Laboratory optical spectroscopy of the phenoxy radical as a candidate for diffuse interstellar bands

Abstract

The gas-phase optical absorption spectrum of the B2A2 ← X2 2 B1 transition of a phenoxy radical (C6H5O), which is a candidate molecule of diffuse interstellar bands (DIBs), was observed in the discharge of anisole using a cavity ring-down spectrometer. The spectrum comprised four peaks having broad and asymmetric profiles, which were regarded as a vibrational progression of ∼500 cm-1. The progression was assigned to the 6a mode, and the broad and asymmetric profiles of the peaks accounted for the sequence of the 10b mode. Each component in both the progression and the sequence had a broad structure of 20-30 Å, which can be explained by a convolution of lifetime broadening and a rotational profile. Based on these assignments of the progression and the sequence, the vibronic transitions from v = 0 in the X2B1 ground state to v6a = 0-3 in the B21excited state were extracted to compare the laboratory bands with DIBs. Although the transitions did not agree with the observed DIBs, the upper limit of the column density for the phenoxy radical in the diffuse clouds toward HD 204827 was evaluated to be 7 × 1014 cm-2.