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The C(2)$^1Π_u$ state in Rb$_2$. Observation and deperturbation

We report a systematic study of the C(2)$^1Π_u$ electronic state in rubidium dimer, observed in polarization labelling spectroscopy experiment through the C $\leftarrow$ X$^{1}Σ^{+}_{g}$ transitions recorded under rotational resolution in two isotopologues $^{85}$Rb$_2$ and $^{85}$Rb$^{87}$Rb. Regularity of the vibrational progressions was distorted by numerous interactions with the surrounding 2$^1Σ^{+}_{\mathrm{u}}$, 2$^3Π_{\mathrm{u}}$ and 3$^3Σ^{+}_{\mathrm{u}}$ states. Deperturbation was performed by coupled-channels analysis taking into account spin-orbit and rotational interactions. Potential energy curves and parameters describing the off-diagonal matrix elements were determined as functions of internuclear distance. About 3000 line frequencies from the present study are reproduced with a standard deviation of 0.07 cm$^{-1}$, in agreement with the experimental accuracy. Along with this, the model is consistent with all high resolution experimental data on the involved electronic states, available in the literature.