CN spectroscopy and cosmic background radiation measurements

Abstract

A source of possible error, not previously considered, in the determination of the cosmic background radiation (CBR) temperature Tγ(CN) from interstellar CN absorption is proposed. This concerns the intrinsic assumption of the validity of using Hönl–London rotational line intensity factors to determine the rotational components of the oscillator strengths of the R(0), R(1), and P(1) lines of the CN B2Σ+–X2Σ+ (0,0) transition. Published data on interstellar CN absorption shows that Tγ(CN) is slightly greater than the standard cosmological CBR temperature, Tγ(COBE) = 2.725 K, determined by the COBE satellite, even when local excitation effects of CN are taken into account. From this difference, an estimation was made of the maximum corrections, a few percent, to the ratio of the line strengths and to the ratio of the Hönl–London factors. Fluorescence lifetime data were shown to give similar values for the corrections as well as providing evidence for intramolecular coupling between the relevant B2Σ+ state rotational levels and close-lying levels of the A2Π state, this being responsible for rendering the standard Hönl–London factors invalid. Key words: CN spectroscopy, cosmic background radiation, Hönl–London factors.