Observation of several new electronic transitions of the SrOH free radical

Abstract

We report four new electronic transitions of the SrOH free radical, the C̃ 2∏–X̃ 2∑+, D̃ 2∑+–X̃ 2∑+, Ẽ 2∑+–X̃ 2∑+, and F̃ 2∏–X̃ 2∑+ transitions. SrOH was prepared in a supersonic jet by laser ablation and spectra were recorded using laser-induced fluorescence. The C̃ 2∏–X̃ 2∑+ excitation spectrum shows complex vibronic structure which is attributed, at least in part, to Renner–Teller activity in the excited electronic state. This is supported by dispersed fluorescence spectra which show substantial bending mode activity in the emission from several different excited vibronic levels. It is suggested that the prominence of nominally forbidden vibrational features arises from a large change in permanent electric dipole moment between the X̃ and C̃ states. In turn, this suggests that the C̃ 2∏ state of SrOH is the analogue of the “reverse-polarized” ∏ states known for the alkaline-earth monohalides, i.e., the highest occupied π orbital points towards the O atom. The D̃ 2∑+–X̃ 2∑+, Ẽ 2∑+–X̃ 2∑+, and F̃ 2∏–X̃ 2∑+ spectra are much simpler than the C̃–X̃ system, being dominated by regular structure in the Sr–O stretching vibration.