The wavenumbers of nearly 8000 transitions in Er I and Er II lying in the visible and near ultraviolet have been measured to an estimated accuracy of ±0.015 cm<sup>−1</sup>. Of these lines, 385 are observed to show self-reversal at extreme excitation conditions. The high precision of the line list, along with these reversal data, have led to the identification of the seven lowest levels of the ground configuration of neutral erbium, 4<i>f</i><sup>12</sup>6<i>s</i><sup>2</sup>. Furthermore, the classification of the eight lowest levels in singly ionized erbium, as reported by McNally and Vandersluis, has been well corroborated, and two more levels have been found.
© 1965 Optical Society of America
LOUIS C. MARQUET and SUMNER P. DAVIS, "Atomic Energy Levels of Neutral Erbium," J. Opt. Soc. Am. 55, 471-474 (1965)