We report the measurement of the electric dipole moment of aluminum monochloride (AlCl) using a cryogenic buffer-gas beam source. Using Stark shift spectroscopy, we derive values for the dipole moments in the body-fixed frame of the two lowest vibrational states for the $X^1{\mathrm{\Sigma }}^{+}$ electronic state, ${\mu }_X(v^{\prime \prime }=0)=-1.679$ D and ${\mu }_X(v^{\prime \prime }=1)=-1.761$ D, and for the $A^1\mathrm{\Pi }$ state, ${\mu }_A(v^{\prime }=0)=-1.030$ D and ${\mu }_A(v^{\prime }=1)=-1.052$ D. We also show that the ab initio calculations of the dipole moment and $T_e$ energy of AlCl are sensitive to the level of treatment of the spin-orbit orbit coupling. We further lay out the implications of these results for astrophysical models of stellar and planetary evolution that have used a substitute value for the dipole moment of AlCl until now.

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• Atomic & molecular structure
• Density functional theory
• Dipolar molecules
• Molecular spectra
• Stark effect
• Ab initio calculations