1934 J. Opt. Soc. Am. B/Vol. 2, No. 12/December 1985
Accurate energy levels for neutral platinum
Rolf Engleman, Jr.
Los Alamos National Laboratory, Chemistry Division, Los Alamos, New Mexico 87545
Received May 13, 1985; accepted July 24, 1985
High-resolution spectra of a platinum hollow-cathode lamp were obtained from 215 to 3100 nm with a Fourier-transform spectrometer. More than 320 Pt I lines were measured and used to derive accurate energy levels for 30even and 52 odd levels. Ritz wavelength standards for Pt in the vacuum ultraviolet were calculated.
INTRODUCTION
General energy-level analyses of many heavy-element atom-ic spectra have not been reported for some time. However,significant advances in experimental techniques have oc-curred that allow greater resolution and improved precisionin wavelength measurements. Earlier studies and the re-sulting energy-level values for Pt I and II have been summa-rized.' Platinum deserves special study because of the gen-eral need for more accurate energy levels and also because ofits important role in a future scientific Earth satellite. Aplatinum/neon hollow-cathode light source was used in thevacuum ultraviolet (VUV) spectrometer aboard the NASAInternational Explorer satellite (IUE), launched in 1977, as awavelength and intensity calibration standard in the farultraviolet down to 115 nm. Platinum was chosen becauseof the many sharp lines in the 310- to 115-nm region and thegood stability of the platinum hollow cathode. 2 A new satel-lite, the Hubble Space Telescope (ST),3 is due to belaunched in 1986 with a spectrometer capable of much great-er spectral resolution than the one used on IUE; it too willuse a platinum standards source. Thus it is important thataccurate line wavelengths be available for this light source.The direct wavelength measurements of the present studyoverlap only the uppermost wavelength range of the STspectrometer; by applying the Ritz principle to the platinumenergy levels derived in the present study, accurate stan-dards in the wavelength region below 220 nm can be ob-tained.
EXPERIMENT
A demountable hollow cathode developed for high-resolu-tion spectroscopic studies 4 was used with flowing argon gas.The platinum was in the form of a cylinder 16 mm long andhaving a 6-mm outside diameter with a 1-mm-thick wall.Current through the lamp was about 0.2 A with an argonpressure of about 2.5 Torr.
All spectra were taken with the McMath 1-m Fourier-transform spectrometer (FTS) at Kitt Peak that is operatedby the National Solar Observatory. This instrument5 wasused with a quartz beam splitter and appropriate detectorsand filters. Spectra were taken in three overlapping re-gions. A broadband scan covered 215 to 1100 nm with aresolution of only 96 mK (1 mK = 0.001 cm-'); narrow-band
scans covered 280 to 525 nm and 660 to 3100 nm with resolu-tions of about 25 mK.
DATA ANALYSIS
Initially, a wave-number and intensity-line list was compiledby conventional procedures, although, as detailed below, theplatinum list was substantially modified by the use of cen-troid values. Small wave-number corrections were appliedby using known argon standards. 6 Single sharp-line posi-tions are believed accurate to 1 mK. Argon lines wereeliminated from this list by using extensive tables of Ar I andII.7 The lines of Pt II were eliminated by using Shenstone'sanalysis8 and a new preliminary Pt II analysis (to be pub-lished separately). Initial selection of strong Pt I lines wasmade with the analysis of Livingood9; thus many relativelyaccurate energy levels could be determined. These levelswere then used to compute, within the limitation of the usualAJ = 0, +1 (but AJ = 0 not allowed if J = 0) selection rule,other possible transitions. A search of the line list for thesetransitions was made, and those that were located were fit-ted into the level structure. Several iterations of this cycleresulted in accurate values for most levels and unambiguousclassifications for many lines.
Most of the Pt I lines were not sharp, and many showedconsiderable resolved structure. The narrowest Pt I lines(full width at half-maximum of about 90 mK in the 280- to525-nm spectrum) correspond to a relatively high Dopplertemperature of 3800 K. An example of a classified narrowPt I line is shown in Fig. 1. A linewidth of around 50 mKwould be more in line with other heavy-atom spectra inhollow cathodes. This excessive width is due not to insuffi-cient spectroscopic resolution or high temperatures but to un-resolved isotope or hyperfine structure. An example of a widePt I line with resolved structure is displayed in Fig. 2 alongwith a tentative assignment of isotopic components. Theseassignments are compatible with early Fabry-Perot stud-iesl0 "' of this transition. A sizable isotope shift separatesthe even platinum isotopes, whereas the odd 95 Pt shows allfour components predicted for a I = 1/2, J = 4 to J = 4transition. It is interesting to note that the two strong 195 Ptcomponents (Fig. 2, peaks 195b and 195c), which appear tobe relatively unblended, have a measured FWHM of about70 mK, corresponding to a Doppler temperature of 2100 K.
At the present stage of analysis, these structured lines,,
Vol. 2, No. 12/December 1985/J. Opt. Soc. Am. B 1935
300 I
200
E--
100
0:30 157
WA!: NIM131 'R
Fig.1. The 30 156.852-cm' line of Pt Iwith a measured half-widthof 95 mK and an assignment of 03-301564. Each interval on thehorizontal scale is 0.2 cm-.
1000
750
Cl)zEZ
500
250
029: 33: 3
WAV NM131R
Fig. 2. Structure of the 29 333.181-cm- 1 Pt I line with tentativepeak assignments. The transition is 8234-301564. The four 195peaks refer to the two major (195b, 195c) and two minor (195a, 195d)hyperfine components predicted for 95 Pt. The measured half-widths of peaks 195b and 195c were about 70 mK. Each interval onthe horizontal scale is 0.2 cm-1 .
caused by isotope shifts and hyperfine splitting of 95 Pt,were not considered in detail. Rather, the Pt I line positionwas approximated by the centroid of the pattern (see Ap-pendix A). Because of the good intensity linearity of theFTS spectra, accurate centroids could be readily derived.Table 1 gives centroid values of 337 Pt I lines found in thisstudy. Intensities were derived from the area of the Pt Ipatterns. The intensities were not corrected for the re-sponse of the FTS system. This becomes particularly no-
a Measured centroid of transition.b Assignments given as integer wave number parts of even and odd levels with J values as superscripts.Intensity as area of centroid scaled to maximum of 1000. Zero intensity denotes a value less than 0.5 on this scale.
d Code is as follows: N, narrow line; W, wide line; P, resolved pattern; B, blended; *, line not used in least-squares-level analysis.Line also given in Ref. 13.
(Line also given in Ref. 2.
ticeable for lines greater than 40 000 cm-', which are muchstronger than indicated in Table 1.
ENERGY LEVELS
Final values of the energy levels were determined by leastsquares fitting using a version of an energy-level least-squares program described in an earlier study.12 Wave-number values and assignments given in Table 1 were used,except for lines marked with an *. Because those lines wereblended or otherwise compromised, they would notaccurately fit into the least-squares analysis. Equalweights were given to all measurements. This analysisresulted in the energy-level values in Tables 2 and 3. Theoverall standard deviation of the least-squares fit was 8mK. This appears reasonable, considering the approxima-tion introduced by using the centroids instead of the trueline positions. A good indication of the accuracy of individ-ual levels is given by the last columns of Tables 2 and 3.Levels connected by many transitions, such as the lower-lying ones, are probably accurate to a few millikeysers,whereas levels connected by only a few transitions may beless accurate. Some of the higher-lying levels are connectedby only a few transitions because of the lack of FTS spectrabelow 220 nm.
DISCUSSION
The new Pt I measurements are so much more accurate thanthose from the early studies that direct comparisons are notwarranted. A meaningful comparison can be made withplatinum standards generated for the IUE program.2 Acomparison of the present wave numbers (Table 1) withthose of Ref. 2 (Table 3 after conversion from air wave-lengths) yields an average difference of 32 + 71 mK for 45lines. There was considerable correlation of the differencewith transition wave number, which suggests possible wave-length-calibration problems in that photographic-gratingstudy. Comparison is also possible with another study thatpartially overlaps the present measurements. Six strongPt I lines measured in a photographic infared-grating study 3
are also found in Table 1. The agreement is 23 ± 10 mK,probably within the estimated accuracy of that study.
The Pt I energy levels derived in the present study aremuch more accurate than those available previously.' Nosystematic search for new levels was made and none were
found. Only one level given in the literature summary,' theodd level at 48 351.9 cm-', was not determined by the FTSspectra.
For the convenience of comparison with VUV measure-ments, Table 4 lists all allowed Pt I lines below 225 nmcomputed from the new-level lists. This list thus predictsaccurate VUV transition to below 180 nm, if those lines aresufficiently intense. A comparison of Livingood's VUV Pt Iline list9 with Table 4 yielded 43 probable matches, although4 were blended in that early list. The average differencebetween Table 4 and Livingood's values for the 39 unblend-
a Configuration assignments from Ref. 1.b The number of transitions connected to this level in the least-squares energy-level fit.This level is given in Ref. 1 but was not determined in the present study.
Calculated using levels from Tables 2 and 3 and the J selection rules: AJ = 0, +1, but AJ = 0 is not allowed if J = 0.b Assignments given as integer wave-number parts of even and odd levels with J values as superscripts.I These calculated lines were matched with platinum lines measured in Ref. 9.d These transitions involve the odd 48 3514 level, for which an accurate value was not obtained in this study.
10
:30
20
10
0
100 200 300 400 500
RELATI\ WV.\\E NMBER (K)
Fig. 3. Isotope structure of a hypothetical platinum transition.The isotope shift between 1
94 Pt and '96 Pt was set at 100 mK. Thevery-low-abundance '90Pt (0.013%) was arbitrarily set at the 100-mK position. The 1
95 Pt line is shown without hyperfine structure.The centroid of this pattern is located at the arrow.
ed lines was 30 d 370 mK, not at all unreasonable since thosevalues are given only to the nearest 0.1 cm-'. There was asignificant correlation of the difference with the line wavenumber, suggesting that calibration problems were presentin this early study. Predictions of width or structure ofthese lines await detailed isotope shift and hyperfine struc-ture analysis of these levels but may be an important factorin using these calculated VUV lines for accurate wavelengthcalibrations.
APPENDIX A
The natural abundances of the platinum isotopes were com-bined with relative isotope shifts14 to generate Fig. 3. Theonly odd isotope, 1
95 Pt, would usually show hyperfine split-ting into several components, but the centroid of any suchpattern would be located as shown. For completeness, thelow-abundance isotope, 190Pt, is shown, although it does not
appreciably affect these results. The weighted mean orcentroid of the pattern in Fig. 3 is calculated to be near thelocation of the hypothetical unsplit 195 Pt component. Theexact location is 0.093 times the isotope shift (between the194 Pt and 196Pt components) greater than the 195 Pt location.Since the isotope shift between these isotopes is about 100mK or less, the centroid of any platinum line is a fairly goodmeasure of the location of the 195Pt component. A bettermeasure of the line position would require a detailed analy-sis of the isotope shifts and hyperfine patterns, which isbeyond the scope of the present paper. No accounting forthe effects of possible self-reversal of strong lines has beenmade, but they would generally be small.
ACKNOWLEDGMENTS
I am indebted to J. W. Brault and R. P. Hubbard for operat-ing the FTS and to the National Solar Observatory for allow-ing me to use it. I also thank B. A. Palmer for developingmany of the data-reduction programs used in this study, M.V. Phillips for help with the data analysis and the figures,and R. J. Winkel, Jr., and the reviewer for useful correctionsand comments. This research was performed under theauspices of the U.S. Department of Energy under contractW-7405-ENG-36.
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