Spectroscopy on the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>A</mml:mi><mml:mmultiscripts><mml:mi mathvariant="normal">?</mml:mi><mml:mprescripts /><mml:none /><mml:mn>1</mml:mn></mml:mmultiscripts><mml:mo>?</mml:mo><mml:mi>X</mml:mi><mml:mmultiscripts><mml:mi mathvariant="normal">?</mml:mi><mml:none /><mml:mo>+</mml:mo><mml:mprescripts /><mml:none /><mml:mn>1</mml:mn></mml:mmultiscripts></mml:mrow></mml:math> transition of buffer-gas-cooled AlCl

Aluminum monochloride (AlCl) has been proposed as an excellent candidate for laser cooling. Here we present absorption spectroscopy measurements on the $A^1\Pi \leftarrow X^1\Sigma^+$ transition in AlCl inside a cryogenic helium buffer-gas beam cell. The high resolution data enables rigorous, quantitative comparison with our high-level ab initio calculations of electronic and rovibronic energies, providing comprehensive picture quantum structure. combination spectral theory permits evaluation spectroscopic constants associated properties, like equilibrium bond length, order magnitude higher precision. Based measured molecular $A^1\Pi$ state, estimate Franck-Condon factor 99.88%, which confirms that is amenable to