The pure rotational spectrum of the X *Z+ ground electronic states of the BaOH and BaOD radicals has been observed using millimeter/sub-millimeter direct absorption spectroscopy. The lesser abundant isotopically substituted species, ""BaOH and 13'BaOH, have been detected as well. The radicals were created by reacting barium metal vapor, produced in a Broida-type oven, with either H2& or D202, The rotational and spin-rotation constants were determined for the molecules from a nonlinear leastsquares fit to the data, using a 2Z Hamiltonian. Hyperline constants were also derived for '"'BaOH, the one species where hypertine structure was resolved. 'These mcasurcments wntirm a linear structure for BaOH.
1. IntrodnctloIt
The spectra of the alkaline-earth metal monohydroxide radicals have been investigated in the past primarily at optical wavelengths. For example, Harris and co-workers have observed both the A *II-X 'E+ and B *2+-X 'IP systems of CaOH [ 11, as well as the B *x+-X *C+ transition of SrOH and SrOD [ 2 1.
Bernath and Kinsey-Nielsen [3] have also studied the BZEf-X *C+ system of CaOH, using Fourier transform spectroscopy, while Kinsey-Nielsen, Brazier and Bemath [4] investigated the B 2Z+-X *Z+ transition of both BaOH and BaOD. In general, these optical measurements have shown these radicals to be linear, except for MgOH, which has some quasi-linear properties [ 5 1. Their linear structure is evidence for chiefly ionic bonding in these species.
Recently, Ziurys and co-workers have observed the pure rotational spectrum of the ground electronic states of several alkaline-earth hydroxide radicals at high resolution, including CaOH [6], MgOH [ 71, and SrOH [ 81. These species were investigated using millimeter/sub-millimeter direct absorption spectroscopy. The ground (0, 0,O) vibrational state of these radicals, as well as several of their low-lying ' NASA Space Grant Fellow.