The A2 (n, 3px) Rydberg state of formaldehyde

Pairs of bands were observed in the absorption spectrum of H₂CO, HDCO, and D₂CO in the 1470-1430 Å region which were characterized by well-defined rotational band contours. A band contour synthesis of the observed band envelopes demonstrated that the bands are described by type C selection rules. From the observed intervals and their shifts on deuterium isotope substitution these bands were assigned to quantum additions of ν₅ and ν₆. The symmetry of the upper state was deduced to be ¹A₂ from the vibrational assignment of the bands and their polarizations, while the results from MO theory lead to the orbital assignment n → 3p_x. The electronic transition then is .