The OH free radical is widely observed in flames, the upper atmosphere, comets, the interstellar medium, the sun, and cool stars. The most recent spectroscopic constants for v = 0-3 of the X 2 state are available from the comprehensive analysis of Mélen et al. (1). For higher vibrational levels v < 11, the best work is that of Abrams et al. (2). A few higher vibrational levels (v = 11-13) are known from the analysis of Copeland et al. (3) of the B 2 + -X 2 i system. In the course of our work on hot water vapor, we have also detected the Meinel system of OH. Starting with our detection of hot water in the spectrum of a sunspot (4), we have reported on a series of laboratory thermal emission spectra (5-8). Unfortunately at 1800 K, the laboratory samples are not a very good match for the sunspot at 3000 K and therefore we have very recently investigated the emission from an oxyacetylene torch, similar to the one used by Flaud et al. (9) in their pioneering work on hot water. The new spectra of hot water now closely match those of the sunspot. In order to make a hot water * Zero energy is the F ie (1.5) level of v = 0.
* * Extrapolated value from Mélen et al.
(1).