Phoenix instrument-to-telescope interface unit for use in focusing and wavelength calibration. The hollow cathode is a commercial tube from Photron run at 10 milliamps. At the same time this hollow cathode was ordered a similar hollow cathode was purchased by the National Solar Observatory (NSO) for use at the McMath-Piece telescope. Mike Dulick (NSO), Dave Jaksha (NSO) and Peter Bernath (University of Waterloo) kindly observed the spectrum of this tube with the 1-m laboratory Fourier Transform Spectrometer. For this observation the tube was operated at 14 milliamps. The spectrum should be very similar to that of the Phoenix hollow cathode. This FTS spectrum (serial number 961113R0.002) provides a map of nearly the entire spectrum available with Phoenix, from 1.13 to almost 5 microns.

Plots (at the bottom of this page) were made by Lloyd Wallace, in collaboration with Ken Hinkle and Dick Joyce (all three KPNO). Brault's DECOMP, MONGO and various FORTRAN programs were used. The first plot is an introduction to the main set; it shows the complete spectrum at full resolution but compressed to a single page. The purpose is to show the prominent thermal continuum, the intervening absorption mostly by CO2 around 2350 cm-1 and H2 around 3755 cm-1, the fringing on the thermal continuum which in this plot appears just to broaden the continuum, and the general increase in emission line strength and density from low frequency out to about 8000 cm-1.

For the main body of the plots, we have apodized the spectrum and then determined the continuum level with its fringing and subtracted it off. The resolution of the apodized spectrum is 0.044 cm-1 (or λ / Δλ ranging from 50000 at 2200 cm-1 to 200000 at 8800 cm-1). The frequencies are from our measurements of this spectrum and the identifications are from the compilation of Outred (1978, J. Phys. Chem. Ref. Data, 7, 1) with updates for Ar I by Palmeri and Biemont (1995, Physica Scripta, 51, 76) and Ne I by Chang and Schoenfeld (1994, Physica Scripta, 49, 26). The sparse work an atomic spectra below ~ 2400 cm-1 is the reason for the many lines labeled "Unid".

We have labeled the lines with emitter if known, the frequency, and if the line goes off scale, the line intensity on the scale of the plots in square brackets. Thus, the strongest line on the plots is "Ar I 7287.394 [44.1]". The intensity scale is "as observed" (and thus includes the transmission of the various optical elements in the FTS and the lamp window) divided by 3.e7. All of the plots are on the same scale so plots in adjacent bands are directly comparable.

Lower resolution plots of a similar lamp have been made by Dick Joyce. The high resolution atlas by Palmer and Engleman (1983, Los Alamos National Laboratory # 9615) of a Thorium-Neon lamp covers the region from 7400 to around 36000 cm-1. The spectrum presented here consists in large part of argon and neon lines. The thorium lines present are weak. A much more fully developed infrared thorium spectrum can be seen in a Th-Ar hollow cathode FTS spectrum taken by Engleman at a cathode destroying current of 320 milliamps (FTS # 820519R0.001).

L.W. & K.H., December 1996

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Updated 09Nov1998