The $A^{1}\Sigma^{+}_{u}$ + (1)$^1\Pi_u$ system of Mg$_2$
  The $A^1\Sigma^+_u$ - $X^1\Sigma^+_g$ UV spectrum of Mg$_2$ has been investigated with high resolution employing Fourier-transform spectroscopy and laser excitation. Computer simulation and fit of line positions to the overlapping structures in the spectra yield precise transition frequencies. Starting with the well characterized ground state $X^1\Sigma^+_g$ from former work we derived excited energy levels and report on the evaluation of the $A^1\Sigma^+_u$ excited state, which is found to interact with another electronic state, which we identify as the lower part of the (1)$^1\Pi_u$ state. A coupled channels fit to the level energies of the upper state yields a reliable potential energy curve for the $A^1\Sigma^+_u$ for the range of vibrational levels 1 $\le$ $v'$ $\le$ 46. A potential energy curve for the (1)$^1\Pi_u$ state is proposed, but the (1)$^1\Pi_u$ state is only characterized by its coupling to the A state, and no direct transition to a level of the (1)$^1\Pi_u$ could be uniquely identified due to the overlapping spectral structures. 
Horst Kn\"ockel, Steffen R\"uhmann, Eberhard Tiemann
physics.atom-ph physics.chem-ph physics.optics