Abstract
The 1A21A1 system of HCHO is explained as vibronic transitions between the pure 1A1 ground state and an excited state which is a superposition of the 1A2, 1B1, and 1B2 states. It is shown that vibronic interaction through the ν4, ν5, and ν6 normal modes couples the excited electronic states. Through symmetry arguments, one comprehensive theoretical treatment explains the parallel bands as magnetic‐dipole transitions between the ground state and the 1A2 component of the excited state, while the perpendicular bands are interpreted as transitions to the 1B1 and 1B2 components of the mixed excited state. Intensity ratios of Type B/Type A bands are explained quantitatively through calculations of excited‐state vibrational wavefunctions.

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