Unravelling the visible spectrum of strontium monomethoxide

► Rotational analysis of A2E-X2A1 and B2A1-X2A1 transitions of SrOCH3. ► Optical-optical double resonance spectra unravel complex spectra. ► Anomalous spin-rotation splitting in B state of SrO12CH3 isotopologue. ► Perturbation in K = 1 F2 component of B state requires modified B value.