Knowledge Bank

The $N=0\to 1$ transition of CrH and the $N=1\to 2$ line of CrD in their $\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}\Sigma +$ ground states have been recorded using submillimeter direct absorption techniques. This study was the first direct measurement of these transitions. These molecules were produced in a discharge plasma of chromium vapor and $H2$ or $D2$ gas. Three fine structure components were measured for each radical, and the proton and deuteron hyperfine splittings were also resolved. Spectroscopic constants were obtained by fitting the data with a Hund's case (b) Hamiltonian including rotation, spin-rotation, spin-spin, magnetic hyperfine, and electric quadrupole interactions. These measurements have resulted in improved spectroscopic constants for both isotopomers. The hyperfine parameters suggest that the bonding in CrH has a nonnegligible covalent character. This molecule may be present in the outer envelope of late-type stars and formed by ion-molecule reactions from neutral or ionized Cr and hydrogen gas. Chromium has a cosmic abundance of $Cr/H=5\times 10-7$.