A stimulated emission study of the ground state bending levels of BH through the barrier to linearity and ab initio calculations of near-spectroscopic accuracy.

The ground state bending levels of BH have been studied experimentally using a combination of low-resolution emission spectroscopy and high-resolution stimulated emission pumping (SEP) measurements. The data encompass the energy range below, through, and above the calculated position of the barrier to linearity. For the bending levels (0,3,0) and above, the data show substantial K-reordering, with the K = 1 levels falling well below those with K = 0. A comparison of the high-resolution rotationally resolved SEP data to our own very high level ab initio calculations of the rovibronic energy levels shows agreement approaching near-spectroscopic accuracy (a few cm). The data reported in this work provide very stringent tests for future theoretical treatments of this prototypical seven-electron free radical.