Spectroscopy of the X1Σ+, a1π and B1Σ+ electronic states of mgs

The spectra of some astrophysical sources contain signatures from molecules containing magnesium or sulphur atoms. Therefore, we have extended previous studies of the diatomic molecule \chem{MgS}, which is a possible candidate for astrophysical detection. Microwave spectra of X$^1\Sigma^+$ , the ground electronic state, were reported in 1989\footnote{S. Takano, S. Yamamoto and S. Saito, Chem. Phys. Lett. \textbf{159}, 563-566 (1989).} and 1997\footnote{K. A. Walker and M. C. L. Gerry, J. Mol. Spectrosc \textbf{182}, 178-183 (1997).}, and the B$^1\Sigma^+$--X$^1\Sigma^+$ electronic absorption spectrum in the blue was last studied in 1970\footnote{M. Marcano and R. F. Barrow, Trans. Faraday Soc. \textbf{66}, 2936-2938 (1970).}. We have investigated the B$^1\Sigma^+$--X$^1\Sigma^+$ 0-0 spectrum of \chem{MgS} at high resolution under jet-cooled conditions in a laser-ablation molecular source, and have obtained laser-induced fluorescence spectra from four isotopologues. Dispersed fluorescence from this source identified the low-lying A$^1\Pi$ state near 4520 \wn. We also created \chem{MgS} in a Broida oven, with the help of a stream of activated nitrogen, and took rotationally resolved dispersed fluorescence spectra of the B$^1\Sigma^+$--A$^1\Pi$ transition with a grating spectrometer by laser excitation of individual rotational levels of the B$^1\Sigma^+$ state via the B$^1\Sigma^+$--X$^1\Sigma^+$ transition. These spectra provide a first observation and analysis of the A$^1\Pi$ state