First Observation of the $2^1\Pi$ state of NaH

The upper levels (to the last bound vibrational level) of NaH $2^1\Pi$ state have been observed for the first time by using pulsed optical-optical double resonance fluorescence depletion spectroscopy. About 20 rovibrational energy levels, $\textit{v}$=2-5 and $\textit{J}$=1-9, were assigned to this electronic state by comparing the successive rotational spectra through selected intermediate levels of the $A^1\Sigma^+$ state. A decreased background fluorescence on the recorded spectra near the atomic asymptotic of Na(3$\textit{d}$)+H(1$\textit{s}$) indicates that the dissociation limit of $2^1\Pi$ state is approaching. Compared with the eigenvalues solved from the potential of Aymer's $\textit{ab}$ initial calculations, the vibrational quantum numbers were assigned. Un-observed lower levels ($\textit{v}$=0 and 1) are due to the lack of Franck-Condon factor under accessible intermediate levels of the $A^1\Sigma^+$ state.