Nature Physics 10, 779–784 (2014); published online 31 August 2014; corrected after print 20 May 2015.

In the version of this Article originally published the assignment of states with symmetric electronic wavefunctions (|eg+〉) and with antisymmetric electronic wavefunctions (|eg〉) to the observed spectral lines was inverted throughout. This has now been corrected in the online versions.