  A high resolution study of the electronic states 1^1\Sigma^+_u and 1^1\Pi_u
which belong to the asymptote 4^1D + 5^1S and of the state 2(A)^1\Sigma^+_u,
which correlates to the asymptote 5^1P + 5^1S, is performed by
Fourier-transform spectroscopy of fluorescence progressions induced by single
frequency laser excitation. Precise descriptions of the potentials up to 2000
cm^{-1} above the bottom are derived and compared to currently available ab
initio calculations. Especially for the state 1^1\Sigma^+_u large deviations
are found. Rather weak and local perturbations are observed for the states
1^1\Pi_u and 2^1\Sigma^+_u, while a strong coupling of the state 1^1\Sigma^+_u
to the component \Omega=0^+_u of the state 1^3\Pi_u, which belongs to the
asymptote 5^3P + 5^1S, is indicated.
