OH spectroscopy and chemistry investigated with astronomical sky spectra

This study summarizes the use of a large catalog of astronomical sky spectra to study different aspects of OH spectroscopy and chemistry in the terrestrial night sky. The sky spectra are unique in that they have high spectral resolution, cover the entire visible wavelength region in one exposure, and are intensity-calibrated with respect to standard stars. The intensity calibration, in particular, allows a significant revision to the OH Meinel band intensity distribution that has been in use for 43 years and permits critical evaluation of the many available sets of OH emission coefficients. The spectra further allow the OH rovibrational population distributions to be monitored throughout many nights. The OH vibrational population distribution is found to change during the night, with the population ratio between the extreme high-v and low-v levels that we can detect, v = 9 and v = 3, varying by as much as a factor of two; the low-v levels being predominant earlier in the night. It has been common to determine the kinetic temperature of the OH emission region by assuming that it is equal to the low-J rotational temperature associated with particular OH bands, typically bands originating in the v = 6 and v = 8 levels. The present calibrated data set reveals that the rotational temperatures are significantly greater for high-v than for low-v levels, the typical difference between v = 3 and v = 8 being 15 K. Previous attempts to establish that a difference existed are consistent with our current observations, although conclusions from those earlier results were limited by relatively wide error limits. The present rovibrational population measurements, which extend to high rotational levels (J' <= 25.5), also reveal that the high-J populations are largely independent of vibrational level - the high-J population in v = 3 is similar to that in v = 7.