576 L E T T E R S T O T H E E D I T O R Vol. 55

New Odd Levels of Neutral Erbium (Er I) NISSAN SPECTOR

National Bureau of Standards, Washington, D. C. 20234 (Received 23 January 1965)

RECENTLY, Marquet and Davis1 reported their first results on the spectrum of neutral erbium, which included four low

levels belonging to the 4 ƒ126s2 configuration. The 3H6 was desig-

TABLE I. Energy levels of 4 ƒ11(4I15/2)5d6s2 of Er I.

May 1965 L E T T E R S T O T H E E D I T O R 577

FIG. 1. Energy level diagram for 4 ƒ11(4I15/2)5d6s2 of Er I.

nated as the ground state. They observed approximately 300 self-reversed lines, and classified 45 of them with these levels.

The great number of self-reversed lines that remain unclassified, however, indicates strongly the presence of another low-lying configuration.

By means of a King furnace, we have observed approximately 1200 lines of erbium in absorption, in the region between 2700-4700 Å. The abundance of those absorptions, despite the limited wavelength region covered by our observations, put the assign­ment of the ground state of Er I in question. It was suspected2

that the large 4 ƒ115d6s2 configuration lies very low, and might compete with 4 ƒ126s2 for the ground state.

By differencing the absorption lines and using wavelengths from Ref. 1, we were able to find 20 levels which we assign to 4 ƒ115d6s2. These include the complete group of 10 levels obtained by adding, in J1-j coupling, a 5d electron to the 4I15/2 level of the 4ƒ11 core.

These 10 levels have J values ranging from 5 to 10. They account for 103 additional self-reversed lines. Thus, together with those classified by 4 ƒ126s2 levels, more than half of the observed self-reversed lines are now classified.

Lines connecting the group of new levels and those reported by Marquet and Davis were observed by us on our infrared emission plates. They put the lowest of the 4 ƒ115d6s2 levels 7176.52 cm - 1 above the 3H6 level of 4ƒ126s2, thus establishing the latter as the ground state of neutral erbium.

Table I gives the list of the ten 4 ƒ11(4iI15/2)5d6s2 levels, and Fig. 1 displays their structure.

Altogether more than 400 new lines were classified. A full list of these lines, along with the upper levels, as well as a preliminary theoretical least-squares calculation, is now being prepared for publication. We wish to thank C. H. Corliss and M. Wilson for taking the King furnace exposures at the Imperial College, London.

1 L. C. Marquet and S. P. Davis, J. Opt. Soc. Am. 53, 1349A (1963); L. C. Marquet, Ph.D. Dissertation, University of California, Berkeley (1964). 2 L. C. Marquet, W. E. Behring, and S. P. Davis, J. Opt. Soc. Am. 54, 1403A (1964).