Analysis of the (5d2+5d6s)–5d6p Transition Arrays of Os VII and Ir VIII, and the 6s 2S–6p 2P Transitions of Ir IX

The spectra of osmium and iridium were photographed in the 300 Å to 1600 Å region on a 3 m normal incidence spectrograph using a triggered spark source. The (5d2 + 5d6s)–5d6p transition arrays of Os VII and Ir VIII were analyzed. All levels of these three configurations in both spectra have been established. There are 77 lines in Os VII and 71 lines in Ir VIII classified. The parametric least squares fitting calculations are used to interpret both spectra. The 6s 2S1/2–6p 2P1/2,3/2 transitions in Ir IX have also been identified.


Introduction and Experiment
The ground configuration of the seventh spectrum of osmium (Os VII) and the eighth spectrum of iridium (Ir VIII) is 5d 2 and the three lowest excited configurations are the 5d 6s, 6s 2 and 5d 6p. They belong to the Yb I isoelectronic sequence, which has been studied through Re VI [1][2][3][4][5]. The extension of the Yb I sequence is a part of our ongoing project of studying poorly known 5dsubshell ionic spectra.
The spectra of osmium and iridium in the 300 Å to 1600 Å wavelength region were excited in a triggered spark discharge and photographed on the 3 m normal incidence spectrograph at St. Francis Xavier University. It is equipped with a holographic grating having a line density of 2400/mm and a plate factor of 1.385 Å/mm in the first order. Osmium or iridium powder was packed into a cavity on the tip of a pure aluminium electrode, which served as the cathode. The anode was a pure aluminium electrode. In later exposures both electrodes contained the sample material. The edges of the electrodes were considerably tapered to avoid low melting point aluminium flowing inwards and blocking the high melting point sample material from getting into the discharge. The electrode gap was set between 2.5 mm to 3 mm. The charging potential was provided by a low inductance 14.3 F capacitor bank. The discharge conditions were varied primarily by changing the number of turns in a series inductance coil. The charging potential was kept at 4 kV to 5 kV. The lines arising from different ionization stages could thus be reliably discriminated. The exposures above 500 Å were taken with Kodak SWR plates 1 whereas those below 500 Å were taken on Kodak 101-05 plates. The plates were measured on semiautomatic comparators either at the Zeemen Laboratory in Amsterdam or at the University of New Brunswick (Canada). The internal standards of C, O, Al, and Mg [6] and known osmium and iridium lines [7] were used for plate calibration. The standard uncertainty (i.e., estimated standard deviation) of the wavelength measurements due to the least squares fitted calibration curve is Ϯ0.01 Å. In the region above 1200 Å there were insufficient wavelength standards and the standard uncertainty increased to Ϯ0.02 Å. These uncertainties are due to the standard deviation of the fit of the calibration lines to a polynomial function of position. We estimate the standard uncertainty from systematic effects to be Ϯ0.005 Å. It is kept low by the presence of well-measured internal impurity lines in the same exposure as the desired spectrum. The combined standard uncertainties (i.e., total one standard deviation estimate) in these two regions are thus Ϯ0.011 Å and Ϯ0.021 Å, respectively.
The strongest lines in the Os VII and Ir VIII spectra appeared on the spectrograms as full length images, stretching from anode to cathode. The weaker lines were polar, stretching from the cathode to the middle of the spark gap. We used the length of the lines for roughly estimating their relative intensities.

Results and Discussion
From fitted calculations of the isoelectronic spectra from Yb I to Re VI for the three configurations 5d 2 , 5d 6s , and 5d6p [1][2][3][4][5], we could accurately predict the scaling factors (the ratio of the least squares fitted (LSF) parameter values to the Hartree-Fock (HF) values of the energy parameters [8].) Nonrelativistic calculations of the transtition arrays with the Cowan code [8] with these 1 Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. parameters showed that for Os VII and Ir VIII the 5d 2 -5d 6p array lies in the region 380 Å to 580 Å and 330 Å to 510 Å respectively, and the 5d 6s-5d 6p arrays lay in the regions 850 Å to 1330 Å and 800 Å to 1200 Å respectively. These arrays could be easily identified on the plates because of the good excitation separation. It was also noticed that the 5d 2 -5d 6p array was much stronger than the 5d 6s -5d 6p array for both spectra.
With these predictions as a guide we classified 77 lines of Os VII given in Table 1, with only one doubly classified. All lines classified in the Os VII and Ir VIII analyses, except the masked lines or very weak lines, exhibit their respective ionization-stage characteristics. The 71 lines classified in Ir VIII are given in Table 2. The mean deviation of absolute differences of measured wave numbers in cm -1 of Os VII lines from those predicted with the final level values is 1.5, and for Ir VIII lines it is 1.6.
All 13 levels of the 5d 2 and 5d 6s even parity configurations and all twelve levels of the 5d 6p odd parity configuration for both Os VII and Ir VIII have been established and are listed in Tables 3, 4 and 5, 6 respectively, along with the two highest eigenvector percentages in LS coupling. The standard deviation between calculated and observed levels for the even configurations of Os VII and Ir VIII are 114 cm -1 and 124 cm -1 , and for the odd configuration 354 cm -1 and 336 cm -1 , respectively. One can unambiguously assign LS designations to all the levels of even parity for both ions. However, for the odd parity levels none is predominantly 1 D 2 or 3 D 3 . The same was also observed in Re VI [5]. In Tables 4 and 6 two levels are arbitrarily assigned these designations for convenience of classifying the lines (see footnotes b and c in Tables 1 and 2). In the LSF calculations configuration interaction was introduced between the 5d 2 , 5d6s and 6s 2 configurations which improved the fit to some extent. The parameters used in Os VII and Ir VIII are given in Table 7 and Table 8 respectively. We should also point out that the G 1 (dp ) and G 3 (dp ) parameters in the 5d 6p configuration in both spectra were fixed at their HF ratios in the iteration process. When they were not linked the fits improved, giving standard deviations of 222 cm -1 and 237 cm -1 , respectively, but the G 3 (dp ) scaling factors were 0.516 and 0.564, respectively, with almost 12 % uncertainty on their values.
The 5d 2 -5d 5f transition array in Os VII and Ir VIII has not been investigated in this research. These transitions lay between 330 Å to 397 Å in Re VI [5] and were much weaker than the (5d 2 +5d 6s )-5d 6p transitions. The corresponding transitions in Os VII and Ir VIII are below 300 Å. We intend to undertake this work shortly. to the level value difference. The intensity is replaced by an estimated value based on the transition probability). 6 unresolved from stronger line. 7 partly blended. 8 asymmetric line, shaded to shorter wavelength side. 9 asymmetric line, shaded to longer wavelength side. c ⌬= (observed) minus (derived from levels).

The 6s 2 S1/2 Level of Ir IX
Kaufman and Sugar [7] identified the 6s 2 S-6p 2 P 3/2,1/2 transitions in the Yb II sequence from Yb II to Os VIII but did not identify them in Ir IX or in the higher members. Our plates show these lines as well as the six Ir IX lines of 5d -6p and 5d -5f identified in Ref. 7. The two 6s -6p lines of Ir IX were predicted to be at 773.5 Å and 1038 Å by extrapolation of the isoelectronic sequence (see Fig. 1). On examining the plate (see Fig. 2) the lines with correct Ir IX characteristic were identified. They are given in Table 9.