The levels of Es I were undetected in the first step of the spectral analysis which involved only 53 lines (70.5) of neutral and singly-ionized einsteinium. In the extended observations by Worden et al. (74.5), eight strong and reversed lines which involve undoubtedly the well-isolated ground level 5f¹¹(4I_15/2)7s² of Es I were classified in the range of 355-313 nm where resonance transitions 7s²-7s.7p are expected. In agreement with the predictions by Brewer (71.1), the lowest energy levels of 5f¹¹7s7p and 5f¹⁰6d7s² were found below 20000 cm^-1. Finally an excited odd level was located at 8759 cm^-1 and labelled 4I_13/2.

The improved knowledge of the whole sequence of 5f^N7s² configurations along the neutral actinides and the subsequent parametric analysis with generalized least-square techniques, led to assign this 8759 level to the lowest j = 11/2 level of 5f¹¹7s² (80.4), and to search for other low levels of the ground configurations. The two levels at 7894 and 10244 cm^-1, found (unp.9) from the line list of (74.5), have been confirmed by systematic measurements of hyperfine structure, as part of an extended analysis of Es I (unp.17). This current analysis make use of spectra photographed at Argonne in the late 70's.

The two lowest levels of 5f¹¹7s8s confirm the regular trends of the 7p-8s transitions in all neutral actinides, as well as the 5f¹¹7s7p label of the lowest even levels.

The hyperfine structure of the ground level has been measured very accurately by the atomic beam magnetic resonance method (75.2), and the hyperfine constants A = 0.02725729 cm^-1 and B = - 0.1439749 cm^-1 lead to a total width W of 1.5428 cm^-1, i.e. 0.011 cm^-1 larger than the optical measurement of (74.5). All values of W reported in the Tables have been corrected accordingly.